§5APT3R til

§§§$lQEil "'lNTlQQUC§I§i

/1-0

Q .-_;

-R __

Indian Ocean is the onl} one not connected to thenorth pole due to the presence of Asiatic Continent,amongst the major oceans of the world. The northernboundary causes differential heating of land and seaduring summer and winter producing the southwest andnortheast monsoons. The water characteristics and

circulation of the upper layers in the North IndianOcean are greatly influenced by the monsoon atmospheric

circulation. Some aspects of the monsoon influence onthe upper layers of the sea are still to be understood,though

of theIndian

considerable advances are made in many aspects

circulation in the upper layers of the NorthOcean in recent years. Using observed wind data

Luther and O'Brien (1985) modelled the seasonalcirculation in she Arabian Sea with remarkable success.

The intermediate layers of the sea which lie belowthe wind—driven layer are dominated by thermohaline effects.

In the Arabian Sea evaporation greatly exceeds precipitationon an annual basis and hence, the salinity of the waterincreases. The Red Sea and Persian Gulf are areas where

i mense evaporation takes place. The warm, saline waterof the Red Sea flow into the Arabian Sea through the Gulf

2

of Aden and disperse to intermediate depths. Besides,comparatively less saline waters are transported to theintermediate depths of the Arabian Sea from south ofequator. The interaction of these waters, which greatlydiffer in characteristics is less understood compared tothe water characteristics and circulation of the upperlayers. Hence, a detailed study of the water characteristicsand current structure of the intermediate waters in theArabian Sea is attempted in the present investigation.

l - 1 - lflartlui @r..i,Situ1d.i.e§ien1 other -i111t.e.r.1vt@diat1e Prater S

1.1.1. Studieswpejoreillog

Much of our knowledge on the water characteristicsand current structure in the Arabian Sea, as indeed of theentire Indian Ocean, was derived on the basis of datacollected during several expeditions, like, Valdiva(1898—99),Planet (1906), Dana (l92O—30), Snellius (1929), John

Murray (1933-34), Swedish Deep Sea Expedition (l947~48),

Norsel I and II (1955-56). However, the informationavailable from these expeditions is rather limited inspace and time.

Using the data of Planet (1906), Dana (1929) andother earlier expeditions in the equatorial and centralIndian Ocean, Moller (1929) identified four water masses

namely, subsurface water of subtropical origin, lmwsalinity intermediate water of Antarctic origin, vannsaline water flowing towards south, and northward movingAntarctic Water. Schott (1926) and Thomsen (1933)

investigated the spreading of Red Sea Water in the IndianOcean from the then available data. Thomsen (1935) gave a

systematic classification of the characteristic watermasses in the Indian Ocean. Clowes and Deacon (1935)

identified Red Sea Water along the African Coast upio4008. Sverdrup et_§l.(1942) while summarising the studieson the water masses in the Indian Ocean attributed the

in ermediate salinity maximum in the Arabian Sea to thesubsurface outflow from the Red Sea.

1 ~ 1 - 2 - _.S.t,11_<'i.i e_si,b,<f=1 sled .I_IO _d__ia_-t'_—a_

The first systematic knowledge of the Indian Oceancame from the IIOE (1960-65) when several ships from

different nations participated; It is the first of itskind for such a large scale oceanographic survey of anyocean that was taken up. During this expedition, anenormous amount of data were collected which greatlyenhanced our knowledge of the oceanography of the IndianOcean. Based on the data collected during and earlier toIIOE, several studies were carried out.

,514.4

Taft (1963) while studying the property distributionson isanosteric surfaces suggested the penetration of lowsalinity water from the Banda and Timor seas into theSouth Indian Ocean on 125—cl/t surface extending westward

to 600E at 1005. He attributed this low salinity water tothe salinity minimum observed in the Somali Basin atintermediate depths. The incursion of Pacific Water intothe Indian Ocean through Banda and Timor seas was earlier

identified by Wyrtki (1957) and furthez confirmed byWyrtki (1961), Rochford (1961, 1966), Sharma (1972) and

Sharma g;_al. (1978).

