Skip navigationGetting the global picture


1. Storms of dust and tanks

2. Big eyes upstairs

3. Eyeing melting ice

4. At the big disconnects

5. Finding fish - by satellite

6. Fire and brimstone




 A giant mixing zone near the Equator controls weather in tropical nations.



The Intertropical Convergence Zone (ITCZ) is the strong band of white clouds in this picture of the tropical Pacific Ocean, taken by a new geostationary satellite. Thunderstorms at the convergence zone bring rain to the tropics. Photo from GOES-11, NOAA.










Surface winds, AKA trade winds, meet at the Intertropical Convergence Zone. In recent years, with help from satellite observations, scientists have detected a southern counterpart to the northern ITCZ. The more powerful northern ITCZ develops where both trade wind systems merge. The southern ITCZ occurs inside the southeasterly trades. Image from NASA.


In the zone
Where do tropical winds meet? To us, the answers might help plan a beach vacation. But to scientists trying to understand Earth as a whole, these questions are critical to understanding the global picture.

Picture shows air circulation patterns in and around two fluffy white clouds. Arrows show cool, dry air circulating from the top toward the bottom,; surface winds circulating from the middle toward the base;  and warm, moist air circulatingThe Hadley cell is a giant heat engine that lofts warm, wet air into the atmosphere near the equator. The cell and the Intertropical Convergence Zone both move north and south with the seasons. The surface winds drawn into the convergence zone gain an easterly component due to Earth's rotation. The winds become the northeast and southeast surface winds shown in next drawing. Graphic from NASA

Convergence zones occur where streams of similar fluids meet, but don't immediately mix. Those streams of cold air blowing at oversized, overheated defensive tackles on the bench during a hot September game don't immediately merge with the sodden, heated air on the sidelines. The Gulf Stream flowing along the Atlantic coast is still a recognizable current when it reaches Scandinavia, due to slow mixing with the colder surrounding water.

Jump to the planetary scale, and consider the mother of convergence zones, the Intertropical Convergence Zone , or ITCZ. Part of Earth's vast heat engine, this zone circles the Earth over the ocean near the equator, at the junction of the northeast and southeast trade winds.

Here, the intense equatorial sun and warm ocean water combine to heat and humidify the air. As this air rises, it cools, and the water comes down in the almost perpetual thunderstorms that are a major source of rain in otherwise-dry tropical areas.

Satellite picture of the strip of land in southern Central America, and the northwest corner of South America. A band of white clouds branches across the center of the photograph, and occasional spots of clouds dot the ocean and land elsewhere.

On the verge of convergence
The convergence zone shifts north and south with the seasons, explaining why the tropical rainy seasons also change with the seasons. But the ITCZ may be changing in other ways.

Recent work with satellite measurements of winds about the ocean and tropical rainfall indicates that the ITCZ also changes every couple of weeks, says oceanographer Semyon Grodsky, a research scientist at the University of Maryland. "We see interesting oscillations in winds, they change back and forth from easterly to westerly during April, May and June."

World map shows how winds circulate along the equator. Northeast surface winds come from the north toward the southwest, while southeast surface winds come from the south toward the northwest. Both sets of winds are directed toward the equator.

The observation, made in the western part of the ITCZ over the tropical Atlantic, coincides with the buildup to the rainy season in the Sahel, a drought-prone strip of West Africa that lies between the rain forest and Sahara Desert.

Eventually, a better picture of quick changes in the northern convergence zone could make for better rainfall predictions in the Sahel. Similarly, Grodsky says, "We also need better understanding of the impact of the southern ITCZ on the ocean and on the precipitation in Northeast Brazil," another drought-prone region that is watered by seasonal rains generated at the convergence zone.

But that's unlikely to happen unless research money becomes available, Grodsky says.

Great fishing in an oceanic convergence zone?

The Why Files

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