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The Persian Gulf Region,A Climatological Study

U.S. Mtrine Corps

FMFRP 0-54

PCN1LiIJ 0005LFII 111)

DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited

DEPARTMENT OF THE NAVYHeadquarters United States Marine Corps

Washington, DC 20380—0001

19 October 1990

FOREWORD

1. PURPOSE

Fleet Marine Force Reference Publication 0-54, The Persian GulfRegion. A Climatological Study, provides information on theclimate in the Persian Gulf region.

2. SCOPE

While some of the technical information in this manual is of usemainly to meteorologists, much of the information is invaluableto anyone who wishes to predict the consequences of changes inthe season or weather on military operations.

3. BACKGROUND

a. Desert operations have much in common with operations inthe other parts of the world. The unique aspects of desertoperations stem primarily from deserts' heat and lack ofmoisture. While these two factors have significant consequences,most of the doctrine, tactics, techniques, and procedures used inoperations in other parts of the world apply to desertoperations. The challenge of desert operations is to adapt to anew environment.

b. FMFRP 0-54 was originally published by the USAFEnvironmental Technical Applications Center in 1988. In August1990, the manual was published as Operational Handbook 0-54.

4. SUPERSESSION

Operational Handbook 0-54 The Persian Gulf. A ClimatologicalStudy; however, the texts of FMFRP 0—54 and OH 0-54 are identicaland OH 0-54 will continue to be used until the stock isexhausted.

5. RECOMMENDATIONS

This manual will not be revised. However, comments on the manualare welcomed and will be used in revising other manuals on desertwarfare. Submit comments to ——

Commanding GeneralMarine Corps Combat Development Command (WF12)Quantico, VA 22134—5001

6. CERTIFICATION

Reviewed and approved this date.

BY DIRECTION OF THE COMMANDANT OF THE MARINE CORPS

M. P. CAULFIELDMajor General, U.S. Marine CorpsDeputy Commander for Warfighting

Marine Corps Combat Development CommandQuantico, Virginia

Di ST: 1L100005L4000

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PREFACE

This technical note documents and consolidates work done to complete USAFETACProject Number 703-30 for 5WW/DNC, Langley AFB, VA 23665-5000. The projectleader was Capt William F. Sjoberg, USAFETAC/ECR. Lead researcher and writer wasMr Kenneth R. Walters, Sr., USAFETAC/ECR. USAFETAC's Operating Location A atAsheville, NC made a major contribution in providing special data summaries. Theauthors are indebted to Mr Keith Grant of the British Meteorological Office for his out-standing cooperation and many contributions of data. Companion documents to thiswork provide detailed electrooptical transmittance climatology and refractivity data forthe Persian Gulf region; see USAFETAC/TN-88/003, Persian Gulf TransmittanceStudy, and USAFETAC/TN-88/004, Persian Gulf Refractivity Study.

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Chapter 1 INTRODUCTION.Area of Interest.,,,,,.,,,,,.Study ContentClimatological RegimesConventionsData SourcesRelated References

Chapter 2 PHYSICAL GEOGRAPHYNaps and ChartsThe Persian Gulf

The Iranian ShoreThe Northwestern Shore.,,.,The Southern Shore

The Strait of HormuzThe Gulf of Oman

The Iranian CoastThe Onani CoastThe Ornani Arabian Sea Coast

Chapter 3 THE MONSOON CLIMATEThe Monsoon Defined.,,,,,,,,.,,,,,,.Seasons of the Southern Asia Monsoon

The Northeast Monsoon.,,.,,The Spring Transition,,,,,.The Southwest Monsoon.,The Fall Transition..,,

Tropical Storm Formation.,Causes of the Monsoon

Chapter 4 THE NORTHEAST MONSOONSemipermanent Climatic Controls,.,.,,,.

The Mediterranean Storm Track,,,,,,,The Persian Gulf TroughThe Saudi Arabian HighNortherly Airflow Cocnponent,,...,.,,...The Siberian HighArabian Peninsula/Iran Upper—Air Ridge.The Polar Jet Stream.,,.,.,..,,,..,,.,.The Subtropical Jet StreanWarm Persian Gulf Waters...,,,,.

Transitory Synoptic FeaturesThe Ornani Convergence Zone (OCZ)

Mesoscale Synoptic FeaturesTypical WeatherSea Surface Conditions

The Strait of HormuzMesoscale Synoptic FeaturesTypical WeatherSea Surface Conditions

The Gulf of Oman and Adjacent LandMesoscale Synoptic FeaturesTypical WeatherSea Surface Conditions

The Reani Arabian Sea Coast,.,.,,,.Mesoscale Synoptic FeaturesTypical WeatherSea Surface Conditions,,,.,,,,,.

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CONTENTS

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Surface Cold Front/Strong Upper-Level TroughsSurface Cyclogenesis

The Persian Gulf Proper and Adjacent Land Areas.,

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Mean May Flow.Transitory Synoptic Features

Tropical Cyclone Frequency .Tropical StormsThe Onset VortexThe Desert FrontThe Omani Convergence Zone (OCZ)Land—Sea Breezes

The Persian Gulf and Adjacent LandMesoscale Synoptic FeaturesTypical WeatherSea Surface Conditions

The Strait of HormuzMesoscale Synoptic FeaturesTypical WeatherSea Surface Conditions

The Gulf of Oman and Adjacent LandMesoscale Synoptic Conditions...Typical WeatherSea Surface Conditions

The Onani Arabian Sea CoastMesoscale Synoptic Conditions...Typical WeatherSea Surface Conditions

Mesoscale Synoptic Conditions..Typical WeatherSea Surface Conditions

The Strait of HormuzMesoscale Synoptic Conditions...Typical WeatherSea Surface Conditions

The Gulf of Oman and Adjacent LandMesoscale Synoptic Conditions...Typical WeatherSea Surface Conditions

The Ornani Arabian Sea CoastMesoscale Synoptic Conditions...Typical WeatherSea Surface Conditions

Chapter 5 THE SPRINGTRANSITLONMajor Climatic Controls

Flow Pattern ReversalStrengthening Continental Low Pressure CentersThe Upper-Level (Above 300mb) Subtropical Ridge....Interaction Between the Polar and Subtropical Jets.

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Chapter 6 THE SOUTHWEST MONSOON REGIMEMajor Climatic Controls

A Massive Thermal Low—Pressure TroughAn Upper—Air RidgeThe Monsoon Trough/Intertropical Convergence Zone (ITCZ)The Southwest MonsoonStrong UpwellingThe Tropical Easterly Jet (TEJ)TwolrnportantAirflowFeaturesMid—Level MoistureA Persistent Mid—Level TroughExtremely Warm Persian Gulf WatersMean July Flow

Transitory Synoptic FeaturesWeak Upper—Level Troughs in the WesterliesNorthwestward Movement of the Monsoon Trough/ITCZThe Onset VortexTropical Cyclonic Storms/Cyclones

The Persian Gulf and Adjacent Land Areas

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.50Strong Upper—Level Westerlies . .50Mediterranean Cyclones 50A Weak Mid—Level Trough 50The Monsoon Trough/Intertropical Cony 50Tropical Cyclones 50Semipermanent Sudan Low 50Mean October Flow SO

Transitory Synoptic Features 54Weak Upper—Level Troughs 54"Winter Shamals" 54

The Persian Gulf and Adjacent Land Areas 55Mesoscale Synoptic Conditions 55Typical Weather 55Sea Surface Conditions.....,,,.,,,,,. 55

The Strait of I4ormuz 57Mesoscale Synoptic Conditions,,,,.,,, 57Typical Weather 57Sea Surface Conditions 57

The Gulf of Onan and Adjacent Land Areas 58Mesoscale Synoptic Conditions 58Typical Weather 58Sea Surface Conditions 58

The Omani Arabian Sea CoastMesoscale Synoptic Conditions...,,.,,Typical Weather 59Sea Surface Conditions

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Chapter 7 THE FALL TRANSITIONMajor Climatic Controls

Flow Pattern ReversalWeak Low—Level Continental Anticyclones..

ergence Zone (ITCZ)

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(1200Z 24 January 1973) .16

Representative 500mb Chart for 24-36 Hour Shamal (12007 24 January 1973) 17

Representative Surface Chart for 3—5 Day Shamal Surface Chart (00007 17 January 1973)...18

Representative 500mb Chart for 3-5 Day Shamal (00007 17 January 1973) 19

Areas of Stronger Shamal Winds 20

Mean May 850mb Flow 26


Mean May 5DOmb Flow 27



Mean Tropical Storm Tracks—-April and May Combined 29

Mean Position of the Desert Front 3D

Mean April Position of the OCZ 30

Mean July Surface Position of the Monsoon Trough/ITCZ 36

Mean Winds, Temperatures, and Geopotentials-—Salalah, 12007, July 1980 37

Mean July 850mb Flow 38





Mean Tropical Storm Tracks——June 41

Low—Level Wind Profileover8ahrain 42

Percentage Frequencies of Visibilities — 3MM, Bahrain 43

Percentage Frequencies of Visibilities — 1,100 Yards, Bahrain 44

Mean Tropical Storm Tracks-—October and November Combined SD

Mean October 850mb Flow 51



Mean October 3ODmb Flow 52

Figure 41. Mean October 200mb Flow 53

FIGURES

Figure 1. Topography of the Persian Gulf Region

Figure 2. Delineation of the 'Monsoon Region'

Figure3. MeanWinterStormTracksFigure 4. Typical Northeast Monsoon Surface Analysis

Figures. Typical Northeast Monsoon 500mb Analysis,,,,,,,,,,.Figure 6. Mean January 850mb Flow

Figure 7, Mean January 700mb Flow

Figure 8. Mean January 500mb Flow



Figure 11. Mean January Position of the 007

Figure 12. Representative Surface Chart for 24—36 Hour Shamal

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Figure 22.

Figure 23.Figure 24.

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Figure 26.

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Figure 32.

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Chapter 1

Ih'TROI%XTION

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AREA OF INTEREST. This study describes theclimatology of the Persian Gulf, the Strait ofHormuz, the Gulf of Oman, and their adjacentland areas for a radius of 150MM, Land areasinclude portions of Iran, Iraq, Kuwait, Bahrain,the United Arab Emirates, and Oman.

STUDY CONTENT. Chapter 2 describes geography inthe areas of interest. Chapter 3 discusses thephenomena of the monsoon climate as it affectsthe Persian Gulf region, Subsequent chapters(one for each of the four 'seasons" or clima—tological regimes listed below) describe the"semipermanent climatic controls' and "transi-tory synoptic features' conmion to each of thefour seasons, "Mesoscale synoptic features,""typical weather," and 'sea surface conditions'are then discussed for each season in each offour major geographical subdivisions (thePersian Gulf proper, the. Strait of Hormuz, theGulf of Oman, and the Omani Arabian Sea Coast)that make up the entire Persian. Gulf region,

CLIMATOLOGICAL REGIMES. The four climatologicalregimes (or "seasons") described here are thesame as those used in t.he "Forecasters' Handbookfor the Middle East/Arabian Sea" and are furtherdiscussed in Chapter 3, They are:

•The Northeast Monsoon (December—March)

SThe Spring Transition (April—May)

•The Southwest Monsoon (June—September)

SThe Fall Transition (October—November)

CONVENTIONS, The spellings of place names andgeographical features are those used by theUnited States Defense Mapping Agency AerospaceCenter (DNAAC) , Distances are in nauticalmiles. Elevations are in feet with a meter (m)or kilometer (km) value ininediately following,Temperatures are in degrees Fahrenheit with aCelsius conversion (°C) following. Wind speedsare in knots. Precipitation amounts are ininches, with a millimeter (em) conversionfollowing,

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DATA SOURCES, The authors used a number of UnitedStates and foreign climatological and meteoro-logical studies from the collections of the AirWeather Service Technical Library (AWSTL) aswell as si.anmarized data from the Air WeatherService Worldwide Climatic Database, Anextensive bibliography is provided. As thereader will note, there are many other excellentstudies on the climatology of the Persian Gulfregion--notably the AWS 100-series ForecasterMemos (FM5) and at least one 'Follow—OnTraining' slide/tape module. Forecastersinterested in this region should acquire andbecome familiar with ct least the following:

• NEPRF Contractor Report 83-06, Forecasters'Handbook for the Middle East/Arabian Sea, AD-A134312.

• NEPRF Technical Report 83-03, Navy TacticalAppl icat ions Guide, Volume 5, Part 1: "IndianOcean Red Sea/Persian Gulf Weather Analysisand Forecast Applications,' AD-A134412.

• NEPRF Technical Report 79-06, Winter Shanial inthe Persian Gulf, AD—A077727.

• "Terminal Forecast Reference Notebook,'USCENTAF ELF One.

RELATED REFERENCES. Station Climatic Swrrnaries--Asia (USAFETAC/DS-XX/O35), published by the USAFEnvironmental Technical Applications Center(USAFETAC) , gives standard weather stationsurmuaries for specific locations in the PersianGulf area; the latest version. of this documentis available from the Defense TechnicalInformation Center (OTIC) . 000 users may orderthrough AWS staff weather officers or directfrom: AWSTL, FL4414, Scott AFB IL 62225-5458.Detailed electrooptical and refractivity datafor the Persian Gulf region are available in twocompanion works to this study:

• USAFETAC/TN-88/O03, Persian Gulf TransmittanceStudy in the 8-12 Micron Band

• USAFETAC/TN-88/004, Persian Gulf RefractivityStudy.

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Chapter 2

PERSIAN 9/IF OGR4 IWV

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NAPS AND CHARTS. Figure 1 shows the generalgeography and topography of the Persian Gulfregion. For more detail , readers are urged toacquire and use DMAAC navigation charts. TheTactical Pilotage Chart (TPC) and OperationalNavigation Chart (ONC) series are recommended.Note that weather stations in Iran and Iraq havetransmitted virtually no weather data since thestart of the fran—Iraq war in 1980. Readersshould consult the current version of the 'AWSMaster station Catalog" for the most recentweather station information.

THE PERSIAN GULF itself extends in a great 530nautical mile concave curve from the Shattal'Arab (Euphrates River) delta to the Strait ofNormuz. Widths from the Shatt al'Arab to AbuOhabi average between I3ONM to a little over1SONM. East of Abu Ohabi, the gulf narrowsrapidly to JONM just east of Dubai. Gulf watersare relatively shallow——average depth variesfrom 65 to 200 feet (20 to 60m) , Deeper watersoccur only on the immediate western side of theStrait of Hormuz where depths reach 300 feet(90m).

The Iranian Shore. Terrain on the Iranian, ornorthern, shore of the Persian Gulf is in sharpcontrast to the rest of the region. Numerousmountain ranges, oriented parallel to thePersian Gulf—Gulf of Oman, front a narrow (20 to4ONM) coastal plain from the Tigris—Ehpratesdelta southeastward for 105MM or to just under2ONM southeast of Bushehr, Southeast and eastof this point to the Strait of Hormuz, the majorranges rise almost immediately inland from thecoast, The highest mountains are inland of thenorthwestern half of the gulf; here, numerouspeaks range from 11,000 feet (3,355m) to morethan 14,000 feet (4,270m). The highest peakreaches 14,465 feet (4,410m) in the Kuh—E Dinarrange near 30°57'N, 51°26'E.

The Northwestern Shore. Salt marshes at thenorthwestern end of the Persian Gulf extend fromKuwait to the Iranian coast near 30°N, 50°09'E,and inland from 15 to 7ONM. The greatest inlandsalt marsh penetration is between Kuwait andBandar-E Mahshahr, where they reach almost toBasrah, Except for iimiiediate river banks,desert lies beyond the salt marshes. Most ofthese deserts are covered with a top layer offine, powdery soil that serves as the source formajor dust and sand storms.

The Southern Shore is nearly flat from the Shattal'Arab southeastward to the On*ani Peninsula.The shoreline itself is smooth, the onlyexception being the Qatar Peninsula on the eastshore of the Gul of Bahrain, Al Khubar(Khobar) , the site of Dhahran InternationalAirport, marks the west side of this narrow,elongated indentation. Bahrain Island lies in

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the middle of the mouth of the Gulf ofBahrain, Terrain in Saudi Arabia inland of theshoreline is a mixture of sand dunes, wadis (drywashes), and low gravel ridges. Sand dunefields are concentrated within 5ONM of the coastfrom Kuwait to Dhahran; south of Ohahran thesefields extend inland from 75 to ZOONM,

Elevations rise slowly inland to about 1,700feet (520m). South of Abu Ohabi, and extendingsouthwest to the mountains of extreme easternYemen, is the 'Empty Quarter." There are almostno permanent human inhabitants here in the sanddunes and ridges.In the Oman Peninsula the sanddune fields become mixed with sand ridges.These extend south and eastward to within 50 to75NM of the mountain range along the Gulf ofOman-Arabian Sea coast. Salt flats are commonwithin 15MM of the coast along the entiresouthern gulf shore,

THE STRAIT OF HORMUL. At Oubai, the Persian Gulfbegins its rapid narrowing into the Strait ofFlormuz. Although the strait averages SONM inwidth, the narrowest part is slightly under30MM. Depth averages from 130 to 260 feet (40to 80m)

THE GULF OF OMAN extends south and thensoutheastward from the Strait of Hormuz into theArabian Sea. The generally accepted boundarybetween the Gulf of Oman and the Arabian Sea isan imaginary line drawn from Jiwani on the Iran—Pakistan frontier south—southwestward to Ras alHadd——the easternmost point of Oman. Widthsrapidly increase from 30MM at the eastern end ofthe Strait of Hormuz to 130MM wide at Muscat andfinally to 200NMwhere it joins the Arabian Sea,

The Iranian Coast, From the Strait of Hormuz tothe Iran—Pakistan border, mountain ranges formthe coast and rise rapidly, Narrow (10—15MM)plains are located around the mouths of therivers flowing into the Gulf, Elevations within4ONM of the coast exceed 5,000 feet (1,525m);within 90MM, peaks exceed 10,000 feet (3,O48m).