Rochford (1964) made an independent study of thesalinity maxima in the upper 1,000 m of the North IndianOcean. He identified five water masses. Three of them are

named according to their origin, Red Sea, Persian Gulf andArabian Sea. The other two could not be assigned the exactorigin although one is confined largely to the northernpart of the Arabian Sea and the other to the equatorialregion of the Indian Ocean.

Warren et_al,(1966) studied the water mass structureof the Somali Basin from observations of temperature,salinity and dissolved oxygen content made during thesouthwest monsoon. They identified the sources of thewaters responsible for the water mass structure in theSomali Basin. They discussed the probable vertical influence

5

of Red Sea Water in the Somali Basin.

Gallagher (1966) described the various water masses

and their movement in the Indian Ocean. According to him,

high salinity water from the Gulf of Aden enters the ArabianSea at depths of 400-600 m and is mixed with adjacent waterand forms the intermediate water of the Arabian Sea. The

salinity of this water mass decreases gradually from westto east and from north to south.

Robinson (1967) demonstrated the spread of Red Sea

Water by means of T—S diagrams. Beneath the surface, the

salinity decreases with depth until the influence of PersianGulf and Red Sea waters are encountered and below 1,500 m,

cold, less saline Antarctic waters are found.

Using the hydrographic data available upto 1965,Duing and Schwill (1967) identified two distinct salinitymaxima in the Arabian Sea below 200 m, contrary to Rochford

(1964). They considered that the Red Sea Water spreads

around 500-700 m depth. They also suggested that thedistribution of salinity in the central part of the ArabianSea is essentially maintained by large scale mixingprocesses.

Wooster gt_gl,(l967) presented the water propertiesat 300, 150 and 100-cl/t surfaces in the Arabian Sea. They

6

considered 100-cl/t surface as representative of Red SeaWater. The fine structure of the Red Sea Water in the

Indian Ocean was investigated by Hamon (1967), Krause (1968)

and Federov (1978).

The results of the IIOB were presented in the

Oceanographic Atlas of the International Indian OceanExpedition (Wyrtki, 1971). Mean distributions of the waf rmasses are given in this atlas. The high salinity waterlamdng the Red Sea through the Strait of Bab-el-Mandebspreads as a well developed core layer into the Gulf ofAden and the Arabian Sea at depths between 500 and 800 m.

The water mass characteristics were described by Wyrtki (1973)

based on the IIOE data. He discussed ifie various sources ofwater masses affecting the intermediate layers of the IndianOcean. Sastry and D'Souza (1972) described the distribut onof salinity in the Arabian Sea during the southwest monsoon‘by presenting vertical sections and spatial distributioncharts.

Sharma (1972) studied the water characteristics at200-cl/t surface in the intertropical Indian Ocean bypresenting the distribution of the physical properties onthis surface. He inferred the transport of low salinitywater from the Pacific through Banda and Timor seas intothe western Indian Ocean along the South Equatorial Current.

7

In a latter paper, Sharma g§_gl. (1978) demonstratedclearly the intrusion of the low salinity water of Pacificorigin in the western Indian Ocean in the layers above100-cl/t surface. They also showed that the Somali Basin

intermediate salinity minimum is obviously of Pacific origin

Sharma (1976) critically examined the transequatorialmovement of water masses in the Indian Ocean and studied

the water characteristics along SON, equator, and 505 usingvolumetric analysis and interrelationships of potentialtemperature-salinity and potential temperature-oxyty inbivariate classes.

Rochford (1964) derived the paths of flow of variouswater masses in the Indian Ocean from salinity maxima.

Warren g;_al.(l966) inferred the probable flow pattern fromthe property distributions in the Somali Basin. The currentstructure in the Somali Basin was discussed by Swallow andBfWfi3(l966) from both direct measurement and indirect method

Currents at depths around 1,000 m are more variable, andappear to have southward components. The southwestward

motion indicated by the geostrophic profile below 500 m seem

to be significant. They pointed out that the flow atintermediate depths was irregular and complex in nature.

