atmospheric circulation-f

8/7/2019 Atmospheric Circulation-f

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General Circulation of the Atmosphere

General refers to the average air flow,

actual winds will vary considerably.

Average conditions help identify driving

forces.

The basic cause of the general circulation

is unequal heating of the Earths surface

yWarm air is transferred from the Tropics to thePoles

y Cool air is transferred from the Poles to the

Tropics


3/23

General

Circulation of the

Atmosphere

Single Cell ModelSingle Cell ModelAssume

uniform water surface

Sun always directly overhead the Equator

Earth does not rotate

Result: huge thermally direct convection cell(Hadley)


4/23

Three Cell ModelAllow earth to spin = three cells (Hadley, Ferrell, Polar)

Alternating belts of pressure starting with L at Equator

Alternating belts of wind with NE just North of Equator

The idealized wind and surface-

pressure distribution over a uniformly

water-covered rotating earth.


5/23

General Circulation of the Atmosphere

Average Surface Wind and Pressure:

The Real World

y Semi-permanent high and lows

y Northern vs. Southern Hemisphere

y Major features shift seasonally with the high sun

North in July

South in December


6/23

Average sea-level pressure distribution and surface wind-

flow patterns forJanuary.

The solid red line represents the position of the ITCZ.


7/23

Average sea-level pressure distribution and surface wind-

flow patterns forJuly.

The solid red line represents the position of the ITCZ.


8/23

A winter weather map depicting the main features of the general

circulation over North America. Notice that the Canadian high, polar

front, and subpolar lows have all moved southward into the United

States, and that the prevailing westerlies exist south of the polar front.

The arrows on the map illustrate wind direction.


9/23

Rising and sinking air

associated with the major

pressure systems of the

earths general circulation.

Where the air rises,precipitation tends to be

abundant (blue shade);

where the air sinks,

drier regions prevail (tan

shade).

Note that the sinking air of the

subtropical highs produces

the major desert regions of

the world.

General Circulation and PrecipitationPatterns


10/23

During the summer, the Pacific high moves northward. Sinking air

along its eastern margin (over California) produces a strong

subsidence inversion, which causes relatively dry weather to

prevail. Along the western margin of the Bermuda high, southerlywinds bring in humid air, which rises, condenses, and produces

abundant rainfall.


11/23

Average Wind Flow and Pressure Patterns Aloft

y North-South temperature and pressure gradient at high altitudes creates West-East

winds, particularly at mid to high latitudes.y Average 500-mb chart for the month January


12/23

Average Wind Flow and Pressure Patterns Aloft

y North-South temperature and pressure gradient at high altitudes creates West-East

winds, particularly at mid to high latitudes.y Average 500-mb chart for the month July


13/23

Average position of the polar jet stream and the subtropical

jet stream, with respect to a model of the general circulation

in winter. Both jet streams are flowing from west to east.


14/23

A jet stream is a swiftly

flowing current of air

that moves in a wavywest-to-east direction.

The figure shows the

position of the polar jet

stream and subtropical

jet stream in winter.

Although jet streams

are shown as one

continuous river of air,in reality they are

discontinuous, with

their position varying

from one day to the

next.


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16/23

Atmosphere Ocean

Interactions

Global Winds and Surface Ocean

Currents

y Ocean surface dragged by wind

y Cold current, flowing north to south, on west

side of continenty Warm current, flowing south to north, on

east side of continent

y Oceanic front


17/23

Average position and extent of the major surface

ocean currents. Cold currents are shown in blue;warm currents are shown in red


18/23

The Gulf Stream (dark red band) and its eddies are revealed in this

satellite mosaic of sea surface temperatures of the western North Atlantic

during May, 2001. Bright red shows the warmest water, followed by orange

and yellow. Green, blue, and purple represent the coldest water.


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20/23

In diagram (a), under ordinary conditions higher pressure over the southeastern Pacific and lower

pressure nearIndonesia produce easterly trade winds along the equator. These winds promote

upwelling and cooler ocean water in the eastern Pacific, while warmer water prevails in the western

Pacific.

The trades are part of a circulation (called the Walker circulation) that typically finds rising air andheavy rain over the western Pacific and sinking air and generally dry weather over the eastern

Pacific. When the trades are exceptionally strong, water along the equator in the eastern Pacific

becomes quite cool. This cool event is called La Nino During Non-El Nino conditions


21/23

(b) Atmospheric pressure decreases over the eastern Pacific and rises over the

estern Pacific. This change in pressure causes the trades to weaken or reverse

direction.

This situation enhances the countercurrent that carries warm water from the west over avast region of the eastern tropical Pacific. The thermocline, which separates the warm

water of the upper ocean from the cold water below, changes as the ocean conditions

change from non-El Nino to El Nino


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A strong ENSO event may trigger a response in nearly all indicated areas, whereas a

weak event will likely play a role in only some areas.

Regions of climatic abnormalities

associated with El NinoSouthern

Oscillation conditions (a) during

December through February

Regions of climatic abnormalities

associated with El NinoSouthern

Oscillation conditions (b) during

June through August

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