The thiani Coast. A range of mountains runs thelength of Oman from Jazireh—al Ghanam in theStrait of Hormuz to 20MM west of Ras al Hadd,Elevations range from 5,000 to 9,000 feet (1,525to 2,745m). The highest known elevation is9,777 feet (2,980m) in the Jabal Akhdar rangenear 23°14'M, 57°17'E. There is a narrow (15 to30MM) coastal plain between Shinas and Muscat.To the west of this mountainous spine,elevations are less than 1,700 feet (52Dm).

The Onani Arabian Sea Coast. A narrow (10-20mile) coastal plain is backed by hilly, some-times mountainous, terrain along the entirecoast. Maximum elevations reach 2,500 feet(760m)

Figure 2. Del meat Ion of the ftnsoon Region (fna Ravage, 1971).Khromov (1957) defined a "monsoonal" area as one with opposing seasonalwind flow. In this simplistic definition, Khromov ignored therequirement that monsoon winds blow steadily from a seasonaldirection. The hatched areas i.n Figure 2 indicate the "monsoonal area'according to Khromov's definition. Klein (1957) postulated that"mo.nsoonal areas' must show low frequencies of surface cyclone-anticyclone alternations in summer and winter. The heavy line withsouthward shading indicates the northern edge of the "monsoonal area"according to Klein. The rectangle encloses (also simplistically) whatis generally accepted as the monsoon region of the world. Ramage,however, qualifies this neatly rectangular expression; those quali-fications are discussed in Chapter 3. Note in the enlarged inset thatKhromov's dividing line arcs across the Strait of Hormuz and separatesthe Persian Gulf itself from the Gulf of Oman and the Arabian Sea..

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Chapter 3

THE MONSOON CLIMATE

THE MONSOON DEFINED. "Monsoon" is the Englishadaptation of the Arabian (and Indian) word"Mausum," the literal translation for which is"season." The term is generally applied toareas where there is a seasonal reversal of theprevailing winds. Unfortunately, some meteoro-logical schools of thought have applied thisidea too loosely. For example, the Russians andthe Germans have implied that the Russian Arcticand Berlin have "monsoon climates," A similardesignation, but with more justification, hasbeen given the United States Southwest by someAmerican meteorologists. The generally accepteddefinition of a "monsoon climate" is one byProfessor Cohn Ramage of the University ofHawaii, who provides these four criteria:

(1) Prevailing seasonal windbetween summer and winter mustleast 120 degrees.

(2) Both summer and winter mean wind speedsmust equal or exceed 10 knots (3 meters/sec)

(3) Wind directions and speeds must exhibithigh degrees of steadiness.

one cyclone/anticycloneJanuary or July in anyany 5—degree latitude/

Applying these criteria to the Persia:n Gulfregion, we find that the area south of July'sMonsoon Trough (or Intertropical ConvergenceZone, ITCZ) position definitely has a monsoonclimate (See Figure 25, page 36). Areas northof the ITCZ, although influenced by monsoonalcirculation, do not have monsoon climates.

SEASONS OF THE SOUTHERN ASIA MONSOON. Thesouthern Asia monsoon in the Persian Gulf regionis characterized by two distinct seasonsseparated by two short (30 to 45 day) transitionperiods.

The Northeast Nonsoon (December through March)is dominated by northeasterly low—level flowcontrolled by a deep Australian thermal lowcenter and an Asiatic high-pressure regioncentered over Siberian Russia. Upper—level flowis sustained westerly, with the subtropical jetover southern Arabia eastward into India.

The Spring Transition. In April and May thenortheasterly circulation rapidly breaks downand is replaced by the Pakistani Low pressurecenter and its associated secondary lows inSaudi Arabia and Saharan Africa. The ITCZ movesrapidly north into the intensifying heat lows,Southern hemisphere southeasterly trade windscross the Equator, recurve to becomesouthwesterly, and flow across the Arabian Seainto India. The initial surge occurs within a

period of 7 to 15 days from the time of thefirst sustained cross—equator flow. It followseither the development of a mid—troposphericvortex in the Bay of Bengal or an "onset vortex'in the extreme northern Arabian Sea, The lattermoves westward to affect Oman.

The. Southwest Monsoon. The onset surge at theend of the Spring Transition marks the start ofthe Southwest Monsoon that lasts from Junethrough September. southwesterly flow has a

direct affect on the Arabian Sea coast of Oman,the Gulf of Oman, the Arabian Sea coast of Iran,southern Pakistan, and India. Low—level flow issustained southwesterly, Upper-level flow issustained easterly; the Tropical Easterly Jet(TEJ) flows across India into East Africa at150mb,

The Fall Transition. In October and November,the Pakistani heat low collapses as days becomeshorter. Cooling over Siberian Russiareestablishes the Asiatic High pressure centersand the ITCZ migrates southward once again.Low—level northeasterly flow is reestablishedover southern Asia and the cycle repeats itself.

TROPICAL STORM FORMATION. Tropical stormsaffecting the northern Arabian Sea are confinedto the spring and fall transitions. These arethe only times when the ITCZ is over the ArabianSea between 15° and 25°N and strong verticalshears are absent-—both of which are necessaryfor tropical storm formation.

CAUSES OF THE MONSOON. The massive circulationreversal referred to as the monsoon is still notcompletely understood. Its ultimate cause——theseasonal variation in incoming solar radiation——is obvious. Solar radiation results in the fallformation and spring decay of the Asiatic High,as well as the spring formation and fall decayof the south Asian Low, What is still .nknownare the threshold gradients that trigger thesetransitions, The latest research suggests thatthe late springtime combination of latent heatreleased by pre-monsoonal thunderstorms oversouthern India, along with late springtimeheating over the Tibetan Plateau, rapidly formsan upper—level high pressure area over extremenorthern India and the Himalayas. The strongwinter westerly circulation is replaced bystrong sumer mid— and upper—level troposphericeasterly circulation. This circulation providesthe sustained outflow necessary for extendedprecipitation over southern Asia, In the fall,diminishing heavy convection over northernIndia, combined with cooling over the TibetanPlateau, has the opposite effect. Low—levelcirculation characteristic of the approachingmonsoon season precedes the reversal in upper—level flow. However, the monsoon has notofficially begun until the characteristic upper—level circulation has been established.

directionschange by at

(4) No more thancouplet occurs during2—year period withinlongitude rectangle.

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Chapter 4

THE AVRTNEAST MONSOON..

(Decarter - March)

SEMIPEmANENT CLIMATIC CONTROLS

THE NEOITERRSEAN STORM TRACK. This is the mainsource of low pressure systems and cool airduring this season. Extensive thunderstormsand heavy rains accompany the movement of a

secondary low across the Saudi Arabianpeninsula. Figure 3 shows mean January stormtracks representative of the NortheastMonsoon. Track A, which moves northeastwardthrough Turkey into the Caucasus mountains, isthe primary track for secondary low formation innorthern Saudi Arabia—Iraq. Track B, acrossLebanon—Israel—Syria into the Persian Gulf, isnormally associated with the 'winter shamals"that will be discussed later, Track C runsacross southern Egypt and the northern Red Seainto central Saudi Arabia, It is associatedwith a mean jet stream position over thesouthern Mediterranean or northern Africa and

often results In formation of a secondary waveover the central Red Sea. This, the leastactive of the three tracks, occurs in latewinter and early in the spring transition.

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THE PERSIAN StilE TROUGH, This relatively weaksemipermanent feature results from a combinationof heating over the Persian Gulf and an induced'lee-side trough' resulting from the highpressure centerc over Iran and northwesternSaudi Arabia, It forms a natural pathway forlow pressure centers moving out of southernIraq, Intensification and movement onto theArabian Persian Gulf coast as a transientfeature often occurs as the result of a

combination of the following three factors:(1) the passage of a cold front down the Gulf,(2) increased ridging over Iran, and (3)temporary weakening of the Saudi Arabian highpressure area. See further discussions in thesection titled "The Persian Gulf Proper andAdjacent Land Masses."

THE SAUDI ARABIAN HIGH is centered overnorthwestern Saudi Arabia as an eastwardextension of the Saharan high pressure ridge.

Figure 3. Mean Winter Stone Tracks.

NORTHERLY AIRFLOW COMPOIENT. The prevailing meanairflow component, as discussed in chapter 3, isnortherly over the entire region. Thisnortherly flow, however, is interrupted by thepassage of upper air troughs and theirassociated surface low pressure areas andsurface fronts/shear lines. See Figures 4 and 5for typical surface and 500mb analyses. Mean

January flow at 850, 700, 500, 300, and 200mb(5,000, 10,000, 18,000, 30,000, and 39,000 feet)is shown in Figures 6 through 10.

THE SIBERIAN HIGH has its primary axis acrosscentral Russia and a mean center over easternSiberia, Since direct northerly flow around thesouth side of this ridge is blocked by theCaucasus-, Northern Iranian, Hindu Kush, andHimalaya mountain systems, the cold Siberian airis forced to spill southwestward through Iranand eastern Turkey into Iraq and the northernportion of the Persian Gulf, Snow cover overthe Zagros mountains maintains (and evenintensifies) this cold air, with consequentstrengthening of downslope winds off the Zagrosinto the northeastern side of the Gulf.

ARABIAN PENINSULA/IRAN UPPER AIR RIDGE. 500mb highpressures centers are located over extremewestern Arabia and over the Zagros mountains Insouthwestern Iran, At 300mb, this ridge isdisplaced southward to between 20 and 22°N.

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THE POLAR JET STREAM is located near 30°N.December core speeds are 90 knots. In Januaryit dips to latitudes between 27 and 29°N withspeeds of 110 knots, then shifts back northwardto just above 30°N with maximum speeds of 80

knots in March, Actual positions varyconsiderably; in January, flow around thebott6m of a deep 500mb low may reach as farsouth as 20°N. Under these conditions, itmerges with the subtropical jet stream,

THE SUBTROPICAL JET STREAM. During this season,the Subtropical Jet flows from west to east overthe Arabian Peninsula. Mean position is between25 and 28°N; at 200mb, core speeds exceed 110knots over the Peninsula, Deep upper air lowsmay displace the jet south to near 2011.

WARM PERSIAN GULF WATERS. By mid-January, surfacewater temperatures average 70°F (21°C).Although considerably cooler than during theSouthwest Monsoon, they are about 3°F (1.6°C)warmer than that of the air passing over them,By April, however, Gulf water temperaturesaverage 74°F (22°C)——only 1°F (0.6°C) more thanthe air temperature. Higher evaporation ratesfrom the Persian Gulf result in a steadynorthwestward flow of surface water through theStrait of Hormuz into the Gulf; the currentjust west of the Strait reaches 2 to 4 knots. A

counter—current flows southeastward along thebottom down the Persian Gulf and out into theGulf of Oman.

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80 °E

Figure 1. Mean January 100mb Flaw.

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Figure 8. Mean January 500mb Flow.

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i'wAwslroar srworrc FEATURES

THE ONANI CONVERGENCE ZONE (OCZ). During normalnortheast monsoon conditions, there is a

convergence zone over Oman south of the Hajarmountains, It lies parallel to, and about 100to 150 miles inland of, the coast of the ArabianSea, Figure 11 shows its mean Januaryposition. Low-level northwesterly flow comingout of southeastern and southern Saudi Arabiameets the northeasterly monsoonal flow that hasbeen deflected inland due to strong sea breezecirculation, This zone does not exist whenthere is southwesterly flow ahead of coldfronts, or northwesterly flow behind.

SURFACE COLD FRONT/STRONG UPPER-LEVEL TROUGHS INTHE WESTERLIES. From mid-December through mid—March these fronts (with their associated upper—air troughs and often accompanied by closed500mb low centers) cross the region routinely.Occurrence is most common when a blocking highover northern Europe results in a high speedcore in the westerlies across the Mediterraneaninto Iraq. The characteristic middle and highclouds associated with these troughs show up

well on both visual and IR satellite imagery,These systems are accompanied by the followingphenomena:

Transitory Saudi Arabian Highs form in responseto the subsiding air beneath upper air ridgesmoving across the area from the west. Thesecells break off from the quasi—permanent SaudiArabian ridge and move eastward andsoutheastward across the Peninsula followingcold fronts.

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Northwestern Arabian Sea Highs form in responseto surface lows that move into northern SaudiArabia. Essentially, these highs are the 'cut-off' southeast portions of the normal ArabianPeninsula ridge. As the low tracks eastward orsoutheastward, high pressure spilling southwardbehind the cold front absorbs the independentsouthwestern Arabian Sea pressure center,

"Aziab is the Arabic name for the hot and drylow—level southerly winds that blow occasionallyover Saudi Arabia in March. These winds occurahead of a secondary low that often results fromthe northeastward extension of the Sudan lowthat moves eastward towards the Persian Gulf asa wave on a slow—moving cold front (Track C inFigure 5) , These wilds cause temperatures torise dramatically; Riyadh has reached 90°F(32°C) during very strung Aziab conditions.

"Kaus" is the local name for the southerly orsouthwesterly winds that blow over Oman, theUnited Arab Emirates, and the Persian Gulf inadvance of a surface low pressure center. Thesewinds cause the most extensive low cloud andprecipitation areas found during this season.

Strung Prefrontal and Frontal Thunderstorms formin and near the Persian Gulf. Satellite imageryhas shown conclusively that thunderstormsassociated with cold front passage down the Gulfare similar to those found in the United StatesMiddle West. The favored formation area forprefrontal squall lines is the central andsouthern Gulf.

Vortex Phenomena in the form of waterspouts havebeen observed in the southwestern Persian Gulfduring December. Although no known reports ofthis phenomenon were made prior to 1970, localArabic words and tradition indicate that suchphenomena have in fact occurred in the past.The recorded cases occurred in areas oflocalized convergence and resultant heavycumulus activity behind cold fronts or shearlines,

SURFACE CYCLOGENESES. Three favored locations forcycloqenesis in this area, in order of mostfrequent occurrence, are:

Southeastern Iraq—Northern PersianSecondary lows form in these regions into cold air advection, warm water, andchanneling through the Iraqui andmountains, notably the Lagros.

The Western Gulf of Oman, Secondary lows formhere for the same reasons as in southeasternIraq and the Northern Persian Gulf. The'channeling' here, however, is through theZagros Mountains of Iran along the northern Gulfof Oman coast and through the Omani PeninsulaMountains along the southwest side off the Gulfof Oman. This combination of airflowmodification induces cyclonic flow if a strongeast—west ridge lies over Iran.

50" so'

20" N

Figure .71. (lean January Position of the OCZ.

6u1 f,responseairflowIranian

Northeastern Sudan—Northern Red Sea—VesternSaudi Arabia. A weak, inverted low—pressuretrough stretches northward into Sudan from EastAfrica during most of the Northeast Monsoonseason. Passage of a cold front southeastwardacross Saudi Arabia, combined with itsassociated upper-level trough approaching Sudanfrom the west, often triggers the formation of asecondary low on the front over northeasternSudan and the Red Sea, The Red Sea serves as a

14

potent but localized moisture source forcyclogenesis here, The low then moves eastwardas a wave on the front, Under properconditions, this center will become the primarysurface low affecting Saudi Arabia and thesouthern Persian Gulf, This location is mostactive during late winter (primarily March) andduring the spring transition as the strength ofcold air outbreaks diminishes,

THE PERSIAN GULF PROPER AND A(UACEN LAND AREAS--WRIHEAST MONSOON

FEATURES. The followingto the Persian Gulf properadjacent land areas:

Gulf Cyclogenesis. Cold frontal stagnation in thesouthern Gulf results in cyclogenesis northwestof the Omani Peninsula—United Arab Emirates,This occurs when the associated upper—air troughslows sharply or becomes stationary in thisarea. Formation of a cutoff low at the base of

The Winter Shamal, "Shamal" is an Arabic word for'north." It refers to the strong northerly andnorthwesterly winds that occur in the PersianGulf and over its ininediate land areas followingpassage of cold fronts southeastward, TheShamal produces the most widespread hazardous

this region,. Its causes arefor the strong northerly winterMcClellan, Mather, Beale, and

NAS Lemoore in California'sLarge pressure gradients

cold frontal passages due toidence rapidly building surface

Arabia andlevel winds

reinforced by northwesterlybehind the middle levelare of two types:

Persistent dust and sandstorms on the scale ofthe Southwest Monsoon normally occur onlyduring these events. A detailed explanationof this phenomenon, including forecastingtechnique, is given in NEPRF Technical Report79—06, Winter Shamnal in the Persian Gulf,Figures 14 and 15 are representative surfaceand 500mb analyses of the 3—5 day shareal,

Gulf Trough intensifies in or justof the Persian Gulf after a low andated frontal system passes the Strait

resulting in high pressure buildingThese troughs maintain stronger than

s over the Saudi Arabian shore andwinds on the Iranian coast thatnormal nocturnal downslope land

If the Iranian high pressure cell iswell developed, conditions similar to

of the Yugoslav Adriatic coast canData on maximum downslope wind speeds

The Persiannortheastits associof Hormuzover Iran,normal winddowns lopereinforcebreezes,extremel ythe 'Bora"develop.Is not available, but dynamic and topographicconsiderations indicate that speeds better than50 knots over and just offshore of the immediateIranian coast are possible, Recent ALPEX

research suggests that only a shallow layeriiwaediately above the ridges accelerates; airbetween the top of this shallow acceleratedlayer and the undisturbed higher level windfield is slow moving and extremely turbulent.Under the synoptic circumstances describedabove, conditions similar to those found inALPEX research might occur over and

southwestward of the Zagros into the PersianGulf northeast of the Persian Gulf Trough.Weakening of the Iranian High and/or the PersianGulf Trough ends the severe downslope wind, or"Bora.' Persian Gulf Trough strength normallydecreases rapidly within 24 to 48 hours afterfrontal passage through the Strait of Hornruz andmarks the end of the 'Shamal

The 24-36 How Shainal. These begin with thepassage of the front. When the associatedupper air troughs are rapidly moving shortwaves, winds die after 24 to 36 hours; hencethe name. Such cases are relatively common,occurring two or three times a month duringthe Northeast Monsoon season. Winds typicallyreach 30 to 40 knots. Figures 12 and 13 arerepresentative surface and 500mb analyses ofthe phenomenon.