8

Bruce and Volkmann (1969) described the geostrophic

and direct current measurements off the Somali Coast. Theyreported an evidence of anticyclonic subsurface motion inthe northern Somali Basin prior to the onset of the southwestmonsoon. They speculated that this might be a remainderof the northern Somali Gyre of the previous year. Dynamictopographic charts of Bruce (1968) indicated that there wasconsiderable eddy activity in the western Arabian Sea.Bruce (1970) suggested that the currents in the Somaliregion were part of the anticyclonic circulation formedduring the southwest monsoon. Duing (1970) derived thesurface circulation of the North Indian Ocean by dynamiccomputations and discussed at length the vertical extentof the monsoon influence on current structure.

Wyrtki (1973) also described the current structureof the Indian Ocean. During the northeast monsoon, thesurface flow does not appear to penetrate much beyond thethermocline whereas during the southwest monsoon, the

circulation appears to penetrate below the thermocline.Large parts of the Somali Current are recirculated in an

intense eddy, the centre of which is about 300 km offshore.This elongated elliptical eddy stretches for about 100 kmparallel to the coast and is intimately connected with the

dynamics of the Monsoon Gyre.

9

1.1.3. §t_'ies after IIOEud

After the IIOE survey, a joint Indo-SovietMeteorological Experiment (ISMEX) was conducted in the

Arabian Sea during the summer of 1973. Subsequently, an

OCEANOVEX Programme was carried out in the northeast

Arabian Sea during l973~74. A monsoon sub—programme known

as MONEX was conducted during 1979 as a part of the GlobalAtmospheric Research Programme (GARP). As a pre—MONEX build

up activity, the Monsoon—77 Experiment on a limited scalewas also conducted during 1977. '

During the First GARP Global Experiment'(FGG3) 1979,

a large oceanographic experiment called the Indian OceanExperiment (INDEX) was conducted in the western Indian Ocean.

Preliminary results of this experiment were reported bySwallow (1980). The behaviour of the Somali Current during

the spring transition period was documented by Leetmaa gt_Ql.(1982). The vertical structure and variability of the SomaliCurrent system were studied using current and temperatureobservations from moored instruments( Schott and Quadfasel,1982; Quadfasel and Schott, 1983). Quadfasel and Schott(l982)

described the water mass distributions at intermediate layers

off the Somali Coast during the southwest monsoon. Theydiscussed the importance of equatorial and near—coastal under­currents in the large-scale redistribution of water massesin the intermediate layers.

10

The waters carried by the South Equatorial Currentand crossing the equator towards north during the south»west monsoon may also be playing an important part in thedistribution of properties at intermediate layers in theArabian Sea. Swallow (1984) suggested that the volume

transport of water coming into the Arabian Sea at inter­mediate depths from south of equator must, generally,exceedthat coming in from the northern source. Godfrey and Golding(1981) suggested that the influence of Banda IntermediateWater in the western Indian Ocean is much greater thanwhat was earlier expected. The intermediate waters t'atare carried by the South Equatorial Current in the 1 .erstages of the southwest monsoon goes directly as far as8ON under the Somali Current (Swallow and Bruce, 1966) but

probably most of it is carried eastwards under EquatorialCounter Current and then westwards in the subsurface west­

ward jets (Luyten g;_§l,, 1980)before it reaches the ArabianSea. Hence, the mixing process at the intermediate watersof the Arabian Sea is much more vigorous than in any otheroceans (Swallow, 1984).

