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Researchers say they have found a clue to why El Niño events in the Pacific Ocean only sometimes lead to a warming of the Atlantic — affecting African rainfall along the Gulf of Guinea.

Writing in Nature today (21 September), they call for a network of monitoring stations in the tropical Atlantic to better understand what governs the variations so that scientists can make better seasonal weather forecasts for Africa.

El Niño events happen every three to seven years and are marked by a dramatic warming of the Pacific's surface water along the coast of South America. This alters the movement of heat and moisture in the atmosphere.

During El Niño years, warm air from the Pacific moves east towards the Atlantic and can warm its surface waters too — a phenomenon known as the Atlantic Niño. But this does not always happen.

In 1982-1983, for instance, a strong El Niño event coincided with cooler temperatures in the equatorial Atlantic and along the African coast.

To try to understand why the Atlantic Niño is so fickle, Ping Chang and colleagues at Texas A&M University in the United States compared the 1982-1983 El Niño to another strong El Niño event in 1997-1998 that did lead to a warming of surface waters in the Atlantic.

They found that the 1982-1983 event was associated with a far stronger easterly wind than the 1997-1998 and all other El Niño events that produced warmer Atlantic temperatures.

Chang and colleagues then verified their findings using computer models.

The researchers suggest that the stronger easterly wind of 1982-1983 drew cooler deeper waters to the surface along Africa's Atlantic coast. The cooler sea-surface temperatures, in turn, reinforce the easterly wind.

This cooling feedback effect competes with the warming effect of El Niño and "is the main cause of the fragile relationship between the Pacific El Niño and the Atlantic Niño", say the researchers.

They say a network of buoys that monitor weather conditions are needed in the equatorial Atlantic to better understand what tips the balance one way or the other.

"So far, we have had a lot of difficulties to achieve any useful [seasonal] predictions" for African rainfall, says Chang.

He says efforts should be made to maintain a network of buoys, known as the Pirata network, that has been temporarily deployed in the Atlantic to monitor temperature, humidity and atmospheric pressure there.

But he concedes that this is not an easy task. Anecdotal evidence suggests that fish tend to group around the buoys, attracting fishing vessels. This has made them vulnerable to damage.

Link to full paper in Nature

Reference: Nature 443, 324 (2006)