Quarterdeck, Volume 6, Number 2, Summer 1998

A varying climate in the tropical Atlantic

	Initial tropical conditions Just north of the equator is the Intertropical Convergence Zone (ITCZ) where heavy rainfall occurs. The tradewinds blow from the northeast in the Northern Hemisphere, and from the southeast in the Southern Hemisphere. At the ITCZ, the tradewinds converge and the air rises, causing heavy convection.
	Positive air-sea feedback When the water in the Northern Hemisphere grows unusually warm, a positive air-sea feedback develops. The sea-surface air pressure difference between the north and south drives an unusually strong wind from the southeast. This wind anomaly weakens the northeast tradewinds. Weakened tradewinds remove less heat from the water in the north, and the water grows warmer. The warmer temperature increases the north-south pressure difference, and causes a stronger wind anomaly. The feedback continues. Stronger tradewinds from the south increasingly cool the southern water and help push the ITCZ northward.
	Results of a temperature change The surface temperature variation affects the climate on both tropical Atlantic coasts. Because of the warm water in the north, and intensified southerly wind near the equator, the ITCZ is displaced further north. The climate in northeast Brazil becomes drier than normal. During a drought, many Brazilians emigrate to wetter areas. Meanwhile, the Sahel region of Africa gets much more rain than usual. Every decade or so, these rainy and dry climates switch places because the sea-surface temperature pattern reverses.   Back to the article

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Last updated August 1, 1998