Planetary Circulation

Today

• Homework in

• Global scale circulations

• Inter-tropical convergence zone

• Horses

• Monsoons

Planetary Scale Circulations• A very complicated pattern that we shall

examine only in a very simple form.

• To begin, imagine the earth as a non-rotating sphere with uniform smooth surface characteristics.

• Assume that the sun heats the equatorial regions much more than the polar regions.

• In response to this, two huge convective cells develop.

Hadley CellSmooth EarthNo Rotation

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Add Rotation• We now allow the

earth to rotate. As expected, air travelling southward from the north pole will be deflected to the right.

• Air travelling northward from the south pole will be deflected to the left.

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Three-Cell Conceptual Model

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Three-Cell Model

• By looking at the actual winds, even after averaging them over a long period of time, we find that we do not observe this type of motion.

• In the 1920’s a new conceptual model was devised that had three cells instead of the single Hadley cell.

• These three cells better represent the typical wind flow around the globe.

General Circulation

Three Cell Circulation ModelThree Cell Circulation ModelA rotating earth A rotating earth breaks the single cell breaks the single cell into three cells.into three cells.

The Hadley cell The Hadley cell extends to the extends to the subtropics, the reverse subtropics, the reverse flow Ferrel cell flow Ferrel cell extends over the mid extends over the mid latitudes, and the latitudes, and the Polar cell extends over Polar cell extends over the poles.the poles.

The Coriolis force The Coriolis force generates westerlies generates westerlies and NE trade winds, and NE trade winds, and the polar front and the polar front redistributes cold air.redistributes cold air.

Figure 11.2AFigure 11.2A

Hadley Cell Circulations

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Horse Latitudes

• Around 30oN we see a region of subsiding (sinking) air.

• Sinking air is typically dry and free of substantial precipitation.

• Many of the major desert regions of the N.H. are found near 30o latitude.– Sahara– Middle East– SW United States

Intertropical Convergence Zone

ITCZ -- Moves south in January

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ITCZ -- Moves north in July

Intertropical Convergence Zone

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Example of these effects: Monsoon

• Wind systems that exhibit a pronounced seasonal reversal in direction.

• Best known monsoon is found in India and southeast Asia.

• Winter -- Flow is predominantly off the continent keeping the continent dry.

• Summer -- Flow is predominantly off the oceans keeping the continent wet.

Monsoon -- Winter

Continents aredry -- dryseason.

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Monsoon -- Summer

Continents arewet -- rainy

season.

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Global Precipitation PatternsGlobal Precipitation Patterns

Global low pressure Global low pressure zones around the zones around the equator and 60° equator and 60° latitude generate latitude generate convergence at the convergence at the surface, rising air surface, rising air and cloud and cloud formation.formation.

Zones of high Zones of high pressure at 30° and pressure at 30° and the Poles experience the Poles experience convergence aloft convergence aloft with sinking, drying with sinking, drying air.air. Figure 11.5Figure 11.5

North American Winter WeatherNorth American Winter Weather

Figure 11.4Figure 11.4

Semi-Semi-permanent permanent highs redirect highs redirect North North American American winds, such as winds, such as cold interior cold interior southerly flow southerly flow from the from the Canadian high.Canadian high.

The Polar front The Polar front develops a develops a wave like wave like pattern as air pattern as air flows around flows around lows.lows.

Summary

• Large climate zones (deserts, monsoons, etc) depend on global wind pattern

• As the Earth tilts with time of year exact position of maximum large scale ascent /

descent of air changes