The 3—5 Day Shamal. Occuring one to threetimes a winter, this phenomenon produces thestrongest winds and highest seas found in theGulf. Over exposed Gulf waters, sustainedwinds reach 50 knots and produce 12— to 15—

foot waves, This shamal arises from thetemporary stagnation of a 500mb short waveover or just east of the Strait of Hormuz, orfrom the establishment of a mean long waveposition in the same area. In either case,the result is a strong northwesterly surfacewind that continues for 3 to 5 days.

TYPICAL WEATHER. The following weather conditionsmay be expected in the immediate Persian Gulfarea during the Northeast Monsoon:

Peninsula. Skies are often clear, but fog canreduce visibilities in coastal regions.Radiational cooling in the relatively moist airnear the water will cause fog to develop aftersunrise; visibilities remain between 2 and 3

miles well into the morning. Patchy stratus isfound during the late night and early morninghours along and just inland of coasts. Favoredlocations are shallow marshy areas——mostly atthe head of the Gulf around the Shatt A1'Arab——and along the United Arab Emirate coastline inthe extreme southern Gulf. Except in thiscoastal fog, visibilities are normally good,

MESOSCALE SYNOPTICfeatures are commonand its irwrmediately

the upper-air trough enhances it. Developmentis rapid over the warm Gulf water, Under suchconditions, strong low—level winds buildrapidly, Wind directions shift from south tosoutheast to northeast, and finally tonorthwest, as the low moves through the Straitof Hormuz into the Gulf of Oman, Precipitation,along with extensive low, middle, and high cloudlayers, occur on both sides of the Gulf. Thegreatest extent and precipitation intensityoccurs on the Iranian side. Moderate turbulenceand icing are found over both the OmaniPeninsula and the Iranian shore.

weather known tosimilar to thosewinds at Travis,Castle AFBs, andCentral Valley.develop behindupper—level subshigh pressure over western SaudiIraq, The strong northwesterly low—are then quicklyupper level windstrough. Shamals here

General Sensible Weather.troughs and associated frontthe only sensible weatherclouds over the Persian Gu

With no uoper—airs over this region,

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Sky Cover. Well—developed high and middle clouddecks are associated with upper air troughs.Light to occasionally moderate icing occurs inthese decks above the 7,000 to 9,000 foot (2.1to 2.7 km) freezing level. Low clouds with1.000 to 1,500 foot (300 to 450 m) ceilingsoccur in the inwediate area of the PersianGulf. These low clouds advect westward aroundthe north side of low pressure cells moving downthe Persian Gulf. They can extend inland overSaudi Arabia as far as Riyadh, Low cloudsnormally clear within 4 to 6 hours after frontalpassage. However, when appreciable rain hasfallen, cold air stratocumulus will form for 1

to 2 days after frontal passage. In the absenceof strong post—frontal surface winds, fogforming in those coastal areas near the head ofthe Gulf will persist throughout for thedaylight hours for the first 24 to 48 hoursafter frontal/trough passage.

Dust, Widespread dust that restricts visibilityto less than 3 miles normally occurs with thefirst frontal passage of the season. Lightprecipitation during the winter binds soilparticles together; winds above 25 knots areneeded to raise dust, Visibility fluctuationsoccur as a function of low—level wind speed--thestronger the winds, the lower the visibility,Visibilities can go as low as 50 metersinmiediately after the first frontal passage, atthe height of a very strong 'Shamal," or with an"Aziab,' described below. Dust withnorthwesterly or northerly winds is initiallypicked up from the Iraqi deserts to thesouthwest and west of Basrah, During March, an'Aziab" (a hot and dry southerly wind) precedesa secondary low moving eastward into centralSaudi Arabia. Dust associated with an 'Aziab'comes from the "Empty Quarter," or extremesouthern Saudi Arabia. By March, almost noprecipitation has fallen over these areas, andmost of the area is dry sand once again,

Winds are generally northwesterly at 10 to 15knots over the northern half of the Gulf. Windsover the southeastern half reflect the west-to-east orientation of the mountains on the Iranianshore. The south-to—north alignment of themountains on the Omani Peninsula of the Arabiancoast combine with these other features to'turn" the prevailing winds——first westerly,then southwesterly, finally southerly——as oneapproaches the Strait of Hormuz from the west.Speeds decrease to 5 to 10 knots. Land and seabreeze circulations, of course, modify thispattern. There are two areas with windssignificantly higher than those observed alongthe Arabian Peninsula coast (See Figure 16).During December and January, the area justnortheast of the Qatar Peninsula sees speeds 10to 15 knots higher than at Dhahran or Bharain,In late February and March, the area just offthe Iranian coast near Lavan Island east—northeast of Qatar has winds ID knots higherthan the rest of the area. Southwesterly orsoutherly low—level high speed wind maximums, or'low level jets' (LLJ5) , occur ahead of lowcenters throughout the area, LLJ5 arereinforced over the eastern side of the Persian

20

Gulf due to the channeling effects of the ZagrosMountains. There is no data that providesvertical profiles, but dynamic considerationsindicate that core speeds may approach 50 knots1,000 to 2,000 feet (305 to 610 m) above thesurface along and just offshore of the Iraniancoast, Sustained strong northwesterly winds——30to 50 knots——occur with the 'winter shamals'that will be discussed later.

Thunderstorms--often severe—-occur along coldfronts as well as in prefrontal squall linesthat form over the northern Gulf from lateDecember through early March; satellite imageryis extremely useful for locating them. Cloudbases are relatively high, averaging 4,000 to5,000 feet (1.200 to 1,500 m) except over thePersian Gulf. Tops often exceed 40,000 feet(12.2 km) and tops of 50,000 feet (15.2 km) havebeen reported. Surface hail and gusty windsnear 50 knots have also been reported. As inthe United States, severe icing and turbulenceare associated with these storms,

Precipitation on the Arabian side of the Gulfaverages 2 to 3 inches (50 to 75mm) a year;almost all during this season and almost allfrom thunderstorms. There is considerableprecipitation variability; some winter monthsmay receive more rainfall than the mean yearlytotals. "Kaus" winds ahead of the storm systemsmentioned above result in extended periods oflow cloud and drizzle over the middle andsouthern Persian Gulf, the Iranian coast, andthe southern Zagros Mountains, Such flowusually occurs ahead of a low center moving downthe Gulf, Higher elevations see thunderstormsand steady rain rsulting from forced lift upthe mountain slopes. Increased icing as well asmoderate mechanical turbulence are found alongthe Iranian shore and over the mountains, The

Figure 16. Areas of Stronger Shasta! Winds.

effects of these winds can be seen in theprecipitation totals--4 to 5 inches (100 to125m) a year, or 60% higher than on the Arabianside of the Gulf, The area around Bushehrreceives almost 13 inches annually——5 inches inDecember alone. As on the Arabian coast, thereis large variability; soee winter monthsreceive more precipitation in a month than themean yearly totals,

Air Temperature. Daytime maximum temperaturesalong the Imediate coast average between 65 and15°F (19 and 23°C). Inland, temperatures riseto 70 to 85°F (21 to 29°C) , Minimum tempe!-a-tures along the coast fall to 45 to 65°F (7 to19°C). Despite the warm Persian Gulf waters,dew points along eveh the imediate northerncoastline range from 30°F (-1°C) to 40°F(3°C). The southern Arabian coast has higherdewpoints--55 to 60°F (13° to 16°C)—-due towinds coming directly off the water, Dew pointsdrop rapidly as one moves inland from theirmeediate coast, Riyadh averages a 30°F (—1°C)dew point through the winter. Minimumtemperatures drop accordingly. Beyond 20 to 30miles inland, minimum temperatures drop to 35 to45°F (3 to 7°C). Dew points, however, riserapidly when a 'Kaus" wind is blowing. Riyadhhas reported rises to 59 or 61°F (15 to 16°C)just before passage of a cold front.

SEA SURFACE CONDITIONS, Persian Gulf.

Water currents in the Persian Gulf have a generalcounter—clockwise circulation, A steady flow ofsurface water enters the Gulf through the Straitof Hormuz, This surface current flows towardthe west and then northwest as it hugs theIranian shore. At the northern end of the Gulf,it turns southeasterly to parallel the Arabiancoast. Finally, it turns northerly along the

21

west side of the Omani Peninsula to join freshGulf of Oman water entering through the Straitof Hormuz, Mean speeds are less than 1 knot,Maximum calculated speed for a 100—year worstcase of combined wind and tidal currents, was 8knots at ,Jubail, As water increases in density.it sinks, moves southeastward, and flows backinto the Gulf of Oman as a bottom currentthrough the Strait of Hormuz. 85% of all waveheights average less than 6 feet in the westernPersian Gulf, Predominant directions along theArabian coast are, as might be expected, from330 through 360 degrees, However, under theinfluence of a "24 to 36 Hour Shamal ,' wavesroutinely reach 8 to 10 feet; for a M3

to 5 DayShamal," heights reach 12 to 15 feet. The usual'decay time" to reach the normal 6 foot heightsafter the ending of a "Shamal' varies from 6 to18 hours, depending on the duration and strengthof the "Shamal," 'Kaus" winds will establish a6 to 8 foot southwesterly wave; greatest heightsare found along the Iranian shore. Note thatthese are 'open water" waves; oil companiesoperating in the Gulf have reported higher wavesiermediately offshore during favorableconditions.

Water Temperatures. In the shallow waters of theGulf, temperatures cool rapidly during late falland early winter. Surface water temperatureranges from 72—76°F (21-24°C) in Oecember to 66-72°F (18—22°C) in February before rising to 68—74°F (20-23°C) in March. Temperatures inextremely shallow coastal waters reach 60°F(15°C) in January. Available temperaturesoundings in the western Persian Gulf show a

bottom temperature of 61°F (16°C) at 60meters, Temperatures show a steady and gradualdecrease in temperature between the surface andthe bottom of about 5°F (3°C). Thermoclines arenot evident.

THE STRAIT & NO1J(UZ--M1RTHEAST MONSOON

NESOSCALE SYNOPTIC FEATURES. The features thataffect the Persian Gulf proper and its adjacentland masses also affect the Strait of Hormuz.

TYPICAL WEATHER. The following weather conditionsmay be expected in the Strait of Hormuz and itsiumiiediately adjacent land areas:

General Sensible Weather. Sensible weather overthe Strait during the northeast monsoon seasonnormally consists of patchy low stratus andstratocumulus, occurring most frequently duringlate evening and early morning, Onshorecloudiness occurs only along the Iranian coastas a function of land—sea breeze circulation.

Sky Cover. Well-developed high and middle clouddecks are associated with upper—air troughs.Ceilings range from 7,000 to 9,000 feet (2.1 to2.4 km). Lower ceilings——1,000 to 1,500 feet(300 to 425 m)-—normally only occur justpreceding or in the imediate area of lowpressure centers. Light or occasionallymoderate icing and turbulence occur above 8,000to 10,000 feet just ahead of and with thepassage of upper troughs or sur lace low pressuresystems and their associated fronts, Icing andturbulence are stronger over the Omani Peninsulaand Iranian mountains.

Winds. Low—level airflow reflects a combinationof: (1) weak outflow from the Persian Gulf,(2) pronounced land—sea breezes (especiallyalong the Iranian shore) , and (3) a generalnortherly to northeasterly flow off the Iraniancoast. Complicating this pattern is theterrain—induced venturi effect. Althoughprecise data for the Strait itself is notavailable, it appears that in the absence of adynamic surface low or 'Shamal ,' wind speeds aredetermined by local land—sea breeze relation-ships and terrain peculiarities.

Winds along the north shore of the Strait arenortheasterly to easterly during the night andearly morning, and southerly to westerly duringlate morning and afternoon. Along the southernshore and under similar conditions, winds aresouthwesterly to westerly east of the :Straitduring the night and early morning, andsoutheasterly to southerly west of the Strait.Speeds average 3 to 7 knots. Late mornings andafternoons have pronounced sea breezes that aresouthwesterly to west—northwesterly west of thestrait and northerly to northeasterly east ofthe strait. Sea breeze speeds reach 12 to 20knots.

During 'Shamal" conditions, low—level winds turnwesterly to west-northwesterly approaching theStrait. Windspeeds over the Strait and alongthe Iranian shore reach 25 to 30 knots (ingusts) during sustained Shamal conditions; on

22

the Omani shore, gusts reach 30 to 35 knots. Asthe 'Persian Gulf Trough" forms and movessouthwestward across the Gulf, downslope windsblow off the Iranian coast. Strong winds arerare and normally confined to the strongeststages of a '3 to 5 Day Shanal

Dust. Almost always advected from Saudi Arabia orIraq, dust is normally confined to the firstcold front passages of the season, However,southwesterly flow late in the season ahead ofdeveloping low pressure areas, either in thePersian Gulf or in central Saudi Arabia, maybring blowing dust or sand into the Strait ofHormuz from the Saudi Arabian "Empty Quarter."Such occurrences rarely last longer than 12hours.

Thunderstor.s occur over the mountains on bothsides of the Strait, but are confined to areasimediately preceding, and with the passage of,cold fronts or upper troughs, Some may besevere. Tops often exceed 40,000 feet (12.2km) ; severe thunderstorm tops have beenreported above 50.000 feet (15.2 km).Prefrontal squall line thunderstorms tend to bestrongest over the Omani Peninsula; embeddedpre—warm front thunderstorms are strongest overthe Lagros Mountains on the Iranian shore. Mostembedded thunderstorms are associated withsouthern Persian Gulf cyclogenesis.

Precipitation. Light rain, rainshowers, andoccasional thundershowers occur with lowpressure areas, cold fronts, and upper troughs,The greatest amounts fall on the Iranian side,As with the rest of the region, precipitation Isextremely variable, While Bandar Abbas averages4.7 inches (12Dm) during the winter, some yearssee no rain at all and others have seen as muchas 13.2 inches (335m),

Air Temperature. Maximum temperatures averagefrom 70 to 85°F (21 to 29°C); minimums from 55to 65°F (13 to 18°C). Temperatures inland aremodified by increased elevation and the seabreeze.

SEA SURFACE CONDITIONS. Strait of Hormuz,

Water Currents. Flow through the Strait consistsof tidal ebb and flood. During ebb conditions,currents of less than 1 knot flow predominantlyfrom the Persian Gulf through the Strait intothe Gulf of Oman, During flood tide, currentsof less than 1 knot flow in the oppositedirection, from the Gulf of Oman through theStrait into the Persian Gulf. 85% of all waveheights average less than 4 feet (1.2 meters),

Water temperatures are similar to those found inthe Persian Gulf. They decrease •from 75—76°F(24°C) in December to 73°F (22°C) in January,but increase to 74°F (23°C) in March.

THE GULF OF OMAN AND ADJACENT LAND AREAS--M)RTHEAST MONSOON

MESOSCALE SYNOPTIC FEATURES. Low pressure wavesmay form in the Gulf of Oman on slow—movingfronts that are oriented northeast—southwest.Extensive low cloudiness with rain or drizzleoccurs on the east side of these waves. Under"Shamal" conditions, the Strait of Hormuz actsas a venturi; winds increase markedly to theinnediate south and southeast of the Stritproper. Once a "Persian Gulf Trough" has moved

TYPICAL WEATHER. The followingof the Strait of Hormuz andthe Northeast Monsoon:

general Sensible Weather. Only high clouds—-oftenjet stream associated—-are normally observedhere during the Northeast Monsoon. Visibilitiesremain excellent, except along the Iranian shorenear dawn, when visibilities often fall into the3 to S mile range, probably caused by very lightwind conditions that allow the moist sea air,advected inland by the prior afternoon's seabreeze, to condense. Morning land breezes,reinforced by heating and the synoptic northerlyflow, dissipate the haze and fog.