Quadfasel and Schott (1982) described an outflow

of relatively high saline water in the depth range of 500 to1,000 m, crossing the equator close to the western boundaryunder the Somali Current. The southflowing current was about100 km wide. Beyond it, there could be another boundary

current flowing in the opposite direction(Swallow, 1984).

ll

Levy et al. (1982) reported that the mean current in al4~month record from a current meter at 750 m depth at00 470E was 4 cm s-1 towards 3500. Farther east alongthe equator, at leas as far as 600E, the mean currentsat 750 m were predominantly westward, with only weak and

variable meridional components. It seems possible thatadvection across the equator at intermediate depth isconfined to the western boundary, may be in the form of apair of opposing boundary currents, with the north-goingone offshore (Swallow, 1984).

From all the above studies it is understood thatthere are some conflicting views on the spatial extent ofthe various water masses at intermediate depths and alsoin the current structure in the Arabian Sea. The salinityminimum observed at intermediate depths of the westernArabian Sea makes a lot of controversy among the variousinvestigators. In view of the diversity of views expressedby various researchers, the author is tempted to study indetail the water characteristics and current structure ofthe intermediate waters in the Arabian Sea.

1 - 1 - 4 - Dsscriprt irt<:_>n,_o f t _thei;=1trea

The Arabian Sea is bounded by the land masses of Asia

and Africa with an opening in the south. It is connected tothe Persian Gulf through the Gulf of Oman through a sill

12

depth of 50 m at the Hormuz Strait. Similarly, it isconnected to the Red Sea through a sill depth ct 125 m atthe Strait of Bab—el-Mandab through the Gulf of Aden.

The atmospheric and oceanic circulation in the

Arabian Sea vary seasonally. During northern summer, a lowpressure area developed over the Indian Sub-continent causesthe winds to blow persistently from southwest, whereas duringwinter the wind system over the Arabian Sea is northeasterlydue to the influence of high pressure developed over TibetanPlataue. The winds are weak and unsteady during the transitionperiods between two n-nsoons i.e. March-April and October­November. Of the two monsoors, the southwest monsoon lasts

over a longer period and the wind speeds are much higher andsteadier than the northeast monsoon, causing profoundchanges in the dynamics of the Arabian Sea.

The surface currents are mainly induced by the monsoon

winds resulting in the reversal of the surface circulation semi­annually. During the southwest monsoon,a strong anticyclonicflow is found whereas a weak cyclonic flow is observed duringthe northeast monsoon. The most notable current in the

Arabian Sea is the Somali Current which is found only duringthe southwest monsoon and perhaps is the strongest current Withspeeds exceeding 350 cm s-1 (Warren Q; gl., 1966; Swallow and

13

Bruce, 1966; Schott, 1983). Besides, strong upwelling

takes place offsouthwest India

the coasts of Somalia, Arabian and

For the present investigation, the area north of theEquator is considered. Red Sea and Persian Gulf are excluded.

SEC TI!) ll :- IYL_*3.Tf5_R__l!?-L_.5r ?~l\7D:_ME.7-_l1l?QD;‘3;

The water characteristics and current structureof the intermediate waters of the Arabian Sea are studied

by presenting the distributions of salinity and accelerationpotential on constant potential thermosteric anomaly surfacesand their topography, vertical sections of potential tempera­ture and salinity along the various latitudes and longitudes,scatter diagrams of potential temperature against salinity atdifferent representative areas, and the distribution ofpotential vorticity between differe.t isanosteric surfaces.

1.2.1. lsanostericganalysisc

The flow

surfaces rathersteric surfacesand circulationParr (1938). Inof conservative

of water takes place along the stericthan along the geometric surfaces. The use of

for the study of the water characteristicswas introduced by Montgomery (1938) and

their studies they worked out the diStfibUtiO1

properties on the surfaces of constant sigma-Tto study the circulation. The method is very useful forstudying the water characteristics and flow pattern as revealedfrom the studies of Montg0mery(l938), Taft(1963), Reid(1965),

14

Tsuchiya (1968) and Sharma (1972). An advantage cf the

steric surfaces for mapping of oceanographic propertieslies in the elimination of depth as an independentvariable, whereby the short term vertical displacements inthe water column, such as internal waves are eliminated.

