Global Atmospheric Circulation
I.	Introductory Material
	A.	Winds blow from H to L pressure at the surface, ultimately powered by differential heating.
	B.	If the earth did not rotate, we could ignore Coriolis effect. Patterns of global circulation would be very simple. Heated air would rise over the equatorial latitudes, cold air would subside over the poles, and winds would circulate in one big thermal cell in each hemisphere.
	C.	Thanks to rotation, this simple, one-celled model does not occur. Instead, the global circulation is better understood as a three-cell model with four surface circulation features.
II.	Hadley cells: the tropical circulation
	A.	The global circulation is powered by the high net radiation surplus of the equatorial latitudes. High sun angles year round and consistent day lengths mean that max net radiation occurs here.
		1.	Results in rising air columns, unstable air with plenty of latent heat (water vapor), rainy/wet conditions. This forms a belt in the equatorial latitudes, called the intertropical convergence zone (ITCZ). Major tropical rain forests found in this zone.
		2.	Air rises in the ITCZ, then moves poleward aloft, ultimately descending around 25° N/S latitude to form the subtropical high pressure zone (STHP). This is a zone of descending air, stable conditions, hot and dry. Major tropical and subtropical deserts found in this zone.
		3.	The ITCZ is a zone of dynamic convergence, with winds blowing into the surface low pressure trough from each hemisphere, moving out of the STHPs. These surface winds are the easterly trade winds. They are northeasterly trades in the N.H., southeasterly trades in the S.H., because of Coriolis deflection.
		4.	These two huge circulation cells over the tropical latitudes are called Hadley Cells. One in each hemisphere.
III.	Midlatitude and polar zone circulation
	A.	The midlatitudes are the zone of temperature contrast, frontal boundaries between warmer tropical and colder polar air. This forms the jet stream and polar front boundary. It is the zone of active energy exchange (by convection), stronger in the winter than in the summer.
		1.	The polar front is a zone of frontal wedging, with warm air being lifted over cold air. This induces low pressure at the surface in the frontal zone. Low pressure cells along the polar front are reinforced by the jet stream aloft, which helps rapidly remove air from the top of these cyclones, promoting stronger uplift and better storm development.
		2.	Because upper level winds are westerly, they steer weather (traveling cyclones) from W-->E. Hence, we call this region the polar front westerlies (PFW). It is generally found around 45° N/S latitude, but it migrates over a large range of latitudes. The migrating cyclones of the PFW bring pulses of unstable, wet weather to this region. The textbook refers to this as the sub-polar low zone.
	B.	The polar latitudes are cold, especially in their winter. This forms a shallow zone of cold, dense air that forms surface high pressure in the winter. These are called the polar high pressure cells (PHP). They are characterized by cold, stable, dry air. (Remember that cold air cannot hold much water vapor.)
IV.	Review global circulation zones:
Circulation feature Latitude Surface pressure Weather ITCZ 0° Low unstable, very warm and wet STHP 25° N/S High stable, hot and dry PFW 45° N/S Low unstable, cool and wet PHP 65-90° N/S High stable, cold and dry
V.	Circulation features migrate seasonally, following the sun.
	A.	Since the global circulation is powered by differential heating, it migrates latitudinally as the sub-solar point and zone of max net radiation moves from hemisphere to hemisphere.
	B.	ITCZ shifts northward in June/July, southward in Dec/Jan. Everything else shifts with it. Let's examine what happens in the Northern Hemisphere at these times.
		1.	January: ITCZ migrates south of equator. STHPs migrates to lower latitudes. PHP becomes very cold and expands to lower latitudes. The PFW become very active (strong jet stream, vigorous storms), and extends to lower latitudes. [We experience effects of PFW on our winter weather here in Athens.]
		2.	July: ITCZ migrates north of equator. STHPs migrate to higher latitudes, as do PFW. PHP all but disappears with longer day lengths, so PFW circulation is much weaker.
VI.	Patterns of rainfall seasonality by latitude.
	A.	Latitudinal zones centered on circulation features experience uniform precipitation patterns year round:
0-10° ITCZ wet 20-30° STHP dry 40-50° PFW wet 65-90° PHP dry
	B.	Latitude zones between circulation features experience pronouced seasonal changes in precipitation:
summer: winter: 10-20° ITCZ - wet STHP - dry 30-40° STHP -- dry PFW -- wet 50-65° PFW -- wet PHP -- dry
VII.	STHP effects are not uniform. STHP cells form over the major ocean basins (Atlantic, Pacific, Indian Ocean in S.H.).
	A.	STHP are much more stable on the eastern side of the ocean (western side of adjacent continent), because of cold ocean currents that stabilize the lower layers of the atmosphere.
			E.g. California is very dry in summer, northwestern Mexico dry year round, because of stabilizing influence of STHP in eastern Pacific
	B.	STHP is more unstable on the western side of the ocean (eastern side of adjacent continent), because of warm ocean currents (like the Gulf Stream) that add lots of sensible and latent heat (water vapor) to the atmosphere.
			E.g., Georgia can be fairly wet in summer with afternoon thundershowers and hurricanes from Atlantic. Florida stays fairly wet year round. Caused by the effects of the Bermuda High (STHP in the western Atlantic Ocean).