'Chemical Equator' Keeps Polluted Air in the North

Sept. 26, 2008 -- Wrapping around Earth's equator like a belt, a boundary of air is keeping the polluted atmosphere of the Northern Hemisphere separate from the relatively pristine south.

A team of researchers led by Jacqueline Hamilton of York University in the United Kingdom have dubbed the peculiar wall of air the "chemical equator." And while scientists have known about the feature for decades, Hamilton's team has just discovered an odd new wrinkle in its behavior.

Typically, where the northern and southern tropical trade winds come together they form a region of stormy updrafts called the Intertropical Convergence Zone (ITCZ). Instead of mixing, the trade winds rise from the ocean or land surface high into the stratosphere, where they diverge again.

The circulation pattern quarantines the Northern and Southern Hemispheres, keeping the pollutant-laden northern air dirty and the southern air clean. There is some exchange between the two, but the process takes a year or two, versus about a week for air to circulate within a hemisphere.

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The team discovered that in January and February, the southern summer, the chemical equator splits off from the ITCZ north of Australia. As hot air rises quickly off the sweltering Australian continent, storms proliferate and the ITCZ is shifted over land, well south of the geographical equator.

Clean air from the Indian Ocean rushes in to fill the gap, pushing north of Australia and into the South Pacific, where warm waters create a second stormy area.