Sky Cover. Extensive middle and high cloud layersoccur only in advance of upper troughs and theoccasional surface low that moves eastward intoPakistan and India. Ceilings range from 7,000to 8,000 feet (2,100 to 2.40Dm). Low cloudceilings of 1,000 to 1,500 feet (300 to 450m)are normally confined to the Iranian shore aheadof developing low pressure waves on slow—movingcold fronts. Patchy low cloud ceilings formover the Gulf of Oman and the Omani coast, butclear within 6 to 12 hours after the surface lowmoves eastward. Middle clouds break with thepassage of the upper trough.

Dust, Widespread dust that restricts visibilityis confined to the coasts and imediate offshoreGulf areas during a strong "Shanal." Dustraised by northerly winds on the back side of a"Persian Gulf Trough' following a Shamal havebeen observed in satellite imagery, Visibilitydrops as low as 3 to 5 miles. Worst conditionsare found at the mouth of rivers or canyons.Dust settles rapidly once winds drop below 25knots, Protected locations, such as SeebInternational Airport on the Omani coast, rarelyexperience visibility restrictions due to dustduring the Northeast Monsoon.

23

imediately southeast of the Strait of Hormuzresult in weak east—northeast to northeastcirculation. Added to this is the effect of aterrain—induced low—level cyclonic eddy east ofthe Strait of I1ormuz, Winds near coastlinesshow a pronounced land—sea breeze effect becauseof markedly higher terrain and temperaturesinland, Although mean surface winds (as shownin climatic summaries) are northeasterly andrelatively light, nighttime land breezes aresouthwesterly to west-southwesterly (downslope)at 4 to 8 knots. The sea breeze establishesshortly after 0900L and persists through 1900Las north—easterly to easterly onshore flow thatreaëhes iS to 20 knots in late afternoon,

Ahead of an approaching Persian Gulf cold frontwinds turn southeasterly, increasing to 15 to 25knots. With cold frontal passage, winds veer tobecome northwesterly at 20 to 30 knots. As the"Shamal' builds, winds increase to 25 to 40knots, Speeds decreasing to less than 25 knotssignal the ending of a 'Shamal,'

Thunderstorms, Isolated thunderstorms form overthe Gulf of Oman and along the Iranian coastinmediately ahead of an upper trough and itsassociated surface cold front, The strongestthunderstorms are found over the Omani mountainswith cold frontal passage, Tops range from40.000 to 50,000 feet (12.2 to 15.2km).

Precipitation. Normal precipitation for theseason is between 3 and 5 inches (75 to l2Snn),but isolated thundershowers may bring muchmore. Jask, on the Iranian shore, has reported5.6 inches (l4Onmi) in January; Jiwani, Paki-stan, 8.7 inches (220mm). Seeb InternationalAirport, on the Omani coast about 25 milesnorthwest of Ras al'Hadd, has also reported 5.6inches (140mm) in January.

Air Temperatures. Maximum temperatures in Januaryrange from 70°F (21°C) to 80°F (26°C) along theIranian coast. Open water temperatures reach70—75°F (21—23°C). Omani coastal temperaturesaverage 75°F (23°C). Minimums range from 45 to55°F (7 to 13°C) depending on exposure.

SEA SURFACE CONDITIOIG, Gulf of than.

Water currents are easterly at 0.4 to 0.6 knots inJanuary, but by late February an anticyclonicgyre establishes itself near 2D°N 65°E; currentflow west of 65°E ranges from easterly at 15°Nto south-southwesterly at 20°N to westerly at24°N, Speeds average 0,5 knots. Wave heightsincrease to 2 to 4 feet during normal northeastmonsoon conditions. Average wind speedsincrease from less than 6 knots just east of theStrait of Hormuz to 10—11 knots as the ArabianSea is reached.

south off the Iranian coast, a bow—shaped cloudline—-with the apex of the bow pointing towardthe Strait of Hormuz—-forms in the middle of theGulf of Oman, This cloud line marks convergencebetween the west-northwesterly winds flowingover the Omani peninsula and the northerly tonortheasterly winds flowing off the Iraniancoast, The cloud line's dissipation marks theend of a "Shamal° episode

weather is typicalits shores during

Winds. Winds over the entire Gulf area are Water temperatures decrease from 79°F (25°C) innormally light and northeasterly, reflecting the December to 73°F (21°C) in February, then risemonsoonal flow. Synoptic scale gradients to 75°F (23°C) in March,

TIE (WANI AIM8IAN SEA COiIST--AVJRTHEAST MONSOON

NESOSCALE SYNOPTIC FEATURES. None affect theOniani Arabian Sea Coast.

TYPICAL WEATHER. The following weather conditionsare to be expected in the Dmani Arabian SeaCoast during the Northeast Monsoon:

General Sensible Weather. In the absence of uppertroughs or fronts, only high clouds associatedwith the sub—tropical jet stream are foundhere. Visibilities are unlimited.

Sky Cover. Isolated heavy cumulus occurs whenstronger onshore winds reach the OmaniConvergence Zone (OCZ)—-see Figure 16.Extensive middle and high cloud decks alsospread over the region ahead and during passageof upper troughs Towering cumulus orthunderstorms occur over higher mountains withthe passage of cold fronts or upper troughs.

Dust. Blowing dust frol Saudi Arabia at theheight of a Shamal poses the only visibilityrestriction. Dust has been observed welloffshore both by satellite imagery and byembarked US Navy aerologists. Visibilities maybriefly fall to 3 to 4 miles.

Winds. Synoptic scale winds east of the OCZ arenortheasterly at 3 to 5 knots; west of the OCZ,northwesterly at 5 to 8 knots. These largescale patterns, however, are distorted and oftenoverridden by mountain—valley breezes and, alongthe coast, by land—sea breezes, Winds offshoreare predominently northeasterly at 5 to 10 knotsalong the coast; further out they increase to 10to 15 knots.

Thunderstorms occur throughout the season witheither surface cold frontal passages or uppertroughs. Bases average 4,000 to 5,000 feet(1,200 to 1,500m); tops can reach as high as50,000 feet (15.2 km) with the rare severestorm. Most thunderstorms occur in the HajarMountains along and just behind cold fronts

24

during January. An occasional thunderstorm ISalso found among the very isolated showers thatoccur when stronger onshore winds reach the OCZ.

Precipitation. Precipitation. along the Irmiediatecoast ranges from 1 to 2 inches (25 to SOn).As with the rest of the Persian Gulf region,precipitation is extremely variable from oneyear to the next, but occasional thundershowerstoward the end of winter have producedi 2 to 4

inches (50 to 100mm). There are few weatherreporting stations in Oman above 1,400 feet(425m) MSL inland to the main mountain range anddata is extremely scarce, However, research byBritish meteorologists using oil company recordsindicates that a total of 0.8 inches (2Onmi) mayoccur on 4 or 5 days during the season. TheHajar Mountains receive up to 4 inches (100mm),most from thunderstorms along and just behindcold fronts during January. Icing and turbu-lence are primarily associated with thecumuliform buildups.

Air Temperatures. Maximum temperatures atelevations below 1,400 feet (425m) average 75 to90°F (23 to 32°C). But they decrease with alti-tude, ranging from 40 to 45°F (4 to 7°C) in thehighest mountains, Minimum temperatures alongthe iranediate coast average 50 to 55°F (10 to13°C); at higher elevations inland, minimumsdrop to as low as 20 to 25°F (-7 to —4°C) on thehighest peaks.

SEA SURFACE CONDITIONS, Itthwestern Arabian Sea.

Water currents average southeasterly at 3 to 5

knots. Wave heights increase from 3 to 6 feetnear the coast to 5 to 10 feet beyond 50 milesoffshore.

Water temperatures cool from 77°F (24°C) inDecember to 75°F (23°C) in January; withincreasing solar radiation, they climb back to77°F (24°C) in March,

C4apter S

THE SPQIHG TRANSITIOH

(April——May)

144.7CR CLIMATIC COEROLS

FLOW PATTERN REVERSAL. The Spring Transition seesthe reversal of synoptic scale flow patterns;completion of this reversal marks the end of theSpring Transition and the start of the SouthwestMonsoon, The actual onset of the SouthwestMonsoon is apparently triggered by a mid—tropospheric cyclone over either the northernArabian Sea (see "Onset Vortex") or the Bay ofBengal. Incoming solar radiation increasesdramatically. Changes in circulation occur atlow levels first. The Northeast Monsooncirculation gradually loosens its grip on theregion as low—level anticyclonic flow over tieArabian Sea is replaced by cyclonic circu-lation. Although the complete transitionaverages 30 to 45 days, actual onsets andendings vary by station location and from oneyear to the next. The change can be rapid, Theend of the transition season is signaled bysoutherly cross—equatorial flow from theSouthern Hemisphere moving northward along theeast coast of Africa, then turning south-westerly, and finally reaching the Indiancoast. Once this moist southwesterly flowreaches western India, numerous pre—monsooralthunderstorms occur over northern and westernIndia. Latent heat of condensation from thesethunderstorms, combined with the increasingeffectiveness of Tibet as a high—level (SOOoib)heat source, builds a high—level anticycloneover the Himalayas and Tibet, The result isthat high—level westerlies are replaced by high—level easterlieg essential to the maintenance ofthe Southwest Monsoon.

STRENGTHENING CONTINENTAL LOW PRESSURE CENTERS,These centers are connected by a thermal troughthat builds through northeast Africa, Saudi

25

Arabia, and central Iran, Northeast Monsoonpressure gradients weaken early in thetransition. By the middle of May, however,strengthening gradients associated withextensive thermal low—pressure areas effectivelyblock the effects of the weakening SiberianAnticyclone and restrict cold air outbreaks toearly April.

THE UPPER—LEVEL (ABOVE 300MB) SUBTROPICAL RIDGE.This ridge, which was oriented east—west between5 and 1O"N during the Northeast Monsoon,strengthens and moves northward. This movementis directly related to the northward shift ofthe Monsoon Trough/Intertropical ConvergenceZone (ITCZ) and rising upper—level pressureabove the shallow continental thermal lows, Bythe end of the Spring Transition, the ridge isanchored by the upper—level high over Tibet.

INTERACTION BETWEEN TNE POLAR AND SUBTROPICAL JETSis greatly reduced, As the mean position of thePolar Jet moves northward, winter short—waveupper—air troughs all but disappear. Occasionalstrong upper—level northwesterly winds arepossible early in the period. Weak upper—levelflow patterns are common over the Arabian Seaduring the transition. The Subtropical Ridgeseparates the easterly flow aloft south of 15"Nfrom the westerly flow between 15 and 35°N. Asthe ridge moves northward, strong upper—leveleasterly flow becomes established over theentire region.

MEAN MAY FLOW at 850, 700, 500, 300 mb, and 200mb(5,000, 10,000, 18,000, 30,000 and 40,000 feet)is shown in Figures 17, 18, 19, 20, and 21.

30°N

20°N

100 N

00

30°N

20° N

100 N

25°E 40°E 60°f 80°f

26

g0

FIgure 11. Mean May 850mb flow.

250 E

Figure 18. Mean May 100mb Flow.

30 °N

20 °N

10 °N

on

300 N

20°N

1O°N

00

27

800 E


80 °E

Figure 20. Mean May 300mb flow.

30'N

20 °N

10°N

28

800£


TRANSITORY SYNOPTIC FEATURES--SPRING TRANSITION

TROPICAL CYCLONE FREQUENCY is highest duringtransition seasons, for these reasons:

increases due to rapiilow—level thermal 'heatAfghanistan, Iran, and

• The Monsoon Trough/ITCZ migrates northward tolocations between 15 and 2511.

TROPICAL STORMS. The meteorological satellite hasprovided for more accurate location and trackingof Arabian Sea storms; reported frequencies arenaturally higher since the early 1970s, butstorms in this area are still relativelyinfrequent. Between 1890 and 1950, only 14 werer&corded in the Arabian Sea during April andMay. Between 1891 and 1969, only eight stormsreached the Omani Coast; favored landfall wasnear Salalah. Storms usually form in theeastern Arabian Sea between 10 and 1514, andbetween 70 and 75°E. The strongest haveextensive layered clouds, low ceilings, heavyrain, and high winds out to 300 miles of thecenter. Flash floods are common. MeanApril/May storm tracks are shown in Figure 22.

flow into the Southernupper-level divergence

devel opment,

• Weak cross-equatorialHemisphere providesfavorable for cyclone

• Vertical shear is weak

• Low—level convergencestrengthening of thelows" in Pakistan,Saudi Arabia.

Figure 22. Mean Tropical Storm Tracks--Ajril and May Combined.

29

TIE ONSET VORTEX. The tropical "onset' vortex,resembling a tropical stor'm, occurs only at theend of the Spring Transition and marks the startof the Southwest Monsoon (hence the name),Statistically, it occurs about about 1 year in 2in the Arabian Sea. In other years, a similarvortex forms in the Bay of Bengal. Occurrenceranges from mid—May to mid-June and isapparently the "trigger" for the deep lower—level southwesterly flow that marks theSouthwest Monsoon, These cyclonic systems format mid—tropospheric levels over the surfaceMonsoon Trough (ITCZ) just •north of the nose ofthe Southwest Monsoon current, Movement isgenerally westward toward the Omani Peninsula,southeastern Saudi Arabia, or the Gulf ofOman, Initial movement, however, is northwardto about 20°P1, then westward or west—southwestward into the Arabian Peninsula,Layered clouds form over the entire regionwithin 300 miles either side of the track.There are heavy cumuliform buildups andrainshowers within 50 miles of the center,Convective clouds and precipitation are greatlyintensified over the Omani Mountains even whenthe vortex does not actually reach them,

THE ISERT FRONT. Actually a mean polar frontalposition over the northern Red Sea, the desertfront is not active every year, It does, however,represent a relatively rare storm track that liesacross southern Egypt and the northern Red Seainto central Saudi Arabia. This storm track oftenresults in the formation of a secondary wave overthe central Red Sea as the mean jet streamposition is displaced southward over the southernMediterranean or northern Africa. Extensivethunderstorms and heavy rains accompany themovement of the secondary low across the SaudiArabian peninsula, This track is only present Inlate winter (March) and during the springtransition. In May 1972, Riyadh, Saudi Arabia,twice experienced passage of a desert front,Although variations in temperature and wind mayhelp identify the surface frontal position, thereare times when this disturbance loses its surfacecharacteristics; when it does, the only indi-cations are in its associated weather and upper-level trough, These fast-moving fronts/troughscan affect a station's weather for 2-3 hours;they are often restricted to the area west of theStrait of Ilormuz. Mean position of the desertfront is shown in Figure 23.

TIE ONANI CONVERGENCE ZONE (DC!), The OCZ marksthe boundary at which the predominant low—levelnorthwesterly flow meets the intensifyingsouthwesterly flow ahead of the SouthwestMonsoon. Although the OCZ is mainly delineatedby wind shear across it, occasional convectiveactivity can be found over higher terrain. Bythe end of the Spring Transition, the MonsoonTrough/ITCZ has moved into the OCZ. Figure 24gives the mean April location of the OCZ.

30

LANO—SEA BREEZES become the dominant feature alongcoastlines during "undisturbed' (normal) condi-tions, Sea breezes are considerably strongerthan land breezes because of strong daytimetemperature gradients. The land—sea breezeeffect can cause abrupt changes in temperature,relative humidity, and wind direction.

20N

•1

Figure 23. Mean Position of the Desert Front.

25!

Figure 24. Mean 'il Position of the XZ.

TIlE PERSIAN GULF PROPER AND ALUACENT LAND AREAS--SPRING TRANSITION

HESOSCALE SYNOPTIC FEATURES, As the SpringTransition progresses, the Omani Convergencelone (OCZ) moves northwestward into the extremesouthern Persian Gulf. This movement is due tothe strengthening thermal lows over Saudi Arabiacombined with increasing southwesterly flow oversoutheastern Arabia and the Arabian Sea.

TYPICAL WEATHER.are typical ofthe Spring Transition:

General Sensible Weather. Cloud cover is nearlynonexistent save for the rare thunderstorm. Thefew instances of jet stream cirrus are almosttotally restricted to upper—level westerliesearly in the period. Fog can occasionally occurearly in the period, but only in early morning.Flooding in the Tigris—Euphrates River Plain isconinon, and caused by melting snow from themountains of Iran and Iraq. Flooding has becomemore of a problem �ince 1980 when flood controllevees were destroyed during the Iraq—Iran war.

Sky Cover. Middle and high cloud decks occur onlywith those few upper—air troughs that penetratethe area early in the Transition, Ceilingsrange from 10,000 to 14,000 feet (3.1 to 4.3km). Lower ceilings are associated withthunderstorms, which see, Light icing andturbulence occur in middle cloud decks near thetrough axis,

Winds. Weak low—level winds reflect themacroscale pattern reversal, Late in theperiod, wind peeds Increase to reflect thedevelopment of strong continental thermallows, April speeds average 5-7 knots,increasing to 10—12 knots in May. Directionscontinue to be northwesterly in the northernPersian Gulf, westerly in the southern Gulf, andturning southerly as one approaches the Straitof Hormuz, On the imediate coast, winds showlarge diurnal changes due to land-sea breezes,,

Dust and Sand. The most significant weather isthe increasing frequency of blowing dust andsand. Late season frontal zones (desert fronts)can advect dust or sand across the entirenorthern part of the Arabian Peninsula, Late inthe Spring Transition, strengthening north-westerly winds cause lower visibilities, withhighest frequencies in the northern part of thePersian Gulf. Visibilities drop to less than 6miles during the day; in the afternoon, withhigher wind speeds, visibilities drop to lessthan 3 miles, Basrah, Iraq, and Kuwait, Kuwait,both average nearly 10 days a month with blowingsand or dust. Blowing dust or sand also occurswith the rare thunderstorm——most of which occurearly in the Transition, Visibilities instronger thunderstorms may drop to as low as a

half mile; near—zero visibilities are possible

31

with severe thunderstorms. When strong windsare not sustained, dust settles rapidly.

