The present atmospheric circulation

Robert FovellAtmospheric and Oceanic Sciences

University of California, Los Angelesrfovell@ucla.edu

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Terms and abbreviations

• NH = Northern Hemisphere• PGF = pressure gradient force• CF = Coriolis force• Geostrophic wind = PGF + CF balance• SLP = sea-level pressure

– Average SLP = 1013.25 mb

• Reanalysis = retrospective weather reconstruction• NCEP = National Centers for Environmental

Prediction

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The sea-breeze circulation

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The sea-breeze circulation(applied to NH)

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One-cell circulation cell

• Surface wind is cold to warm & northerly

• “One cell” model• “Thermally direct”• Vertical motions help

reduce T gradient– Adiabatic

compression & expansion

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One-cell circulation cell

• Rotunno experiment provided glimpse that Earth rotation complicates matters

• Rotation proxy: Coriolis force

• Consequence: 1-cell model fails to explain global circulation

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Geostrophic adjustment

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Horizontal pressure gradient

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PGF impels motion

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Coriolis acts to right in NH

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Air starts curving away from L pressure

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Coriolis keeps tugging rightward…

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So air curves even more…

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Eventually PGF & Coriolis come into opposition…

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Eventually PGF & Coriolis come into opposition…

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Geostrophic adjustment

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Geostrophic balance:PGF + Coriolis

Note NO flow towards low

Buys-Ballot’s “Law”:In NH, with wind at back,low pressure is to your left

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Geostrophic with friction:some component towards low possible

Geostrophic equations

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How Earth rotation impacts the global circulation

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From one cell to…

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Step #1: three cells

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Step #1: three cells

Polar Ferrel Hadley

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Step #1: three cells

cool warm

Ferrel cell is thermally indirect

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Step #1: three cells

storms deserts storms

Consequence ofvertical motion

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A cloud-free Earth

http://visibleearth.nasa.gov/view_cat.php?categoryID=1484

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Zoom in on Africa

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Focus on surface PGFs

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Step #2: geostrophic adjustment

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Step #2: geostrophic adjustment

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Step #2: geostrophic adjustment

Coriolis weakin tropics…

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Both hemispheres

Coriolis actsto left…

Coriolis actsto right…

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Note surface convergence at Equator

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ITCZ

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Note ITCZ resides a bit north of Equator… more soon35

Theory vs. practice

Atmospheric circulation as seen in reanalyses

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NCEP global reanalyses

• Reanalyses are retrospective reconstructions of weather, averaged to deduce climate

• Not purely observations– Before satellite era (1979-present) a lot of

guesses

• http://www.esrl.noaa.gov/psd/cgi-bin/data/composites/printpage.pl

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Annual average SLP

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Making that plot…

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Annual average SLP

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Expect: surface L at 60˚

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Avg. SLP1013.25 mb

Expect: surface H at 30˚

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NH winter SLP

Lows prominent over ocean… but high over inland Asia Aleutian & Icelandic Lows

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NH summer SLP

Highs dominate, again overocean. Low over Asia Pacific & Bermuda Highs

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Seasonal variation in Asia

summer

winter

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Seasonal variation in Asia

summer

winter

Regional scale sea-breeze:the monsoon (Arabic for “season”) 46

• Dominates SoCal weather

• Note also large west-east SLP gradient, owing to “regional sea-breeze”

• Establishes Walker circulation

Pacific High in NH summer

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Pacific Ocean as “bathtub”

Walker circulation

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Trade winds help pile up water in west Pacific. Sea level about a half meter higher there

Walker circulation

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Downwelling in west Pacific leads todeep layer of very warm (even hot) water.

The ‘Maritime Continent’ of Indonesia, Philippines

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Warm water supports storms, establishes Walker circulation, which looks a lot like a sea-breeze.

Subsidence branch reinforces Pacific High

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Periodically, the trade winds relax, weakening Walker circulation. Storms shift eastward. Drought strikes

Maritime Continent. El Nino.

Midtropospheric vertical motion: NH summer

Ascent = purple; descent = yellow/red 53

• Rising motion near Equator… except:– Ascent maximum NORTH of

Equator– Somalia receives subsidence

• Sinking motion at 30˚N, 30˚S– Subsidence near Crete

especially large– Flow over Alps causes

descent

• Note Somalia resides on western side of Walker circulation

Africa

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Ascent = precipitation

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Midtropospheric vertical motion: NH summer

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ITCZ

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ITCZ

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ITCZ in East Pacific (NH summer)

• Note surface wind convergence at ITCZ

• Note ITCZ is NORTH of Equator

• ITCZ position “follows the sun”

• Mean annual position resides in NH– more land in NH

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EQ

Midtroposphericascent NH summer

Surface precipitationNH summer

To a reasonable approximation:where air rises,precipitation occurs

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Surface precipitationNH winter

Surface precipitationNH summer

Northern oceanstorm tracks appearIn NH winter;ITCZ migrates south

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storm tracks seen

Theory vs. practice

• Three-cell per hemisphere model is a reasonable starting point for understanding atmospheric circulation

• Deficiencies/complications– Continents cause regional/seasonal-scale sea and

land breezes (monsoons)– Influences of oceans and storms (Walker

circulation, El Nino)– Topography also plays an important role

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Reanalysis vs. EdGCM

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Reanalysis vs. EdGCMEdGCM’s modern vs. modified

climate

Compare & contrast:

•SLP patterns, annual & seasonal•Precipitation patterns, amounts•Monsoon and Walker circulations•ITCZ migrations•More…

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Reanalysis and EdGCM will differ…

• Reanalysis is average across decades with trends; EdGCM control run is statistically steady (next slide)

• EdGCM cannot represent El Nino and some other weather phenomena reanalysis captures

• EdGCM topography very coarse (affects vertical motion and precipitation)

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Global averageprecip rates (mm/day)

Reanalysis

EdGCM66

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Annual average SLP

EdGCMEquatorial L appears moreprominent

ReanalysisEmphasizes 60˚S and 30˚ Highs

Changing color tablescan shift perceptions

[Color tables differ]

1005

1011

1018

1022

995

1000

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Annual average precipitation

EdGCM

Reanalysis

Changing color tablescan shift perceptions

[Color tables differ]

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Surface T difference:2xCO2 – control runs

ALL VALUES ARE POSITIVE

Using default options

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Surface T difference:2xCO2 – control runs

Zero-centered color scale

Changing color tablescan shift perceptions

[end]

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