The movement of water from a particular source

along the steric surface may form, in the maps of salinityand temperature, patterns that indicate the path of flow.The interpretation of tongues of high and low concentrationsas evidence of flow is in many cases sound, but it has beenrecognized that tongues need not always represent the axisof flow in every case and that under some conditions the flow

may parallel the isolines of a property (Sverdrup Q; gl.,1942).Thus, core mdmfi always need not give" an unambiguous picture.It is all the more inapplicable in the Indian Ocean becauseof the inflow of various water masses at different levels(Sharma, 1976). But Montgomery and Wooster (1954) a ggested

thermosteric anomaly surfaces to study the water characteristicsand flow pattern in place of steric surfaces with the assumptionthat the pressure effect is much less. Nevertheless, tocompensate the pressure effect, constant potential thermostericanomaly surfaces are employed in the present study.

The main aim of the present investigation being the

J-_»

study the water characteristics and current structure in

('2'CT‘

FD

the intermediate waters, it is found from literaturethat the intermediate waters are covered within theisanosteric surfaces of 100 and 4O—cl/t and therefore itis preferred to present the distribution of propertiesand the flow pattern on 100, 80, 60 and 40-cl/t potentialthermosteric anomaly surfaces.

1J%2-Qmwmwyisfhw1

The geostrophic flow along the isanosteric surfaceswas deduced from the gradient of acceleration potential(Montgomery, 1937; Montgomery and Sphilhas, 1941; Montgomery

and Stroup, 1962), commonly called Montgomery function

(Reid, 1965). The expression of acceleration potential usedfor numerical computation is

69I P d6 + P 66 6 o 6.9°

where 5 is potential thermosteric anomaly at the6°

reference pressure (Po), The reference pressure for thisnumerical integration has been chosen to be 2,000 db.

1-2-3- Yertisal sections Qfrppteniial temperature and salinitr

The surface and intermediate layers of the ArabianSea are composed of several high and low salinity water masses

16

Each water mass has originated from the surface water ii a

particular area and has carried the characteristics of thatarea to the positions appropriate to their density, wherethey lie beneath the sea surface in the Arabian Sea. Mixingbetween and along these layers modifies the originalcharacteristics, but some of these layers can be recognizedover very long distances in the Arabian Sea.

In order to study the water characteristics of theArabian Sea, seven vertical sections have been chosen. Fiveof them are zonal; along the eguator, SON, 1OON, l5ON, 2OON,

while rest of the two are meridional; along 580E and 700Efrom equator to 15oN and 18ON respectively. Station positionsare given in Fig.1. The water properties presented arepotential temperature and salinity.

1 - 2 - 4 - tS,¢.<1.t.tsri_d.ia_qr.am.s__ Oi pcotcentia l..._iciamper.a.t.vr.eiasa.i-an S t

salinityThe potential temperature—salinity characteristics

offer a useful view of the qualitative description of thewaters of the region under study. The value of potentialtemperature is plotted against salinity at the levels of30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150,160, 180, 200, 240, 280, 320, 360, 400, 440, 480, 520,580, etc., cl/t at the surface and the deepest sampling

level. Scatter diagrams for 94S are prepared at different

mfg_mwZO_6% 4g_:m_) gt fig MWZOEWOQ Z92; _ O;_2 _2 _£ _8 E _3 E F8maIt_‘OFF_’-m V]1___Q_T_1_F.2]7_+__o~ I____.1_ _2%“TA _ |_‘_||* lfil \+_:__F||i_ __ * ‘ _ IQ‘ :_ __ :+i|Tii_|| +ll _ * T|H‘ J_K_JHHJ * A _ __|I:|+ _ I_H]\_]|___ _|+é|j:T| __IO11 Ilolm ‘T Ui||LuF‘ N LM {Y , LT > I ‘hf ) ||O\| \+\ O iOI\\J°H 298%J1 _ W13 _ I 1 *4 4 3 4 zomI_I I PI ‘ I N P |fh‘|||lI_I_l‘, i Vhi q EzO__Umm___15) 296$ ; 296%w 9‘4* 2 zO_Bwm_ 0 ! ¢ _“‘ if\‘ ‘ ‘ii I. J _ \<_QZ_ J11” /‘_m<N_<> 295%/é |_[|r[*é I ? L{|_\\_‘lé + _| _| _| _ ‘* |[__é: _ ‘Ii *_ I * L fl _ FLT‘: * Ii‘: * r T__é[_1 A ___ T TL ‘F :1; M‘_§|+|L TILJ__ILB_It_A4_‘L%_+ACQI__II?L4A_Q _LF3‘A: ‘fig .3 _8 _£_ .8 %'~ FQ4m khm.0722FmQ