Thunderstor.s, Tops often exceed 40,000 feet(12.2 km); bases average between 4,000 and6,000 feet (1,200 and 1,800 meters.) The usualsevere Icing and turbulence can be expected,Hail should be assumed when the classic KillerAWSTR-200 wet bulb zero heights above terrainare met. Thunderstorms during the SpringTransition have several causes:

• Fronts. An occasional Mediterranean frontalsystem reaches the region early in the trans-ition season, Although considerably weakerthan fronts that occur during the NortheastMonsoon, these can be strong.enough to triggerthunderstorms.

• Intense heating over coastlines allowsconvection to penetrate low—level inversions.

•Strong sea breezes lift moist air orographi-cally over nearby coastal mountain ranges.

S Mid—level unstable air associated with anupper—level trough (or with a weak Mediterra-nean front) is lifted orographically over theZagros mountains.

Late in the transition, but very rarely,thunderstorms may also form over mountains alongand southeast of the OCZ,

Precipitation. Rainfall decreases steadily duringthe transition, Its convective nature resultsin high variability from one location to anotherand from one year to another, Mean values of0.50 to 1.0P inches (13 to 2Gm) early in thetransition decrease to less than 0.50 inch(13mm) by early May, then to near zero by thestart of the Southwest Monsoon, Much more mayfall over the Zagros during isolatedthunderstorms; flash floods have occurred.

Air Temperatures. Temperatures rise significantlyduring the Spring Transition, Daytime maximumsin coastal regions climb to between 95 and 97°F(35 and 36°C). Mean May temperatures on thenorth shore of the Persian Gulf are even higher;for example, 101°F (38°C) at Abadan, Iran, and102°F (39°C) at Kuwait. Inland temperatures inSaudi Arabia stay within a few degrees ofcoastal temperatures, Zagros Mountain tempera-tures are nearly 5°F (2.8°C) cooler than coastalregions. Minimum April temperatures at mostlocations range froe 62 to &5°F (16 to 19°C).warming rapidly to 73 to 75°F (22 to 23°C) inMay. Iranian mountain locations are nearly 10°F(5°C) cooler in both months. Dewpoints incoastal regions range from 50 to 53°F (10 to11°C). Dewpoints drop dramatically inland,

The following weather conditionsthe Persian Gulf proper during

SEA SURFACE CONDITIONS. Persian Gulf.

Water currents in the Persian Gulf circulatecounterclockwise. A steady flow of surfacewater enters the Persian Gulf through the Straitof Hormuz. The surface current flows toward thewest, then northwest as it hugs the Iran Ianshore. At the northern end of the Gulf. itturns southeasterly to parallel the Arabiancoast. Finally, it turns northerly along thewest side of the Omani Peninsula to join freshGulf of Oman water entering through the Straitof Hormuz, Speed is less than 1 knot. Maximumcalculated speed for the 100—year worst case ofboth wind and tidal currents was 8 knots atJubail, As water increases in density, itsinks, turns southeastward, and flows back intothe Gulf of Oman as a bottom current through theStrait of Hormuz.

32

Water temperatures rise rapidly during the SpringTransition. Surface water temperatures rangefrom 73 to 76°F (22 to 23°C) in April to 80—82°F(26—27°) in May. Temperatures In extremelyshallow coastal. waters may get 5 to 7°F (2.8°to3.8°C) warmer. With the warming surface waters,there tends to be an increasing temperaturedifference between the surface and the bottom(about 195 feet or 60m); by May the bottom isabout 10°F (5.6°C) cooler, There Is no evidenceof thermoclines. Wave heights are less than 4feet (1.2m) more than 80% of the time.

TIE STRAIT OF 1101 WIUZ--SFQING TRANSITION

MESOSCALE SYNOPTIC FEATURES. None applyspecifically to the Strait of Hormuz.

TYPICAL WEATHER. The following weather conditionsare typical of the Strait of Hormuz during theSpring Transition:

Sky Cover. Middle and high clouds are found onlywith the infrequent late winter front ortrough. Ceilings average 12,000 to 14,000 feet(3.7 to 4.3 km). Light mixed icing and light tomoderate turbulence——especially over the Zagrosmountains on the Iranian shore——are associatedwith these layers, Convective clouds may formin late afternoon, especially along the StraitsIranian shore. These clouds are caused by a

combination of strong land-sea breezeinteraction and mechanical lifting over nearbyhigher terrain. They have relatively high basesat 5,000 to 7,000 feet (1,500 to 2,100 m).

Winds. General flow is southwesterly to west—northwesterly west of the Strait and northerlyto northeasterly east of the Strait. Low—levelflow reflects a combination of the weak inflowfrom the Gulf of Oman, pronounced land—seabreezes (especially along the Iranian shore),and a general easterly to southeasterly flowaround the southern end of the terrain—inducedleeside Gulf low, Bandar Abbass (on the Iranianshore) maintains the mean southerly flow foundin the other seasons while Dubai, United ArabEmirates, finds its mean wind direction changingfrom westerly to southerly. The absence ofdisturbances leaves the flow pattern of thesouthern shore of the Strait to the localpatterns of the leeside low and land—sea breezecirculation, Complicating these patterns is thepronounced venturi effect through the narrowestpart of the Strait, Sea breezes strengthen inMay as thermal lows build over the continents.

Dust is confined to rare cases of advection fromthe northern Persian Gulf or central SaudiArabia following frontal passages, Visibilitiesnormally remain above 6 miles, but visibilitiesas low as 3 miles are possible if the sourceregion is central Saudi Arabia, Lower visi—bilities are possible only with an active desertfront,

Th.mderstorms. Although rare, thunderstorms canoccur over the mountains on either side of theStrait. Storms are stronger over the Zagrosmountains. Causes and characteristics are muchthe same as for those in the Persian Gulfproper, which see.

Precipitation occurs only with the rarethunderstorm and is confined to mountainousareas without reporting stations, No

measurements are available, but crude estimatesderived from similar situations in Arizona givevalues of 0.25 to 0.75 inches (6 to l9nvn).

Air Temperatures. Maximum temperatures averagefrom 97 to 99°F (36 to 37°C). Minimums risefrom the lower 70s (about 22°C) on the Iranianshore in April to the upper 70s (about 24°C) byMay, On the Arabian shore, temperatures areslightly lower, warming from the upper SOs (near20°C) in April to the low lOs (near 22°C) inMay, Temperatures farther inland are cooler dueto increased elevation and the diminishing seabreeze effect,

SEA SURFACE CONDITIOI'6, Strait of Hormuz.

Water currents through the Strait are mainly tidalebb and flood, During ebb conditions, currentsof less than I knot flow predominantly from thePersian Gulf through the Strait into the Gulf ofOman. Flood tide currents (also less than 1

knot) reverse to flow from the Gulf of Oman intothe Persian Gulf. Waves heights are less than 4feet (1.2m).

Water temperatures warm from near 77°F (24°C) inApril to 82-83°F (27—28°C) in May.

General Sensible Weather. Cloud cover decreasesfrom near 3 eighths in April to 1 eighth inMay. This is the only season of the year inwhich the usual late evening and early morningpatchy low stratus and stratocumul us areabsent, Visibilities are excellent,

33

THE OJLF OF (flN AND ALUACENT LAND AREAS--SEWING TRANSITION

NESOSCALE SYNOPTIC FEATURES.

Tropical Cyclones. Tropical cyclones thatoriginate in the Arabian Sea only reach the GulfOf Oman about once every 4 years, They producelayered clouds, low ceilings, heavy rains, andhigh winds. In the rare cases of landfall.expect flash floods in and near mountains oneither coast. Moderate icing and moderate tosevere turbulence are common, but turbulence andicing are heavier over the mountains on eithershore. Ceilings can be as low as 1,000 to 1,500feet (300 to 45Dm) with layers through 40,000feet (12,2km). Visibilities can be as low as

•half a mile, and winds can be of hurricaneforce. There are no records of maximum windspeeds; not many of these storms passed overreporting stations, and those that did destroyedthe anemometers. Although more than 10 inches(250mm) of rain were recorded in a storm that

made landfall near a reporting station, therehave undoubtedly been higher amounts.

Omani Gulf Low, This mesoscale low is located inthe western Gulf of Oman; it dominates duringthe early part of the transition season. It iscaused by a combination of a Zagros Mountain leeside effect and the mountainous terrain alongthe Omani coast, As the Northeast Monsoonweakens, so does the low. Establishment of theSouthwest Monsoon destroys this mesoscalefeature,. See 'Winds.'

TYPICAL WEATHER.

General Sensible Weather, This is perhaps themost settled season of the year in and aroundthe Gulf of Oman, Skies are normally clearwith excel lent visibil ities. Mid—latitudedisturbances from the Mediterranean rarely havethe support to reach this far south, The OCZmoves into the extreme southern Persian Gulf;the Monsoon Trough/ITCZ has yet to appear.

Sky Cover. Late in the Transition season, cirruslayers are sometimes advected westward fromtropical disturbances in the eastern ArabianSea. These layers reach the area only when theupper—level easterlies have set up south of thesubtropical ridge. The cirrus dissipates nearthe eastern portion of the Gulf of Oman due tostrong mid—tropospheric subsidence,

Winds. The light and shifting synoptic gradientduring the first part of this season leaveswinds over land at the mercy of local effectssuch as land—sea breezes and/or mountain—valleybreezes. Low-level winds along the innediate

34

Iranian shore turn east—northeasterly andincrease in speed during daylight hours as oneapproaches the Strait of Hormuz. This is due tothe strengthening sea breeze influence as wellas the Omani Gulf Low described earlier. Towardthe end of the season weak southwesterly flow,often masked by local effects, appears along theOmani coast. Speeds average 3 to 5 knots,

list is almost unknown. Blowing dust or sand mayoccur with the rare thunderstorm over highermountains on either side of the Gulf Of Oman.

Thunderstorms, A very rare thunderstorm may occurover higher mountains on both the Iranian andOmani shores of the Gulf. The most common causeis locally induced convection resulting from thelifting of moist Gulf air over nearby coastalmountain ranges. Although infrequent, the mostconmion location is in Oman's Jabal Akhdarrange, Thunderstorm tops often exceed 40,000feet (12,2 km); bases average between 4,000 and6,000 feet (1,200 and 1,800 meters.) The usualsevere icing and turbulence should beexpected. Hail should be assumed when theclassic Miller AWSTR-200 wet bulb zero heightsabove terrain are met.

Precipitation. Except for that from the raretropical cyclone, precipitation is thunderstorm—associated, almost always falling in mountainswhere there are no reporting stations. Crudeestimates based on similar situations in Arizonaindicate amounts to be 0.25 to 0.75 inches (6 tol9nm.

Air temperatures increase rapidly due to thecombination of decreased cloud cover andundisturbed (normal) weather conditions. Max-imum temperatures on both shores of the Gulf ofOman warm from between 87° and 89°F (30 to 31°C)in April to between 94 and 96°F (34 to 35°C) inMay, Minimum temperatures on the southern coastrise from near 80°F (26°C) in April to 85°F(29°C) in May. Maximum temperatures on thenorthern coast are nearly 5°F (2.8°C) cooler.

SEA SURFACE CONDITIONS. Gulf of Oman,

Water currents become southwesterly, averagingonly half a knot throughout the Gulf of Oman andnorthern Arabian Sea. Wave heights are lessthan 4 feet (1.2m) nearly 90% of the time,

Water temperatures range from 78° to 79°F (near25°C) in April to 83 to 85°F (near 29°C) inMay.

OMAN! ARABIAN SEA COAST--SPRING TRANSITION

NESOSCAIE SYNOPTIC FEATURES.

Tropical cyclones originating in the Arabian Seamay affect the Omani Coast, but incidence is nothigh. On the average, one of these storms maymake landfall on the Omani coast in only 1 yearout of 4. For characteristic conditions, see'Mesoscale Synoptic Features' for the Gulf ofOman and Adjacent Coasts.

Onset Vortex. The onset vortices that mark theend of this transition season and the start ofthe Southwest Monsoon (see 'Transitory SynopticFeatures") reach the Omani Arabian Sea coastless than 25 percent of the time. Layeredclouds occur from 1,000 to 3,000 feet (300 to1.000m) up to 35,000 to 40,000 feet (10.7 tb12.2km) over the entire region within 300NM ofthe vortex center. There are heavy cumuliformbuildups and rainshowers within 50 miles of thecenter, and moderate icing and turbulence within150 miles of the center. Flash floods in andnear the Omani mountains are common even whenthe vortex does not actually make landfall.

TYPICAL WEATHER.

General Sensible Weather is excellent. Thisthe most cloud-free season of the year here;visibilities are excellent.

Sky Cover. Multilayered cloud decks are confinedto those associated with the very rare tropicalcyclone (see "Mesoscale. Synoptic Features").

Dust. As wind speeds rise late in the season withincreasing southwesterly flow, and the groundbecomes baked by high temperatures, the numberof days a month with blowing dust or sandincreases. By season's end, blowing dust orblowing sand occurs on 4 or 5 days a month,

Winds. Winds switch to southwesterly in late Mayor early June to herald the end of theTransition Season, Northeasterly winds at

Nasirah become southwesterly by May. Winds atSalalah move from the southeast to southwest,Wind speeds are near their annual minimum.Early morning light winds increase throughoutthe day to 7—9 knots by mid-afternoon. Windsover the northwest Arabian Sea becomesouthwesterly by mid-April at 4 to 7 knots.Directions stay constant through May, withspeeds increasing to 10 to 15 knots just beforethe onset of the Southwest Monsoon.

Thunderstorms. Although none have actually beenrecorded here, they probably do occur. Britishmeteorologists working with oil explorationcompanies in the interior have observed rarecumulonimbus over the mountains to the south-west,

Precipitation occursthunderstorms, orvortex. Amounts inhave been recorded,

only with tropical storms,the passageexcess of 10

of aninches

onset(255nn)

Temperature, This is the warmest season of theyear. Maximum and minimum temperatures are attheir highest annual mean at both Salalah andMasirah. Maximums average from 90 to 96°F (32to 35°C); minimums from 77 to 78°F (near

SEA SURFACE CONDITIONS, Northwest Arabian Sea.

Water currents begin to establish themselves assouthwesterly at 1 to 2 knots by the end of theseason.

Water temperatures climb to their highest valuesof the year, reaching between 79 and 81°F (26and 27°C) . The fact that water temperatures areat their warmest now, rather than later in thesurmeer, is a result of the low cloud cover andextensive upwelling that establishes almostcoincidentally with the onset of the SouthwestMonsoon,

is 24°C)

35

Chapter 6

THE SOUTHWEST MONSOON

(.June--SeptSier)

MAJOR CLIMATIC CONTROLS

A MASSIVE ThERMAL LOW—PRESSURE TROUGH dominatesthis season. This trough has its primary axisalong the Persian Gulf, with a secondary centerover south central Saudi Arabia, It is a

westward extension of the summer thermal lowcentered in extreme western Pakistan, Theentire system is anchored by a combination ofterrain (the Zagros mountains along the IranianPersian Gulf shore) and the extremely hot anddry air over the Arabian Peninsula,

AN UPPER—AIR RIDGE has. formed over the Arabianpeninsula and Iran, 500mb high pressure centersare located over extreme western Arabia and overthe Zagros mountains in southwestern Iran.

THE NONSOON TROUGH/INTERTROPICAL CONVERGENCE ZONE(ITCZ) enters the study area about 200NM Inlandof Salalah on the southwestern DreaM Arabian Seacoast and follows the Omani mountain crest to

36

the Strait of Hormuz, After crossing theStrait of Hormut it lies at or near the 1,640foot (SOOm) contour line of the ZagrosMountains--or about 25 to 30 miles inland of theIranian coast, Upon reaching extreme westernPakistan near Jiwani, the Monsoon Trough/ITCZturns north—northeastward. Unlike in othertropical regions, the Monsoon Trough/ITCZ heremarks the boundary between moist tropical air atlow levels and hot, subsiding air above, It is,therefore, normally inactive. It slopes gradu-ally upward towards the southeast, and is thecause of the extremely strong elevated inversionsouth and east of Its surface position. Insoutheastern Saudi Arabia and Oman, the MonsoonTrough/ITCZ absorbs the Omani Convergence Zonefound during the Northeast Monsoon and theSpring Transition, Figure 25 shows the meanJuly surface position of the MonsoonTrough/ITCZ,

Figure 25. Hem, July Surface Fositi3n of the ?bnsoon Trough/lTd.

