lecture 15 general circulation of the atmosphere and ocean · general circulation, hadley...
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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.