17

areas as shown in Fig.2. This method is advantageous inthat the number of representative points is the same foreach station, and the relative distribution of propertiesat each steric level is well depicted (Sharma, 1976).

1 . 2 - 5 P_OteI31Q_i§1.l,_ f\LQI.tbi_§?i1?)L

Vorticity is a characteristic of the kinematics offluid flow which expresses the tendency for portions ofthe fluid to rotate (Pond and Pickard, 1983). when it ismeasured relative to the earth, it is called relativevorticity. For a rotating solid object,the vorticity istwice the angular velocity, 29 Sin Q and is known asplanetary vorticity. The sum of the relative and planetaryvorticities is the absolute vorticity.

For a layer of thiciness D in the sea, where thedensity is unform, the absolute vorticity divided by D isa constant for the motion of a water body, provided thatthere is no input of vorticity such as might come fromwind stress or other frictional effects. The quantityabsolute vorticity divided by D is called the potentialvorticity of the water (Pond and Pickard, 1983).

Behringer (1972) produced maps of potential vorticityfor two density intervals in the intermediate depths in theSouth Atlantic Ocean. Sarmiento gt Q1, (l982)presented maps

.0.o:I_@__QNzmaOwhzwmwflfl MK4 WI4”Gd__O 5:43 I91; Kn: m<ME4 M_>___4__Zm_mmEn_WE N_O_&_li1LlW3i1HIa1III.IW_1Q (“K4 "(mg “:2 _(H_fi<<_l_<IOm“ is w (“E4 mg”;\ I\|___0\__l\\Q (:4 Q fig m_ _f \Q\,CI _4 _ O Z 4 W _ \_ < 2 m 4 N“ 4____2 3% ,_‘__ T _ _| __ I __} +I|_1|__*|:_!_\%_\\ |‘_ €lU_|'!FH¥|_ _'|\ _L _1\‘i[ _ “ r *\‘_rl% F _r iT|h_l h;_4 _,\|__é|_ L“.0” _mN _ok _mo _Dm ‘mm _om _mQ w_D“‘Q _ lip i WW‘ “I ___|__|: ii _ _ _ ‘ * “My _ _‘+| _ é_‘_‘é||éé‘*W||é‘|] _ _h|_‘| |é|_| ‘_ _ _| 41* * * é_|_“|l_ Q4AI%LMAE1__ItlIl__L_A1‘_ _I1__A1IT‘whoat _2 _£ _8 _£ .3 _m_\ we$1_Q_%__oNTrmu

18

of potential vorticity on density surfaces to studythe distribution of tritium in the North Atlantic Ocean.The vertical minimum in potential vorticity was used asthe primary tracer of Labrador Sea Water by Talley andMcCartney (1982). McDowell gg_§l.(l982) presented maps

and sections of the large scale North Atlantic potentialvorticity in relation to the general circulation. Mapsof potential vorticity derived from smoothed densityprofiles were studied by Stramma (1984).

In the interior of the ocean, for large-scaleprocesses, relative vorticity is negligible compared toplanetary vorticity. Under these conditions McDowell Q; al'(l982) showed that the potential vorticity, q , can bedetermined from hydrographic measurements of the potentialdensity, 5 (in 103 kg m_3) alone. In this case,

—- .f.. £6q =" O0 az

where f is the coriolis parameter, Q is the mean densityof thg layer and z is the vertical coordinate(positivedownward).