THE SOUTHWEST MONSOON, for which this season isnamed, is a deep and strong sustained flow ofsouthwesterly to westerly winds over the entireIndian Ocean north of the Equator, Flow depthover the Central Arabian Sea and the IndianOcean extends from the surface to above 500mb,This flow lies on the equatorward side of theMonsoon Trough/ITCZ. Strongest speeds are overeastern Africa and immediately offshore ofArabia, The core current of this high—speed airis known as the "Somali Jet,° Normal maximumdepth of the moist tropical air in this airflowalong the Arabian Coast and in the Gulf of Omanis 2,000 to 3,000 feet (0.6 to 0.9km). it iscapped by a strong inversion that rises towardthe southeast over the Arabian Sea. Thisinversion marks the Monsoon Trough/ITCZ,

STRONG UPWELLING along and just off the OmaniArabian Sea coast assists in the maintenance of

the high—speed winds of the Southwest Honsoon,It also results in a stratocumulus deck offshorethat turns onshore in the lee of peninsulas,This deck behaves like California coastalstratus.

THE TROPICAI EASTERLY JET (TEJ), The northernedge of t he TEJ crosses the sou them ArabianPeninsula and the northern Arabian Sea from eastto west, Maximum speeds (in the area of thisstudy) are 50 to 75 knots between 40 and 45,000feet (12.1 and 13,6km),

TWO IMPORTANT AIRFLOW FEATURES.

• A northwesterly current at all levels over theArabian Peninsula, Extremely dry, this flow isthe primary atmospheric cause for the prevailingdust layer found throughout the region,Embedded in this current is the Sunner Shamalthat occurs over and just west of the PersianGulf, This enhancement of the prevailingnorthwesterly and northerly flow is normallystrongest during June and July.

• Hid—level easterly to southeasterly flow at 700through 500mb over Iran and at 600 through 500mbover the Omani Peninsula. It originates withthe mid— and high—level anticyclone centeredover Central Iran and western Pakistan,

Mb—LEVEL MOISTURE over the Irani coast of theGulf of Oman, the Zagros Mountains and westwardover the Omani Peninsula. During periods ofstronger flow, isolated thunderstorms developover all three areas.

A PERSISTENT MID-LEVEL TROUGH at 600 and 500mbover the southern portion of the eastern ArabianPeninsula separates the northwesterly andsoutherly flows. Experienced British meteoro-logists think this relatively weak trough is thecause of the occasionally thick altocumuluslayers found here.

EXTREMELY WARM PERSIAN GULF WATERS. The shallowwaters of the Gulf heat rapidly and remain hotthroughout the suirmier. Surface watertemperatures by mid—July are above 90°F(32°C) . High evaporation rates result in a

steady northwestward flow of surface waterthrough the Strait of Hormuz into the Gulf; thecurrent just west of the Strait reaches 2 to 4

knots, A counter—current flows southeastwardalong the bottom down the Persian Gulf and outinto the Gulf of Oman. Note that the surfacecurrent into the Persian Gulf flows against theprevailing northwesterly winds,

MEAN JULY FLOW at 850, 700, 500, 300, and 200mb(S,000, 10,000, 18,000, 30,000, and 40,000 feet)are shown in Figures 27 through 31.

Figure 26 is a mean July 1200Z sounding forSalalah, on the Arabian Sea Omani coast.Although the sounding is based on limited data,the inversion profile shown is believed to berepresentative of the southeastern ArabianPeninsula and innnediately offshore that lie inthe 'Southwest Monsoon,' discussed next,

Figure 26. Mean Winds, Tenqrratures, andGeopotentials--Salalah, 12001, July 19&0.

37

30 °N

20 °N

1O°N

30 °N

200 N

10°N

00

38

E

Figure 27. Mean July 850mb Flow.

80° E


30°N

20 °N

10°N

g0

300N

200 N

10 °N

00

39

800 E



30°

N

20 °N

10 °N

40

80° E


TMP6ITLWY SYNOPTIC FEATURES--SOUTHWEST MONSOON

WEAK UPPER-LEVEL TROUGHS IN THE WESTERLIES. liiJune and early July these troughs may penetratesouthward into the Arabian Peninsula when a

blocking high over northern Europe results in a

high—speed wind core in the westerlies acrossthe Mediterranean into Iraq. The characteristicmiddle and high clouds associated with thesetroughs show up well on visual and BR satelliteImagery.

NORTHWESTWARD MOVEMENT OF THE MONSOON TROUGH! ITCZintO the southern Persian Gulf and ArabianPeninsula. The Monsoon Trough/ITCZ normallylies over southeastern Arabia and the Strait ofHorniuz, and along the Iranian Gulf of Oman

shore, but under certain conditions, it willmove further northwestward into the southernpart of the Persian Gulf. This movement resultsfrom pulses in the Southwest Monsoon that aremost prevalent in July, August. and earlySeptember. Pulse origins are;

Easterly troughs moving westward over the IndianOcean in the southern hemisphere trades. Thesetrade winds, recurving toward the east aftercrossing the equator, become the "southwestmonsoon," Increased mass transport behind thetroughs reinforces the Somali Jet and results inhigher wind speeds.

Shear lines moving northeastward up theMozambique Channel between Africa andMadagascar. These shear lines originate assouthern hemisphere cold fronts, The ridgesfollowing them provide the additional mass that,after merging with the ex-southern hemispheretrade winds over eastern Kenya and Tanzania,results in increased wind speeds of the Somalijet.

THE ONSET VORTEX has been thoroughly discussedunder the Spring Transition in Chapter 5.However, readers should remember that thisfeature actually straddles the end of the SpringTransition and the start of the SouthwestMonsoon. It is an Arabian Sea tropical vortexassociated with the onset of the SouthwestMonsoon. It occurs in 1 out of 2 years. Eitherthis vortex, or a similar one in the Bay ofBengal , seems to be a necessary condition forestablishment of the upper—level anticycloneover Tibet and the Himalayas. This anticyclone,which acts as the monsoon outflow, in turn isnecessary for formation and maintenance of theupper—level tropical easterlies, For furtherdetails, see "Transitory Synoptic Features"under 'The Spring Transition,"

TROPICAL CYCLONIC STORMS/CYCLONES are late spring-early suriner phenomena, almost always theremains of Bay of Bengal cyclones that crossIndia, regenerate In the Monsoon Trough—ITCZover the extreme northern Arabian Sea, and thenmove toward the west. June storm tracks areshown In figure 32.

41

Tropical storm occurrence drops off markedlywith the onset of sunner as the monsoontrough/ITCZ moves northwestward to a liflethrough the southeast Arabian Peninsula acrossthe Strait of Hormuz and eastward along theIranian Gulf of Oman coast into extreme westernPakistan, Eleven storms were recorded duringthe month of June between 1890 and 1950.Between 1891 and 1969, five made landfall on theOmani coast, Occurrences In July throughSeptember are rare; only seven were observedover the Arabian Sea between 1890 and 1950.Storms that do form and surv lye the infl uênce ofcolder water found in the extreme eastern Gulfof Oman and along the northeastern Arabian coastmake landfall near Salalah on the central Omanicoast.

One storm is known to have crossed the northernArabian Sea during August; It made landfall atMaslrah, Winds of 47 knots and a total stormrainfall of 1.77 Inches (45.0mm) wererecorded. The surface pressure gradienttightened rapidly in the southern Persian Gulfat the same time; Abu Dhabai and Dubai recordedeasterly winds gusting to 37 knots withvisibility 5/16 mile (500m) in blowing sand,Masirah has recorded much higher stormrainfall; for example, 17 inches (43Ormn) fellduring the June 1977 storm. Although there areno observing records for the Omani mountains,precipitation amounts are doubtless higherthere.

Figure 32, Mean Tropical Storm Tracks, June.

THE PERSIAN GJLF PROPER AND ADJACENT LAND AREAS--SOUTHWEST IICASOON

MESOSCALE SYNOPTIC FEATURES. The followingfeatures are common to the Persian Gulf proper andits immediately adjacent land areas:

The Sumier Shamal describes the prevailingnorthwesterly winds that occur throughout theSouthwest Monsoon season but are strongest fromearly dune through mid- to late July, TheSummer Shamal winds result from a combination offlow around the Pakistani semipermanent lowpressure area, the Saudi Arabian heat low, the"valley' of the Persian Gulf, and the barriereffect of the Zagros Mountains that induces a

mesoscale low pressure area over the gulf nearthe Iranian shore.

A Nesoscale Lee—side Induced Surface Low lies overthe southeastern Persian Gulf, anchored againstthe Iranian coast. Caused by complexinteractions between terrain features (theZagros and Omani mountains) and the SummerShamal winds, the low results in low—leveleasterly and southeasterly winds along theIranian coast as far northwest as Busheir, The

low causes the tightened gradient that ispartially responsible for the low-level jetdescribed below.

The Low—Level Jet, From the combination ofterrain and synoptic features already described,a high—speed, low—altitude wind maximum, or"low—level jet' (LU) , persists for most of thesouthwest monsoon season along the Arabian Coastfrom about 75 miles southeast of Kuwait to 100miles southeast of Doha. The changing terrainorientation between Doha and Abu Ohabi resulsin relaxation of conditions favorable for LU

formation, Speeds within the LU can exceed 50knots; altitudes of the strongest wind speedcores are between 800 and 1,500 feet (245 and460m) MSL, As with all LU.Js, altitudes and corespeeds are subject to diurnal variations; thelowest altitude of the wind maximum, and thehighest speeds, occur just before dawn. Figure33 shows a representative case, In lateafternoon, speeds drop to those of the gradientwind.

Figure 33. Low-Level Wind Profile Over Bahrain.

42

WIND DIRECTIONI I I

300 320 340 360

WIND SPEED (KTS)

FEET

3000

2000

1000

0

10 20 30 40 50 KTS

Dust Devils, These vortex phenomena occur overland areas throughout the Persian Gulf region,The strongest occur along coast lines in thevicinity of sea breeze "fronts.' During latemorning and afternoon, rising thermals in thehot air just inland of advancing sea breezes arereinforced and twisted by the moist, cooler airof sea breezes. The resulting dust devils riseto heights of over 330 feet (100 meters) , Theycan be advected inland several miles along thesea breeze "front" before dissipating,Extensive damage has been reported.

TYPICAL WEATHER. The following weather conditionsmay be expected in the immediate Persian Gulfarea during the Southwest Monsoon:

General Sensible Weather over the Persian Gulf andthe Arabian Peninsula normally consists of highclouds, Patchy low stratus will form along andjust west of the Arabian coast from Dhahransoutheastward in late evening. Stratus formsdue to a complex combination of land breeze, seabreeze, and summer Shamal strength. The"Terminal Forecast Reference Notebook forDhahran, Saudi Arabia," contains an excellentdiscussion of the subject. Similar conditionshave been noted by forecasters at Doha, Oubaiand Abu Ohabi. Visibilities throughout theregion during June and early July average from 3to 5 miles in suspended dust. Visibilitiesafter mid—July normally improve to 3 to 7 miles,still in suspended dust.

Sky Cover. Upper—air troughs early in the seasonproduce patchy layered middle and high clouds inwhich light icing is found. Bases average from14,000 to 16,000 feet (4.3 to 4.9 km). Veryrare thunderstorms may form along the troughaxis; tops may reach 45,000 feet (13.7 km)

Dust. The major Southwest Monsoon weather hazardthroughout the Persian Gulf region is thesuspended dust and sand raised by the strongnorthwesterly low—level winds of the "SurmuerShamal." Suspended dust in the atmosphere hererestricts visibility to 3 to 5 miles. The topsof this persistent dust layer are between 13,000feet (3.9km) and 16,500 feet (5.1km) MSL.Visibility in the dust varies as a function oflow—level wind speed: the stronger the wind,the lower the visibility. Much of the dust hasbeen picked up from the Iraqi deserts to thesouthwest and west of Basrah, Critical windspeed for lifting the dust into the atmosphereis about 15 knots—-a condition that is nearlyconstant during June and July. The criticalspeed is raised to 20 knots after showers, butheavy vehicular traffic lowers it to 10 knots.

Considerably lower visibilities——sometimes nearzero——can occur throughout the season as a

result of stronger sustained winds caused byshear lines embedded in the westerlies thatpenetrate into southern Iraq and/or extremenorthern Saudi Arabia, These conditions aremost common in June and early July and aresurface reflections of the weak upper-leveltroughs that produce the rare thunderstorms tobe discussed later, Winds behind these shear

Sustained occurrences of very low visibility aretied to the existence of an Omega block overnorthern Europe, and the resulting depression ofthe westerlies to the latitude of the southernMediterranean. Bahrain visibility distributions(shown in Figures 34 and 35) are representativeof normal conditions along the Arabian PersianGulf coast.

Extremely dry soil. Windsdry soil particles tosuspended dust that is a

of this region duringMonsoon. Winds aboveheavier sand particles.

above 15 knots pick upform the ever-presentcharacteristic featurethe entire Southwest

25 knots pick up the

lines can45 knotsRiyadh andgenerate athe dust19,000 to"clearing"winds drop

exceed 30 knots; peak winds of 35 tohave been recorded as far west as

as far south as Dubai. Such winds"haboob" type wall of dust. Tops of

wall have been reported as high as20,000 feet MSL. In such cases,can be said to have occurred oncebelow 15 knots and visibility returns

to the "normal" 3 to 5 miles

Figure 34. Percentage Frequencies of Yisibilities � 3If, Bahrain.

43

I-• —

4.

I

I — — . _L. ' II

A

0500 0700 0900 lao 1.300 'SOC 100 900 2'OO 2300 t

Figure 35. Perca9tage Frequencies of Visibilities S 1100 Yards, Bahrain.

Winds over the northwestern half of the Gulf aresustained northwesterly at 10 to 20 knots withmuch higher gusts, all reflecting the °SurmiierShamal,' A low-level jet (LU) along theArabian Persian Gulf shore from about 75 milessoutheast of Kuwait to 100 miles southeast ofDoha is a common occurrence, The changingterrain orientation between Doha and Abu Ohabiresults in weakening of conditions favorable forLU formation, Wind speeds within the LU canexceed 50 knots; altitudes of the strongestwind speed cores are between 800 and 1,500 feet(245 and 460m) MSU. Average surface speeds are15 to 20 knots, Winds in the southeastern halfof the Persian Gulf reflect both the circulationaround the southern end of the leeside—inducedmesolow and the west-to—east orientation of themountains on the Iranian shore. The south-to—north alignment of the mountains on the OmaniPeninsula of the Arabian coast combine withthese other features to 'turn" the prevailingwinds——first westerly, then southwesterly,finally southerly——as one approaches the Straitof Hormuz from the west, Land and sea breezecirculations , of course, modify this pattern,which can only be overcome by a strong'Shamal.' Even under these conditions, windsover water and along the Persian Gulf coast ofthe Omani Peninsula will back only as far aswesterly. Average speeds over the Iranian siderange from 10 to 15 knots, or 5 to 15 knotslower than on the Saudi Arabian shore. Thehigher speeds on the Arabian—Omani side are notsurprising; this is the region of tightermesoscale gradient in phase with the prevailingmacroscale northwesterly flow,

Thunderstorms. The extreme northern portions ofthe Persian Gulf (including Riyadh, Kuwait, andBasrah, as well as the area of the Zagrosmountains southward to Bushehr) will experiencea rare mid—level thunderstorm associated withweak troughs in the westerlies. These are mostcommon during June and July; all are associatedwith troughs in an enhanced and southward—depressed high-speed current in the mid-latitude'westerlies, These thunderstorms are similar tothose in the southwestern United States; basesare high (above 8,000 feet——2,500 meters) , withvirga and associated downburst phenomena.

Isolated thunderstorms can form over thesouthern Zagros Mountains along and inland ofthe Iranian coast, This activity results from

44

an increase in low—level moisture from either ofseveral rare sources, These sources include:enhancement of the persistent easterly andsoutheasterly mid—level flow around the upperair Iranian anticyclone; a Bay of Bengal monsoondepression that maintains itself westward intothe northwestern Arabian Sea; and a

northwestward movement of the MonsoonTrough/ITCZ, The latter allows southwestmonsoon moisture to feed northward (or evennorthwestward) into the Zagros. Intense summerheating provides the necessary trigger in allthese cases.

Precipitation reaching the ground, except fromvery isolated showers over the Zagros Mountains,is nil. Even with the rare thunderstorm, much,if not all, precipitation evaporates beforereaching the ground.

Air Te.perature. Daytime maximum temperaturesalong the irrmiediate coast average between 105and 111°F (40 to 44°C). Inland, temperaturesrise to 115 to 120°F (45 to 50°C), Minimumtemperatures along the coast fall to 75 to 79°F(23—26°C). Despite high Persian Gulf watertemperatures, dew points along even theirrmied iate northern coastl me range from 56°F(13°C) in June to 66°F (18°C) in September. Thesouthern Arabian coast has marginally higherdewpoints because of flow off the water, butthey drop rapidly as one moves inland. Riyadhaverages a 35°F (3°C) dew point through thesummer, with correspondingly lower minimumtemperatures. Beyond 20 to 30 miles inland,minimum temperatures drop to 75 to 85°F (24 to29°C)

SEA SURFACE CONDITIONS, Persian Gulf,

Water currents generally circulate counter-clockwise. As mentioñd earlier, shallow waterin the gulf is due to evapOration and an almosttotal lack of water flowing off the land. Sincethe water level is lower than the Gulf of Oman,a steady flow of surface water enters thePersian Gulf through the Strait of Hormuz. Thissurface current flows west, then northwest, asit hugs the Iranian shore, At the northern endof the Gulf, it turns to the southeast andparallels the Arabian coast. Finally, it turnsnorth along the Omani Peninsula to join freshGulf of Oman water entering through the Strait

4

4

of Hormuz, Speeds are less than 1 knot.Maximum calculated speed for a 100—year worstcase of both wind and tidal currents, was 8

knots at Jubail, As water increases in density,it sinks, turns southeastward, and flows backinto the Gulf of Oman as a bottom currentthrough the Strait of Hormuz, 85% of all waveheights average less than 6 feet. Predominantdirection along the Arabian coast is, as mightbe expected, from 330 through 360 degrees.

