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Lecture 15 General Circulation of the Atmosphere and Ocean

Simon de Szoeke

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sea breeze(afternoon)

land breeze(morning)

General CirculationThe atmosphere is a heat engine, driven by radiative heating.

Equator

Pressuresurfaces

The thickness of pressure layers is proportional to temperature.

TT

WarmCold

LOWHIGH

HIGHLOW

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Non-rotating model of atmospheric general circulationHadley (1735) Cell is induced by uneven distribution of solar heating.

• Conceptual models idealize to capture the essence (e.g., Earth is a solid sphere rotating around the axis between the poles).

• Numerical models are mathematical equations derived based on physical understanding and verified against observations (weather & climate prediction).

Three-cell modelconsiders earth rotation effects and explains the easterly trades in the

subtropics and prevailing westerlies in the mid-latitudes.

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General Circulation and Weather• Raining near the equator• Subtropical deserts like

Sahara• Precipitation in the mid-

latitude storm tracks

Wind velocity at 300 hPain January, viewed from the North Pole

Westerly Jet Streams• Maximum (> 60 m/s) over

Japan & secondary max (40 m/s) on the US east coast.

• Responsible for longer return flights from North America.

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http://www-paoc.mit.edu/labweb/experiments.htm

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Winter surface circulation (geostrophy + friction)Aleutian low and Pacific high are both a result of the Tibetan Plateau’s

blockage of the subtropical westerly jet.

Summer surface circulation (geostrophy + friction)Thermal lows over continents and thermal highs over oceans in the NH.

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Fig. 7-22, p.188

See movie at http://solomon.physics.sc.edu/~tedeschi/demo/demo13.html

A ring of air shrinks its radius as it moves poleward (at the level of tropopause).

The jet spins faster than the earth by conservation of angular momentum, like a spinning skater pulling her arms inward.

Subtropical Jet

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Winter

Summer

Surface Pressure & Wind over the North PacificBased on historical ship reports

m/s

• Cool temperature forms surface high over ocean.

• Oceanic highs produce dry/cool climate on the west coast and hot/humid climate on the east coast in summer, by advection.

Role of Ocean in Climate

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Ekman flow• Important in the top 10 m• Surface flow in response to wind• To the right of the wind (in northern

hemisphere)W

ind

Cor

iolis

Ekman Current

Ekman (1905) explained Nansen’s observation that icebergs moved to the right of the wind direction in the Arctic.

Summer upwelling off CaliforniaNortherly winds along the coast

Offshore surface Ekman flowDeep cold water rises to compensateFog formation on Californian coast High nutrients and good fisheries

(Coastal water is not much warmer, but still warmer, in summer than winter.)

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Key WordsGeneral circulation, Hadley circulationIntertropical convergence zone (ITCZ), doldrumsSubtropical high, trade windsWesterlies, subtropical jet streamAleutian low, Pacific high, Siberian highIcelandic low, Bermuda highUpwelling, Ekman flow

Lecture 16

El Niño, and Ocean-Atmosphere Interaction

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El Nino: abnormal warming in the central/eastern equatorial Pacific Ocean (including the Peruvian coast).

Southern Oscillation: seesaw in sea level pressure between the western (Darwin, Australia) and central (Tahiti) Pacific.

(sea surface temperature changes)

ENSO

Why does sea surface temperature matter?

It determines the location and intensity of atmospheric deep convection (cumulonimbus): e.g., minimum temperature for hurricane formation ~ 27oC.

wind

Time series of El Niño

present

El Niño of the Century

°C

warm

cool

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Sea Surface Temperature

Dec 1997El Niño

Dec 1998(normal)

El Niño of the Century

TRMM Rainfall Anomalies Jan 1998

El Niñonow?

SST anomalyanimation 2005-2006

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Positive Feedback between ocean & atmosphereBjerknes (1969)

Warming in the eastEastward shift of deep convection & reduced air pressure in the eastReduced easterly tradesReduced upwelling & deepened thermocline (as water flushes back toward the east)

El Nino is a coupled ocean-atmosphere phenomenon.

Global Reach of El Niño’s Influence

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Pacific-North American (PNA) Pattern:A global wave train El Niño rides in winter

Pressure anomalies in the upper troposphere

Number of seasons with anomalous rainfall in Hawaii among 20 El Ninosanalyzed (Chu 1995, J. Climate)

El Niño’s Influence on Hawaii Climate

Enhanced subsidence due to strong convection in the

central/eastern equatorial Pacific

Dry in the El Niño Winter

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El Nino’s influence on the US winter climate

ENSO Cycle• A swing between El Niño and La

Nina.• Basin-scale ocean waves are the

switch.• This wave switch is constantly

monitored by an array of equatorial buoys and satellites

• Significant skill in predicting El Niño six months ahead.