Using this relationship, potential vorticity iscomputed and maps of potential vorticity between theisanosteric surfaces, 110 ami9O cl/t, 90 ami7O cl/t and70and5O cl/t are prepared.

19

1.2.6. Data

"The data used in the present investigation are fromthose collected on board various research vessels during theInternational Indian Ocean Expedition and subsequently.The

details of the oceanographic data used are given in Table 1and their geographical positions are shown in Fig.3. Thestations were chosen on the basis of their geographicaldistribution, maximum depth of observation and verticalspacing of samples. Stations for which observations, madebeyond 1,000 m were only selected.

The study of Duing (1970) reveals that the seasonalinfluence in the North Indian Ocean is limited only to theupper 400 m. Since the present study of the water characteri~stics and current structure of the intermediate waterspertains to water below 400 m, the seasonal variation isexpected to be negligible. Therefore, no attempt is made toselect the data on seasonal basis.

1.2.7. §r0¢eQ§;Q

For each hydrographic station, the potentialtemperature for each sample are computed using the equationgiven by Fonoffof (1962).

Potential temperature 9 = T -“ATJTR_ 1

where AT‘- ZZZAijk p S

ZMNTI T‘? T ‘LN L ‘ATLQ LU*:"‘n\‘L““ r LwLH_ P‘? +l_%’_\ WWMLEIVM L \lT'L%\ r F \__‘ U_ UTL\ ‘Ty? L‘? L[ _1pg MN E Mm hm mm om m, \ y \ W3mzO_____.WO& ZOT2; no ZO_:_UO|_ OZ_;OIm Q4: M_O__“_mg w EM“ AA _2 Mm B Mm .3 _$ W.0: aid G H Q G G\ G‘@\é\\q|G]q|@hQNé\Jw\qJH ql \ W |_|H7_O O G _ D O O D O OQ0 0 _ 0jfi 0 Do Q o®_®_ 0° _Q‘ O Q O 0 0 ‘ 9 Q0 0 __1 D __ ‘ 0OO OO+G ‘ O OOOOOOO6 ‘DOOIOD 4Q _ O _DD_“QO6Qq. O0 +0 O0‘m 0......‘ o..D> . ...O OO 4 O do­®4 DU ‘DU ‘DODD b OD__DbGoDD <_J<zQm.0.+ 0 9O . . D O Db 0 O0 __ 0G @9460+05? DO DQC 00 ‘.. O. O .gr 4 __..7 ‘ DOD Q1 D I O .W_m__*LaJ‘T.G Q . bG0 ‘¢ b Q ‘$ ' . 0 ‘Q + OD ‘ ‘ ‘ . R0D + - . ‘ 9_* ‘ ‘ D 90 . D __ 4 ' ‘ 90 . . . . . D I_ @ . . . . ‘ .D000.‘-ooQooQ_0‘ .00 xv. “_*_.QQ ob ‘Or 4 .i Q0 9 D 9 9 ' 9 9.“®do (_m_<”_<U.Nr_D 8 __ a.‘ 0i+W‘ SQ4GOD.‘abLl <_oz_ so2 DdIiLb __7 JItLLIL‘L____J_8LibA.____41­L____:L_ _A4MYL2m$

TABLE 1

Number ofstationsusedShip Months and Year

Symbol Abbre­used viation

Atlantis II 112

Anton Bruun 26

Argo 64Commandant R. 15Giraud

I

Columbus Iselin21

Darshak 7

Discovery 58

Diamantina 3Kalva 2Kistna 8Meteor 7Mikhal 6Lomonosov

Okean 54

Aug.—Sep.Feb.-Apr.

Mar.,May,Aug., Oct.Nov.

Jan.,May

Jul.-Aug.Aug.