Water Temperatures, The temperature of theshallow Gulf water here increases rapidly during

45

late spring and early suimner, Surface watertemperature ranges from 82-85°F (27—29°C) InJune to 90-92°F (42—43°C) In August beforedropping to 84—87°F (29-31°C) in September.Temperatures in extremely shallow coastal watersreach 104°F (40°C) on July and Augustafternoons. Available temperature soundings Inthe western Persian Gulf show a bottantemperature of 73°F (23°C) at 195 feet (60meters). Temperatures show a steady and gradualdecrease between the surface and the bottom ofabout 13°F (7°C). There is no evidence ofthermocl ines.

THE STRAIT OF HOWIUZ--SO1JTHWEST MONSOON

NESOSCA(.E SThOPTIC FEATURES. The mesoscalefeatures that affect the Persian Gulf proper andthe Gulf of Oman also affect the Strait ofHormuz; there are no known mesoscale featurespeculiar to the Strait itself,

TYPICAL WEATHER. The following weather conditionsmay be expected in the Strait of Hormuz duringthe Southwest Monsoon:

General Sensible Weather over the strait duringthe Southwest Monsoon season consists of patchylow stratus and stratocumulus occurring mostfrequently in late evening and early morning.Onshore cloud occurs only along the Iraniancoast in the very shallow Southwest Monsoonair. Visibilities average 5 to 7 miles in dustand salt haze.

Sky Cover. During incursions ("pulses') ofSouthwest Monsoon air into the southern PersianGulf, thunderstorms occur along the mountains onboth the Iranian and Omani shores, Bases rangefrom 2.000 to 3,000 feet (610 to 915 m); topsreach above 45,000 feet (13,7 km). Britishmeteorologists stationed at Abu Dhabi and Dubaihave reported seeing these thunderstorms overthe Omani mountains. Unconfirmed reports havebeen received of mountain flash floods duringsuch 'pulses."

Winds reflect a combination of weak inflow fromthe Gulf of Oman, pronounced land—sea breezes(especially along the Iranian shore) , and a

general easterly to southeasterly flow aroundthe southern end of the induced leeside Gulflow. Complicating this pattern is the venturieffect through the narrowest part of theStrait, Precise data for the Strait itself isnot available, but it appears that wind speedshere are directly rel ated to "Summer Shamal"strength over the central and northern PersianGulf. In the absence of a well-developed

Shamal , winds along the north shore of theStrait are northeasterly to easterly during thenight and early morning, and southerly towesterly during late morning and afternoon,Along the southern shore, "normal' winds aresouthwesterly to westerly east of the Straitduring the night and early morning, andsoutheasterly to southerly west of the Strait.In late mornings and afternoons, strong south-westerly to west—northwesterly sea breezes blowalong the Omani shore west of the Strait andnortherly to northeasterly east of the Strait.

Precipitation is confined to the rare thunder-storm. No statistics are available, but rain-fall amounts of 0,5 to 1 inch (12 to 25mm) inthe higher mountains are probable, Heavieramounts have cauted flash floods.

Air Temperatures. Maximum temperatures along theimediate shore on either side of the Straitaverage from 90 to 95°F (32 to 35°C). Minimumsaverage from 80 to 82°F (26 to 27°C) . Tempera-tures inland are modified by increased elevationand the diminishing sea breeze,

SEA SURFACE CONDITIONS. Strait of Hormuz,

Water currents through the Strait consists of atidal ebb and flood, During ebb conditions,currents of less than 1 knot flow from thePersian Gulf through the Strait into the Gulf ofOman, During flood tide, currents of less than1 knot flow from the Gulf of Oman through theStrait into the Persian Gnlf. Wave heightsaverage less than 2 feet (O.6m).

Water temperatures are similar to those found inthe Persian Gulf. They increase from 85—86°F(29-30°C) in June to 90—91°F (32-33°C) in Julyand August. Temperatures fall to 86—87°F (30—31°C) in September.

Thunderstorms, Isolated thunderstorms occur overthe interior Zagros mountains; they are veryrarely found in late afternoon over the higherOmani ranges. None have actually occurred overweather reporting stations. Bases are estimatedat 4,000 to 5,000 feet (1,200 to 1,SOOm) MSL;tops at 40,000 to 45,000 feet (12.2 to 13,7 km),

Dust is advected into the Strait of Hormuz fromthe Persian Gulf. During strong "SummerShamals' in the Gulf, visibilities may drop toless than 3 miles in the Strait, Visibilitiesbelow 1 mile——though rare——have occurred, almostalways associated with near zero visibilities inthe southern Persian Gulf caused by extremelystrong "Summer Shamals."

46

THE GULF OF 0*4% AND ADJACENT LAND AREAS--SOUTHWEST MONSOON

*SOSCALE SYNOPTIC FEATURES.

The Gulf of Oman low is a result of:

Terrain features and coastline orientation ofboth the Iranian and the Omani shores

The position of the Monsoon Trough/ITCZ alongthe Omani mountains-Strait of Hormuz

The cyclonic eddy from the mainMonsoon current. This eddy effect isthe one found in summer along thecoast from Santa Barbara westConception.

TYPICAL WEATHER. Typical weather conditions inand near the Gulf of Oman during the SouthwestMonsoon are:

General Sensible Weather over the Iranian coastand the open waters of the Gulf consists ofpatchy low clouds with bases from 1,000 to 1,500feet (305 to 460 m) and tops of 2,500 to 3,500feet (760 to 1,070 m). From early through latemorning, ceilings of 800—1,200 feet are coiTmion

along the irmmediate coast and just inland,Visibilities along the shore range from 5 to 7

miles, but over open water, from 7 to 10miles, Salt haze and dust are the mainrestrictions. The Omani Coast normally hasclear skies, with visibilities ranging from 6 to10 miles in dust and suspended salt haze.Patchy low clouds form over the inmiediate coastsjust after dawn about a third of the time, butskies clear rapidly with morning heating.

Sky Cover. Under the effects of a SouthwestMonsoon "onset vortex" or tropical storm, theusual multilayered low, middle, and high cloudsare found over an area within 300 miles of thestorm track or landfall, But, as has alreadybeen discussed under Transitory SynopticFeatures," these storms rarely enter the Gulf ofOman or make landfall on either shore. The rareoccurrence here is in June or early July, butone was observed in August.

3ust. Dust and salt haze are normally presenthere, restricting visibility to 5 to 10 miles.Lower visibilities are only found during a

strong "Summer Shamal ,' Dust raised by theShamal has been observed on satellite imagery asfar east as the central Gulf of Oman, Lowestknown visibilities (in extreme cases) wereobserved as less than 3 miles, but then onlybriefly.

Winds. A complex wind pattern results from theinteraction of the Gulf of Oman low and thepronounced land—sea breeze, Iranian shore windsare east—southeasterly at 9 knots in July and

August. The normal land breeze is reinforced bythe 'fall wind" effect of the high mountainsthat back the irmnediate coast. Winds on thecoast closer to the Strait of Hormuz show a

marked land—sea breeze circulation, Macroscalegradients irrvnediately southeast of the Strait ofIlormuz are extremely weak. As a result, nightand early morning winds are southwesterly towesterly at 4 to 6 knots; late morning,afternoon, and early evening winds are east—northeasterly to easterly at 8 to 12 knots,Over the Gulf of Oman itself, winds turnsoutheasterly and increase in speed markedlyduring daylight hours as the Strait of Hormuz isapproached. Night winds along the Iranian shoreeast of the Strait and over the northern Gulf ofOman tend to be variable due to the opposinginfluences of the land (mountain) breeze and thegradient winds. Average wind speeds increasefrom less than 8 knots just east of the Straitof Hormuz to 13—14 knots as the Arabian Sea isreached, Surface winds over the southern halfof the Gulf of Oman and on the Omani coast arewesterly to west-southwesterly (downslope) at 4

to 8 knots at night. The sea breeze establishesshortly after O900L and persists through1900L. This onshore flow is northeasterly toeasterly and reaches peak speeds of 15—20 knotsin late afternoon.

Thunderstorms are normally only found in thehigher mountains on both the Iranian and theOmani coasts; these very Isolated storms havethe usual high bases. On rare occasions, thereis widespread thunderstorm activity (with lowcloud bases and heavy rainshowers) over thenorthern Omani mountains and the Zagros rangesimmediately north of the Iranian coast as farnorthwest as Bandar-e—Lengeh. This activity iscaused by a rare strong pulse in the southwestmonsoon that combines with the approach of a

weak upper-air trough over the Persian Gulf.This combination advects low—level monsoon airfrom over the Gulf of Oman and the Strait ofHormuz into the extreme southeastern PersianGulf.

Precipitation. The only preci pitation of anysignificance is from the rare tropicaldisturbance strong enough to make it to thecoast. Die precipitation can be heavy——a bitless than 15 inches (J8Ormni) was recorded in onestorm that made landfall. Extensive flashflooding has also been reported.

Air Temratures. Along the Iranian coast andover the Gulf of Oman, maximum temperaturesrange from 92°F (33°C) in June to 88°F (31°C) inSeptember. Dew points drop from 18°F (25°C) inJune to 73°F (22°C) in September. Along theOmani coast, maximum temperatures in July andAugust are 95 and 91°F (35 and 33°C); dew pointsaverage 70 to 75°F (21 to 23°C).

Southwestsimilar toCal iforniato Point

47

SEA SURFACE CONDITIONS, Gulf of (Ran.

Water currents in the Gulf of Oman maintain a Water temperatures decrease from 86°F to 90°F (30cyclonic gyre, reflecting the low—level wind to 32°C) at the Strait of Hormuz to 82—84°F (27-pattern. Speed averages 1 knot. Wave heights 29°C) at the eastern end of the Gulf of Oman.average 4 to 6 feet.

48

THE (SIANI ARABIAN SEA COAST--SOJJTIfEST MONSOON

NESOSCALE SYNOPTIC FEATURES. None have a directand specific affect on the Omani Arabian Seacoast.

TYPICAL WEATIER. The following conditions are tobe expected in the Omani Arabian Sea Coastduring the Southwest Monsoon:

General Sensible Weather. Extensive night andmorning low clouds (bases from 600 to 1.500feet, tops between 3,000 and 4,500 feet)increase rapidly after mid—June along the coastand inland about 50 miles. Seaward extent isbeyond 100 miles from the shore. These cloudsoccur after the full onset of the SouthwestMonsoon and establishment of the MonsoohTrough/rTCZ inland of the Omani Arabian Seacoast. Low cloud occurrence decreases rapidlyafter mid—September as the Southwest Monsoonweakens and the Monsoon Trough/ITCZ shiftssouth. The greatest occurrence of low cloud ison the southwestern part of the Omani coastproper during July and August nearest thoseregions of the Arabian Sea with the strongestupwelling, or approximately abeam Salalah.Here, stratus ceilings lift froe 600 to 1,000feet (180 to 300m) before dawn to become 2,000to 3,000 feet (610 to 915m) by mid—afternoon,Low ceilings persist over the coldest water off-shore for a distance of 50 to 75 miles from thecoast, Drizzle in the predawn hours occursnearly half the time. Low cloud ceilingsdecrease rapidly the closer one gets to Rasal'Hadd, due to decreased upwelling; low cloudsdissipate onshore after 0900L. Skies are nearlycloudless beyond 50 miles inland.

Sky Cover. Pulses in the southwest monsoontemporarily deepen the maritime moist layer.Under these conditions, maritime air forcedonshore can result in isolated thunderstorms,layered clouds, and low ceilings over the mainOmani mountain range that leads toward theStrait of Hormuz, These late afternoon andearly evening summer thunderstorms over themountains southwest of Abu Dabi and Dubai havebeen observed by British meteorologists at bothstations and are not unusual,

Dust, Blowing dust is common along the north-eastern coast. Strong Southwest Monsoon windspick up enough dust and sand to reduce visibili-ties to less than 3 miles on I day out of 3.Weather observations over the interior of thisregion are not available, but suspended dustthat restricts visibility to 7 to 10 miles hasbeen reported by oil company exploratory teams.

Winds beneath coastal stratus and stratocumulusdecks average 10 to 15 knots, with higher

Thunderstoras occur over the intermountain ranges in the northeastsouthwest, but they are primarily conthose rare occasions when pulsessouthwest monsoon current are strongforce moist tropical air inlandmountains.

Precipitation along the coast and offshore isconfined, to drizzle underneath stratocumulusdecks. Amounts are less than 0.05 inches(1mm) Weather records for the interior are notavailable, but reports of rare flash floodssuggest that precipitation there can be heavy.

Air Temperatures. There is a distinct moderationin temperature. The highest temperatures occurin May and June before the onset of the stratusseason (July through early September) . AlthoughSalalah (17°N, 54°E) is outside the area of thisstudy, its temperatures are representative ofthose in the Omani—Arabian seacoast's 'stratusarea.' Salaiah's maximum temperatures in Julyand August average 81—82°F (27°C); minimumsaverage 74-75°F (23°C). Masirah (20°41'N,58°53'E) , 100 miles southwest of Ras al Hadd,has a much lower low cloud frequency, withresulting higher temperatures; maximumtemperatures average 86—88°F (30—31°C) in Julyand August. Thumrait (17°40'N, 54°02E) can beconsidered representative of inland areas eventhough it is southwest of the study region.Maximum temperatures here average between 104°F(37°C) in June and 95°F (33°C) in July.

SEA SURFACE CONDITIONS. Northwestern Arabian Sea.

Water currents are southwesterlyknots. Highest current speedsmiles offshore,

Water temperatures in June, before full onset ofthe Southwest Monsoon, average between 80 to82°F (27°C). By late June, upwelling inresponse to the Southwest Monsoon has droppedtemperatures markedly. During July and August,surface water temperatures average 77 to 78°F(24 to 25°C) at Ras al Hadd. They decreasedramatically to 68 to 70°F (20 to 21°C) at theintersection of the coast and 20°N. Watertemperatures at Salalah are even lower—-about65°F (near 18°C). The coldest water throughoutthis portion of the Arabian Sea is inmiediatelyoffshore.

gusts. After clearing, speeds25 knots, with higher gusts.beyond 25 to 30 miles offshoreknots, with some gusts above 30

increase toOverwa ter

average 20knots

15 towindsto 30

Oman ithe

tothe

tothe

io randfinedin

enoughto

at 5 to 10are about 60

49

Chapter 7

THE FALL TR.4NSTTION

(October--Noveirter)

MAJOR CLIMATIC CONTROLS

FLOW PATTERN REVERSAL. Like spring, The FallTransition is a period of major synoptic patternreversal. Although the complete transitionaverages 30 to 45 days, actual onset and endingvary by station location and from one year tothe next, The change can be very rapid. SeeChapter 3.

WEAK LOW—LEVEL CONTINENTAL ANTICYCLONES replacethe massive heat lows of the SouthwestMonsoon. Surface high pressure centers aretypically located in• southern Saudi Arabia,central Iran and northern Africa.

STRONG UPPER—LEVEL WESTERLIES delineate thenorthern extent of the Subtropical Ridgecentered near 15°N. Core speeds between 30,000and 40,000 feet range from 60 to 75 knots. Thesubtropical jet is now centered just south ofthe Himalayas over India. Variation in thisupper-level flow comes from the infrequentappearance of the Polar Jet, Jet maxima of 40to 60 knots will move southeastward with a

cyclonic disturbance from the Mediterranean.

MEDITERRANEAN CYCLONES and their associated coldfronts begin to penetrate into the Persian Gulfby mid-November as the Mediterranean storm trackbegins to reestablish itself.

A WEAK KID—LEVEL TROUGH in the northern Red Seaarea acts as a steering mechanism and helps toshift Mediterranean cyclones into the PersianGulf. The trough becomes a persistent mid—levelfeature throughout the winter months.

THE MONSOON TROUGH/INTERTROPICAL CONVERGENCE ZONE(ITCZ) retreats southward from the extremesoutheastern part of the Arabian peninsula andthe Arabian Sea coast of Iran and becomesoriented east-northeast to west-southwest nearthe equator.

TROPICAL CYCLOIES can form in the MonsoonTrough/ITCZ while it is still north of 10'N.These rarely effect the Persian Gulf region,averaging only one occurrence every 10 years.The primary area affected is the Gulf of Oman

50

and immediate coastlines, These storms bringmultilayered clouds, poor visibilities, heavyrain, and embedded thunderstorms. Turbulenceover coastal mountains is intensified, Flashfloods are possible. Figure 36 shows meantropical storm tracks for October and Movember.

