The General CirculationThe General Circulation

The large-scale wind patterns of the earthThe large-scale wind patterns of the earth Mission: to mitigate global temperature Mission: to mitigate global temperature

contrasts (decrease temperature gradients, contrasts (decrease temperature gradients, redistribute warm and cold air across the redistribute warm and cold air across the globe)globe)

Caused by the unequal heating of the Caused by the unequal heating of the earth’s surfaceearth’s surface

The General Circulation (High Latitudes)The General Circulation (High Latitudes)

““Polar Cell” = sinking/descending air at Polar Cell” = sinking/descending air at highest latitudes highest latitudes

Purpose is to bring warm air poleward (aloft) Purpose is to bring warm air poleward (aloft) and cool air equatorward (below)and cool air equatorward (below)

Atmospheric “Dump”Atmospheric “Dump”– Mid-Latitude pollution carried thereMid-Latitude pollution carried there– Very stable troposphere, on average, so there is Very stable troposphere, on average, so there is

little mixing and a build-up of little mixing and a build-up of pollutantspollutants

– Nearly permanent high Nearly permanent high pressure, sinking air, lead pressure, sinking air, lead to very little precipitation to very little precipitation (which would help clean (which would help clean the air)the air)

The General Circulation (Tropics)The General Circulation (Tropics)

The Hadley Cell: large convection cell where The Hadley Cell: large convection cell where air rises near the equator and sinks near 30 air rises near the equator and sinks near 30 degrees north and south latitudedegrees north and south latitude

Purpose is to bring warm air poleward (aloft) Purpose is to bring warm air poleward (aloft) and cool air equatorward (below)and cool air equatorward (below)– Trade windsTrade winds– Eckman transport (ocean)Eckman transport (ocean)– ITCZITCZ– Subtropical jet streamSubtropical jet stream– Subtropical highsSubtropical highs

G.C.: Hadley Cell: Trade WindsG.C.: Hadley Cell: Trade Winds

Fairly constant tropical winds that transport Fairly constant tropical winds that transport air equatorwardair equatorward– Blow from the NE in Blow from the NE in

the northern the northern hemisphere hemisphere

– Blow from the SE in Blow from the SE in the southern the southern hemispherehemisphere

G.C.: Hadley Cell: Eckman TransportG.C.: Hadley Cell: Eckman Transport Vast stores of oceanic heat are distributed Vast stores of oceanic heat are distributed

through this processthrough this process Upwelling: bring cold water up, heat it and Upwelling: bring cold water up, heat it and

send it polewardsend it poleward

G.C.: Hadley Cell: ITCZG.C.: Hadley Cell: ITCZ Where NE and SE trade winds meetWhere NE and SE trade winds meet Convergence into belt of weak low pressureConvergence into belt of weak low pressure Ascending branch of the Hadley CellAscending branch of the Hadley Cell NOT a straight line at the equatorNOT a straight line at the equator

– Unequal heating of land and waterUnequal heating of land and water– Follows the sunFollows the sun– Monsoon: seasonal reversal of wind directionMonsoon: seasonal reversal of wind direction

India, SW India, SW United States United States (wimpy one)(wimpy one)

NOT a NOT a synonym for synonym for rain storm!!!rain storm!!!

ITCZ ITCZ in Julyin July

ITCZ in ITCZ in JanuaryJanuary

G.C.: Hadley Cell: Subtropical Jet StreamG.C.: Hadley Cell: Subtropical Jet Stream Farthest poleward (upper troposphere) part of Farthest poleward (upper troposphere) part of

Hadley CellHadley Cell Eastward momentum is transferred from tropicsEastward momentum is transferred from tropics Coriolis effect turns winds, resulting in a fast-Coriolis effect turns winds, resulting in a fast-

moving ribbon of air moving ribbon of air generally moving west- generally moving west- to-east near 30 to-east near 30 degrees latitudedegrees latitude

G.C.: Hadley Cell: Subtropical HighsG.C.: Hadley Cell: Subtropical Highs Descending branch of Hadley Cell caused by Descending branch of Hadley Cell caused by

upper tropospheric convergenceupper tropospheric convergence Very persistentVery persistent Convergence causes semi-permanent regions Convergence causes semi-permanent regions

of surface high pressureof surface high pressure– World’s deserts are found underneath and eastWorld’s deserts are found underneath and east– Horse LatitudesHorse Latitudes

El NinoEl Nino An irregular warming of the topmost layers of the eastern An irregular warming of the topmost layers of the eastern

Pacific oceanPacific ocean– Generally occurs once every 3-7 yearsGenerally occurs once every 3-7 years– Trade winds weakenTrade winds weaken

Upwelling of cold, nutrient-rich water off Peru ceasesUpwelling of cold, nutrient-rich water off Peru ceases Warm water sloshes eastward from western PacificWarm water sloshes eastward from western Pacific Upwelling near equator weakens, water warmsUpwelling near equator weakens, water warms

– Not fully understoodNot fully understood– TeleconnectionsTeleconnections– NOT a weather eventNOT a weather event– La NinaLa Nina

Key FiguresKey Figures

10.2, 10.9, 10.11, 10.15, 10.16, 10.19, 10.2, 10.9, 10.11, 10.15, 10.16, 10.19, 10.33, 10.40, 10.41, 10.4510.33, 10.40, 10.41, 10.45