Sep.Apr.—MayJul.-Dec.

Aug.-Sep.

Dec.00 0Mar., May,Jun., Aug.Sep.

May

Feb.Apr.

Nov.-Dec.Mar.

Febo 0May-Jul.

Jun.-Jul.Jun.-Jul.

19631965

19631964

19621964

196019611962

1970

19731974

19631964

1965

19581959

19621965

1965

1966

19731977

. AN

I AOQ AR® CG

5 CIA DA DI

D DM0 KA+ KIQ MEQ MLO OK

Number ofShip stations Months and

usedYear

Symbolused

Abbre­viation

Priliv 20§obert D. Conrad 4

Requisite 5Shakalsky 7Varuna 17Vema 3Vityaz 29

May,Jul.Jun.

Jun.

Feb.-Mar.

Jun.

Sep.-Dec.

May-Jun.

Jan.-Apr.I 0Sep.

19731977

1965

1961

1973

1962

1958

1960

1962

9

0

B

I0

Y

V

PL

RD

RE

SH

VA

VE

VI

468Total

TABLE 1 The details of the oceanographic data used

20

Using the equation, a computer program was written inPDP-ll/60 computer in FORTRAN IV PLUS language, the input

values being temperature T in OC, salinity S in %» , depth Din metres and latitude Q in degrees.

For each station, curves are drawn with a common

abscissa of potential temperature against the ordinate ofsalinity and depth with an overprinted isopleths ofthermosteric anomaly. Smooth curves are drawn through the

plotted points. Some apparently questionable observed values,such as those indicating hydrostatic instability or abnormaldeviation from neighbouring stations are rejected. The valuesof depth and salinity at each chosen potential thermostericanomaly surface are read directly from the station curves.

Station V&ll€S, thus, obtained are plotted on eachmap, and smooth isopleths are drawn. If a station value isincompatible with nearby stations, the station curves arerevised without ignoring the observed values, in such a waythat the station value fits better with the nearby stations.The isopleths on the maps are drawn not strictly followingthe station values, and some points are ignored in the interest

of smoothness. This is particularhtrue for the salinitj anddepth maps on which the station values show certain deviations.

21

Since, temperature on an isanosteric surface isuniquely defined by salinity, the salinity maps can bealternatively interpreted as temperature maps. The valuesof temperature corresponding to the values of salinity forthe isohalines drawn on the maps are listed in Table 2.The value of sigma~9 equivalent to those of potentialthermosteric anomaly are also listed in Table 2.

The numerical integration of the accelerationpotential is carried out at intervals of l0#cl/t to yieldthe acceleration potential at chosen potential thermostericanomaly surfaces. The unit of acceleration potential chosenis joule per kilogram (J/Kg). Like the salinity and depthmaps, prepared for the chosen four isanosteric surfaces,the maps are also prepared for the acceleration potential.Since, 2,000 db has been chosen as the reference pressurefor the computation of acceleration potential and not allstations extend to 2,0 O m depth, the number of stationsused in the presentation of acceleration potential are lesscompared to those of the depth and salinity.

6e,cl/t 10O I6 g/l 27.07 27.28 27.49 27.70

0 80 60 40Salinity» per mil __ .P.9,t§¢_n_t.ia_lr_t<2mp@1:§*-2tu:1e.11C.Ql§-i.us

34.8

34.9

35.0

35.1

35.2

35.3

35.4

35.5

35.6

35.7

35.8

9.41

9.88

10.33

10.77

11.19

11.60

12.01

12.41

12.81

6.03

3.47

vb

f‘~J

U.)

3.01 6.62 4.948.59 7.18 5.609.08 7.72 6.229.55 8.24 6.80

10.01 8.7410.46 9.22

10.89 9.6911.31 10.15

TABLE 2 Values of potential temperature corresponding tothe values of salinity for the isohalines drawnon the maps of the isanosteric surfaces. Che valuesof 06 equivalent to the values of potentialthermosteric anomaly are included.