SEMIPERMANENT SUDAN LOW, Late in the FallTransition, this low acts as a source region forcyclogenesis associated with upper—level troughsin the westerlies. As these troughs movesoutheastward from the Mediterranean, they maytrigger a transient low center that moves east—northeastward into the northern ArabianPeninsula,

MEAN OCTOBER FLOW at 850, 700, 500, 300, and 200mb, (5,000, 10,000, 18,000, 30,000, and 39,000feet) are shown in Figures 37, 38, 39, 40, and41.

Figure 36. Mean Tropical Storm Tracks--Octoberand M,venter Cotined.

30° N

200 N

10°N

00

300 N

20 N

10°N

00

25°E 40°F 60°E 80°E

25°E 400 F

51

Figure 37. Mean October 850mb Flow.

GOOF 80UE


30° N

20°N

100 N

25°E 40°E 60°E 80°E

30° N

20°N

100 N

00

40°f 60°E 80°E

52

Figure 39. Mean October 5OUmb Flow.

25°E


30°N

20°N

10° N

25°E 40°E 60°E 80°E

53

Figure 41. Mean October 200mb. Flow.

TRANSITORY SYNOPTIC FEATURES--FALL TRANSITION

WEAK UPPER—LEVEL TROUGHS in the westerl iesnormally begin penetrating southward through thePersian Gulf Region by mid-November, oftenaccompanied by typical surface pressure andfrontal systems, Associated weather includessand and dust storms, squall lines, andthunderstorms throughout most of the region. Atleast one severe squall line can be expected tomove southeastward down the Persian Gulf duringthis period,

"WINTER SHANALS" are strong northerly ornorthwesterly winds that can begin in mid-November, This phenomenon is often the resultof intensification of a cyclone (or 'low')moving east-southeastward out of the easternMediterranean Into Syria. As the upper-leveltrough moves into the Arabian Peninsula it may

54

trigger a secondary transient low in Sudan.This combination often results in cyclogenesisand frontogenesis in the Arabian Peninsula, Thesecondary low becomes the dominant pressurecenter and tightens the pressure gradient in thePersian Gulf, Cold air advection over themountains of Turkey and Iran can drive the coldfront as far south as the northern ArabianSea. The rate of progress of the Winter Shamalis directly dependent on the movement of theupper-level trough, Typically these earlyShamals last 24 to 36 hours. However, if theupper-level trough stalls near the Strait ofHormuz (due to a strengthening ridge east oranother low forming to the west) , the Shamal maylast from 3 to 5 days, For a more detaileddiscussion, see Chapter 4, The NortheastMonsoon,

THE PERSIAN GULF AXE) AIJJACENT LAND AREAS--FALL TRANSITION

NESOSCALE SYNOPTIC FEAIURES. Land-sea breezesbecome increasingly important as they become thepredominant feature along coastlines duringundisturbed (normal") conditions. Gradientwind direction and speed are often greatlymodified by land-sea breezes. The effects aregreatest near coastlines with steep terraingrad i en ts,

The following weather conditionsin the Persian Gulf and its

General Sensible Weather. Patchy high clouds(mean coverage 1/8) and unrestricted visibiltiesbecome the rule.

SkyCover. The only cloud cover of significanceis that caused by low pressure systems or theinteraction of Subtropical and Polar jetstreams. The lows move into the area fromeastern Europe or the Mediterranean. Clouds areusually middle and high; scattered areas of2,000-3,000ft (610—915m) ceilings and rain-showers occur in the southwesterly flow ahead ofthe low. Isolated 1,SOOft (460m) ceilings occurwith rain squalls and thunderstorms in theiimnediate vicinity of the low and its coldfront. Such systems are relatively weak,affecting area weather for 24—36 hours. TheSubtropical and Polar jet streams interactionover the Arabian Peninsula often results in twodistinct cirrus bands that are easily discern-ible on satellite imagery. One band curvescyclonically with the Polar Jet; the otherturns slightly anticyclonically with theSubtropical Jet. Strong wind shear near the jetmaxima are likely to produce complicatedvertical and horizontal turbulence patterns.Terrain features can further compound turbulencepatterns.

Dust. Blowing dust and sand frequencies decreasesharply as the season progresses. Averagevisibilities rise dramatically to above 7

miles. The northwestern shore of the PersianGulf maintains a higher incidence of blowingdust and sand until the first significant rainbecause of the extreme dryness of its very finesoil. The occurrence of blowing dust and sandis tied to strong convective activity associatedwith increasing numbers of cold fronts passingthrough the area.

Winds remain northwesterly, but speeds decrease.Winds back from the northwest to the southwestahead of troughs or cold fronts moving into theregion from Iraq. Winds turn westerly, thensoutherly, as one approaches the Strait ofHormuz. Speeds are stronger on the Arabian sideof the Gulf (10—15 knots); the Iranian coastaverages 5—10 knots. The 24— to 36-hoursustained high surface winds characteristic of"Winter Shamals" begin in November. Largepressure gradients develop behind cold frontsdue to upper—level subsidence that rapidlybuilds surface high pressure over western Saudi

Arabia and Iraq, The strong northwesterly low—level winds are then quickly reinforced bynorthwesterly upper-level winds behind theupper-level trough. The associated upper-airtroughs are almost always rapidly moving shortwaves during this transition; surface winds dieafter 24 to 36 hours. Speeds in such casestypically reach 30 to 40 knots. For a nmredetailed discussion, see "Winter Shamals" inChapter 4, "The Northeast Monsoon."

Thunderstorms. Thunderstorms occur alongahead of cold fronts moving southeastwardthe Gulf. Severe thunderstorms occasionoccur with cold fronts or squall lines. Thearea affected is the central and southernand its inrediate coastlines. Thunderswinds above 50 knots have been recorded inUnited Arab Emirates. Although surface hailnot been reported over the Gulf or alongimmediate coasts, it has been reported withstorms over the Omani and Zagros Mountains.

Precipitation shows a dramatic increase in thisarea as the season progresses. The SiberianHigh strengthens as the upper—level westerliesbuild; this combination shifts a storm tracksouthward into the Persian Gulf. Averageamounts of less than 0.25 inch (6mm) increase tobetween 0.75 and 1.00 inches (19 and 25mm) byNovember. Most of the increased precipitationfalls over the Zagros Mountains.

Air Temperatures drop significantly. Daytimemaximum temperatures along the immediate coastcool to between 85 and 95°F (29 and 35°C).Inland, temperatures decrease even more due tothe significant loss of solar insolation and thestrengthening of continental anticyclones. TheZagros Mountains bear the brunt of thesechanges; maximum temperatures there fall toless than 80°F (27°C) near Shiraz and Jahram,Minimum coastline temperatures average 55 to65°F (13 and 18°C) Inland temperatures can fallto below 45°F (7°C), again at Shiraz andJahram. Dewpoints on both coasts of the PersianGulf drop to between 45 and 55°F (7 and 13°C)during this period. Inland, dewpoints fall tobetween 35 and 45°F (2 and 7°C).

SEA SURFACE CONDITIONS. Persian Gulf.

flowing off the land into the Persian Gulf,which is lower than the Gulf of Oman.Consequently, a steady flow of surface waterenters the Persian Gulf through the Strait ofHormuz. This surface current flows toward thewest and then northwest as it hugs the Iranianshore. At the northern end of the Gulf, itturns southeasterly to parallel the Arabiancoast. Finally, it turns northerly along theOmani Peninsula to join fresh Gulf of Oman waterentering through the Strait of Hormuz, Speed is

TYPICAL WEATHER.may be expectedadjacent land areas: and

downallymainGulfto rmthehasits

such

Water currentscirculation.evaporation

show a general counter-clockwiseShallow water levels are due to

and an almost total lack of water

55

less than 1 knot. Maximum calculated speed fora 100—year worst case of both wind and tidalcurrents was 8 knots at Juball . As waterIncreases in density, it sinks, turns south-eastward, and flows back into the Gulf of Omanas a bottom current through the Strait ofHormuz.

56

Water Te.jratures. By November, watertemperatures range from 80 to 82°F (27 and28°C). They are actually a few degrees higherthan the mean air temperature. Water temp-erature soundings show a temperature decrease of10 to 15°F (6 to 9°C) in the upper 195 feet (60m) of the Gulf,

THE STRAiT OF HOsTIUZ--FALL TRANSITION

NESOSCALE SYNOPTIC CONTROLS. Land-sea breezes areIncreasingly important here as they become thepredominant feature along coastlines duringundisturbed ("normal") conditions. Gradientwind direction and speed are often greatlymodified by the addition of land—sea breezes,These effects are greatest near coastlines withsteep terrain gradients.

weather conditionsStrait of Hortnuz

General Sensible Weather during normal conditionsis similar to that of the Southwest Monsoon,Patchy early morning low stratus and strato-cumulus ceilings decrease on the southerncoast; percent frequencies drop from 15-20% inAugust to less than 5% by late October. Thenorthern coast still sees upslope conditions dueto the mountains limiediately inland; frequen-cies decrease more slowly, For example, atBandar Abbass, Iran, ceiling frequency continuesto average 15-20% during October, but drops tonear 10% in November, Visibilities, except withthunderstorms and precipitation, are excellent,

Sky Cover, Middle and high cloud ceilings occurahead of and during the passage of low pressurecenters. Ceilings of 2-3,000 feet (610 to 915m)and visibilities of 3—5 miles (5—8km) in rainshowers are confined to the irmnediate vicinityof the low.

Dust. Blowing dust or sand virtually disappearsafter the first rains of the season. The rareblowing dust that is seen has been advected fromthe Persian Gulf,

Winds, except during low pressure center passages,resemble those found in the Southwest Monsoon.This flow results from a combination of weakinflow from the Gulf of Oman and pronouncedland-sea breezes, especially along the Iranianshore. The venturi effect through the narrowpart of the Strait increases wind speeds at that

point, Strongest winds continue to be on thesouthern coast with speeds of 10—12 knots fromthe northwest. Winds on the Iranian coast, asreflected by Bandar Abbass observations,continue from the south at 7—8 knots.

Thunderstorms occur with squall lines ahead ofcold fronts or with cold frontal passages.Although still rare, thunderstorm frequencyincreases towards the end of November as theMediterranean storm track becomes established.Bases are between 2,000 and 3,500 feet (610 to1,070 m); tops can reach 45,000 feet (13.8km). Isolated thunderstorms——usually associatedwith squall lines——may approach, or even exceedsevere limits.

Precipitation increases dramatically. Averageamounts of less than 0.25 inch (6mm) increase tobetween 0.75 and 1.00 inches (19 and 25m) byNovember, Higher amounts (up to 3 inches——7Smm)occur over the Zagros Mountains, almost allassociated with prefrontal and frontal showersand thunderstorms.

Air Temperatures along the immediate shore oneither side of the strait average 85 to 90°F (29and 32°C) , Minimums drop to between 65 and 70°F(18 and 21°C). Temperatures inland are coolerbecause of increased elevation,

SEA SURFACE CONDITIONS, Strait of Hormuz,

Water currents through the Strait consist of tidalebb and flood. During ebb conditions, currentsof less than 1 knot flow predominantly from thePersian Gulf through the Strait into the Gulf of0mn. During flood tide, currents of less than1 knot flow from the Gulf of Oman through theStrait into the Persian Gulf.

Water temperatures are similar to those found inthe Persian Gulf. Sea temperatures cool frombetween 85 and 87°F (29 and 31°C) in October tobetween 81 and 82°F (27 and 28°C) in November.

TYPICAL WEATHER, The followingare to be expected in theduring the Fall Transition:

57

THE GULF OF OMAN AND AD.JACEFIT LAND AREAS--FALL TRANSITION

NESOSCALE SYNOPTIC FEATURES. •Land-sea breezes areincreasingly important as they become thepredominant feature along coastlines duringnormal conditions. Gradient wind direction andspeed are often greatly modified by the additionof land-sea breezes, These effects are greatestnear coastlines with steep terrain gradients.

TYPICAL WEATHER, The following weather conditloAsare to be expected in the Strait of Hormuzduring the Fall Transjtion:

General Sensible Weather in this area is nearlyalways good. Except for wispy cirrus, skies areclear and visibilities excellent,

that move through the Persian Gulf to the Gulfof Oman near the end of November, These rarelows originate in Eastern Europe or theMediterranean. Multilayered low, middle, andhigh clouds occur with the rare tropical cycloneas it moves westward out of the Arabian Sea.Occurrence of tropical cyclones in this area isless than 1% in any given year, The usualweather conditions (low ceilings and visi—bilities, heavy rain, and, in very rare cases,huricane force winds) may occur with thesesystems.

Winds are predominantly land-sea breezes. Themountains that rise ininediately inland of thecoast on the western side of the Gulf of Omangive such winds increased strength, Mean windspeeds drop throughout the area, averaging 5 to8 knots east of the Strait of Hormuz. Strongerwinds are associated with increased pressuregradients south of t low pressure center in thePersian Gulf,

Thunderstons are confined to passages ofprefrontal squall lines and cold fronts over the

latter.

Precipitation shows a dramatic increase in thisarea as the season progresses, Average amountsincrease to between 0.75 and 1.00 inches (19—2simn) by November. Precipitation from the raretropical storm is enhanced over the mountainsinland of the coasts. Exact rainfall recordsare not available, bUt 10 to 15 inches (255 to380nm) has been reported with spring and suimnertropical storms. Rainfall over the highermountains could be in excess of 20 inches(SlOimn)

Air Temperatures cool gradually, but not asrapidly as along the shores of the Persian Gulfand inland over the Arabian Peninsula andrran, Diurnal ranges, on the other hand,increase markedly. These go from 5 to 10°F (3to 6°C) at the end of September to 10 to 15°F (6to 9°C) by November. Temperatures on theIranian coast are cooler than those on theArabian side of the Gulf of Oman due to theeffects of the Zagros Mountains. Maximumtemperatures are between 80 and 85°F (27 and29°C) north and between 85 and 90°F (29 and32°C) south. Minimum temperatures range from 70to 75°F (21 and 24°C) north and from 75 to 80°F(24 and 27°C) south.

SEA SURFACE CONDITIONS, Gulf of Oman.

Water currents slowly reverse (from thesouthwesterly and westerly flow of lateSeptember to easterly by late November) andreflect the change in monsoorial flow. Speedaverages 0.4 to 0.7 knot,

Water temperatures cool slowly. However, as inthe Persian Gulf, mean sea surface temperaturesbecome I to 2°F (0.6 to 1.2°C) warmer than meanair temperatures, Average sea temperatures coolfrom between 82 and 85°F (28 and 29°C) inOctober to between 80 and 81°F (27°C) inNovember.

higher Zagros and Omani Mountains,severe thunderstorms are possible

Isolatedover the

Sky Cover, Patchy middleoccur with the infrequentof low pressure centers

and high cloud decksbut increasing numbersand associated fronts

Dust is almost nonexistent untilShamal ,° in late November,bilities normally remain above

the first 'WinterEven then, visi—3 miles,

58

THE &IANJ ARABiAN SEA COAST--FALL TRANSITiON

MESOSCALE SYNOPTIC CONTROLS, Land-sea breezes areincreasingly important as they become thepredominant feature along coastlines duringnormal conditions, Gradient wind direction andspeed are often greatly modified by the additionof land—sea breezes. These effects are greatestnear coastlines with steep terrain gradients.

TYPICAL WEATHER. The following weather conditionsare to be expected on the Omani Arabian SeaCoast during the Fall Transition:

General Sensible Weather improves dramatically,There are normally only patchyvisibilities are unrestricted.

high clouds;

Sky Cover rapidly becomes 1 to 2/8 cirrus andcirrostratus. Flow from the northeast causes adramatic reduction in the upwelling of coolerwaters along the Omani Coast and eliminates thestratus and stratocumulus decks characteristicof the Southwest Monsoon.

Dust ceases to be a problem as winds becomenortheasterly and speeds drop drastically fromthose of the Southwest Monsoon, The change inwind direction during this period shuts off thesource of the blowing dust or sand, Occurrencesat Masirah, for example, decrease from 8—12 daysduring September to zero by late October.

Winds along the coast and offshore reflect thestrengthening of the Northeast Monsoon behindthe Monsoon Trough/ITCZ. The sustained, strong

southwesterlies end with the retreat of theMonsoon Trough/ritz southward. Behind it, windsswitch to and steady at north—northeast. Speedsdecrease to between 7 and 10 knots.

Thunderstorms are very rare, occurring only in thehigh Omani mountains in late November, and onlywith the passage of Mediterranean cold fronts.

Precipitation is found onlyin late November withthunderstorm, No rainfall

in the high mountainsthe rare frontal

data is available,

Air Temperature. With virtually clear skies, meanmaximum temperatures in October' rise 1 to 2°F(0,6 to 1.2°C) to 87-88°F (31°C). Conversely,October minimum temperatures cool to between 68and 72°F (20 and 22°C) , November temperaturescool still more. Maximum temperatures rangefrom 78 to 82°F (26 and 28°C) while nights coolto 63—67°F (17-19°C).

SEA SURFACE CONDITIONS, Northwestern Arabian Sea.

Water currents remain southwesterly until onset ofthe full northeast monsoon. Speeds, however,decrease to 3 to S knots by late November. Waveheights average 3 to 5 feet.

Water temperatures rise rapidly as upwellingceases. Mean temperatures are 81°F (27°C) byearly October and remain warm through theseason.

SI

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