EES215

Lecture 20

Climate Zones:

The distribution of global climate zones largely reflects the decrease in solar energy from the low to high latitudes in both hemispheres. Superimposed on this decrease is the distance to oceans and the wind and current patterns. The major climate zones are shown in Fig. 1. Mountain areas are exceptions to the general decrease in temperatures from the equator to the poles are; they have developed their own climate zones based on altitudes.

The major ecosystems (Fig. 2) reflect largely the distribution of climate zones, but also are strongly influenced by precipitation patterns. Because a persistent pressure high exists at around 30^o (the horse latitudes), these areas have generally low precipitation, in contrast to the equatorial regions with their persistent pressure low and plenty of precipitation. In many areas, strong seasonal variation exists, as, for example, in India with its dry season during the winter and the Monsoon season (strong rains) during the summer. This seasonality is caused by the shift in the ITC (Inter Topical Convergence) which allows either cold dry air masses (winter) or warm, humid air masses (summer) to dominate (Fig. 3).

This distribution of ecosystems and climate zones has been mostly stable since end of the last ice age.

El Nino: the interaction of oceans and atmosphere
Normal conditions: Large gyre south of equator:  Fig. 4
- correlated with wind direction - brings deep water to surface at coast of S America; slope in thermocline, Fig. 5; water temperatures colder than average for these latitudes; nutrient rich deep waters - fishing industry off the coast of Peru.

Wind pattern: surface wind - SE trade winds; high pressure zone off the coast of Peru; low pressure zone in Indonesia; warm moist air rises over Indonesia - precipitation; rain forest; return flow at high altitudes; cool dry air sinks off the coast of Peru; dry area parallel to equator.  Fig. 6

El Nino:  it is not clear what sets off this phenomenon (changes in temperature of ocean surface or changes in wind pattern), but the consequences can be described.
Low over Indonesia is shifted to the east; wind direction in mid-Pacific is reversed; surface currents follow wind direction - thermocline goes deeper; zone of anomalously high surface temperatures expands to the east; no upwelling of deep waters - food chain is disrupted - collapse of fisheries; low pressure zone close to coast of Peru; high precipitation in usually dry area; different cloud cover.  Fig. 7

Influence on weather pattern in northern hemisphere: change in convection pattern of winds; dislocation of jet stream; influx of dry air from northern Canada; drought in mid-western states; high precipitation in coastal areas of California; storms.