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Earth Sense: Tropic moisture zone affects weather

POSTED: February 3, 2013 2:00 a.m.

In the research about global climatic change, one weather system is of particular interest: The ITCZ. Its name may look like the license plate of one of those new European nations, but it stands for Intertropical Convergence Zone.

Satellite images like those on Google Earth show it. Look for the equator, and there will be a band of white clouds running from west to east. This marks the zone where the sun deposits a huge amount of energy (or heat) year-round.

The oceans are large near the equator, which results in massive amounts of evaporation. That moisture then condenses again and is transported some distance, to come down again as rain. The inner tropics are known for their daily rain showers, a feature that has brought the tropical rain forests into existence.

Like all the climate zones on the planet, the ITCZ shifts north and south with the seasons. Right now, its summer in the southern hemisphere, so it is found south of the equator. Six months from now, it will have moved northward.

Some African countries rely heavily on its summer rains for agriculture and ranching, especially the ones in the so-called Sahel (“borderlands”) zone such as Niger, Mali, Chad and South Sudan.

Recent studies have shown that changes in Arctic sea ice affect the position of the ITCZ, and therefore the availability of rain in places that were starved by drought during the last century. For example, shrinking ice covers due to climatic warming would allow the rain belt to expand northward, and bring relief to the African regions where starvation is still a huge problem. But the situation is very complex because weather over land is influenced by mountain ranges and different wind patterns than weather over the oceans.

If those models hold true, though, we could also see an expansion of the high pressure cells that are north of the ITCZ, like the Bermuda High. A stubborn high in the summer would mean more drought for the Southeastern States, because rains are sparse when the air pressure is high.

Georgia isn’t ever under the ITCZ. We get summer rain when the Bermuda High eases up and local thunderstorms form, or when a rare cold front manages to make it this far south. Most of the assumptions about our area therefore expect global warming to cause changeable weather, but also more hot, dry summers with episodes of drought.

Rudi Kiefer, Ph.D., is a professor of physical science and director of sustainability at Brenau University. His email column appears Sundays and at gainesvilletimes.com.


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