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 Cell Smooth Earth No Rotation ©1997 Prentice-Hall, Inc.©1998 Prentice-Hall, Inc.

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. ©1997 Prentice-Hall, Inc.

Three-Cell Conceptual Model ©1998 Prentice-Hall, Inc.

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 Model A rotating earth breaks the single cell into three cells. The Hadley cell extends to the subtropics, the reverse flow Ferrel cell extends over the mid latitudes, and the Polar cell extends over the poles. The Coriolis force generates westerlies and NE trade winds, and the polar front redistributes cold air. Figure 11.2A

Hadley Cell Circulations ©1998 Prentice-Hall, Inc.

Horse Latitudes Around 30 o N 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 30 o latitude. –Sahara –Middle East –SW United States

Intertropical Convergence Zone ITCZ -- Moves south in January ©1998 Prentice-Hall, Inc.

ITCZ -- Moves north in July Intertropical Convergence Zone ©1998 Prentice-Hall, Inc.

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 are dry -- dry season. ©1998 Prentice-Hall, Inc.

Monsoon -- Summer Continents are wet -- rainy season. ©1998 Prentice-Hall, Inc.

Global Precipitation Patterns Global low pressure zones around the equator and 60° latitude generate convergence at the surface, rising air and cloud formation. Zones of high pressure at 30° and the Poles experience convergence aloft with sinking, drying air. Figure 11.5

North American Winter Weather Figure 11.4 Semi- permanent highs redirect North American winds, such as cold interior southerly flow from the Canadian high. The Polar front develops a wave like pattern as air flows around 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