The General Circulation
 The large-scale wind patterns of the earth
 Mission: to mitigate global temperature
contrasts (decrease temperature gradients,
redistribute warm and cold air across the
 Caused by the unequal heating of the
earth’s surface
The General Circulation (High Latitudes)
 “Polar Cell” = sinking/descending air at
highest latitudes
 Purpose is to bring warm air poleward (aloft)
and cool air equatorward (below)
 Atmospheric “Dump”
– Mid-Latitude pollution carried there
– Very stable troposphere, on average, so there is
little mixing and a build-up of
– Nearly permanent high
pressure, sinking air, lead
to very little precipitation
(which would help clean
the air)
The General Circulation (Tropics)
 The Hadley Cell: large convection cell
where air rises near the equator and
sinks near 30 degrees north and south
 Purpose is to bring warm air poleward
(aloft) and cool air equatorward (below)
– Trade winds
– Eckman transport (ocean)
– Subtropical jet stream
– Subtropical highs
G.C.: Hadley Cell: Trade Winds
 Fairly constant tropical winds that transport
air equatorward
– Blow from the NE in
the northern
– Blow from the SE in
the southern
G.C.: Hadley Cell: Eckman Transport
 Vast stores of oceanic heat are distributed
through this process
 Upwelling: bring cold water up, heat it and
send it poleward
G.C.: Hadley Cell: ITCZ
Where NE and SE trade winds meet
Convergence into belt of weak low pressure
Ascending branch of the Hadley Cell
NOT a straight line at the equator
– Unequal heating of land and water
– Follows the sun
– Monsoon: seasonal reversal of wind direction
 India, SW
United States
(wimpy one)
 NOT a
synonym for
rain storm!!!
 ITCZ in
in July
G.C.: Hadley Cell: Subtropical Jet Stream
 Farthest poleward (upper troposphere) part of
Hadley Cell
 Eastward momentum is transferred from tropics
 Coriolis effect turns winds, resulting in a fastmoving ribbon of air
generally moving westto-east near 30
degrees latitude
G.C.: Hadley Cell: Subtropical Highs
 Descending branch of Hadley Cell caused by
upper tropospheric convergence
 Very persistent
 Convergence causes semi-permanent regions
of surface high pressure
– World’s deserts are found underneath and east
– Horse Latitudes
El Nino
 An irregular warming of the topmost layers of
the eastern Pacific ocean
– Generally occurs once every 3-7 years
– Trade winds weaken
 Upwelling of cold, nutrient-rich water off Peru ceases
 Warm water sloshes eastward from western Pacific
 Upwelling near equator weakens, water warms
– Not fully understood
– Teleconnections
– NOT a weather event
– La Nina
Key Figures
 10.2, 10.9, 10.11, 10.15, 10.16, 10.19,
10.33, 10.40, 10.41, 10.45
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