El Nino's cousin

FOR THE past four months the tropical Atlantic has warmed on a geographic scale not seen since the 1960s, leaving scientists mystified.

As storms and hurricanes generated by El Nino batter North and South America, NASA scientists have uncovered sea temperatures in the Atlantic that they think could have a bearing on El Nino.

El Nino involves periodic changes in sea surface temperature, currents and winds on an east-west plane in the Pacific. A similar self-generating oscillation between warm and cool ocean temperatures also exists either side of the equator in the Atlantic.

Though less regular and intense than El Nino, the Atlantic dipole can dramatically affect rainfall in Brazil and the Sahel. About every ten years scientists have found sea surface temperatures oddly warm north of the equator in the Atlantic and cool to the south, or vice versa, causing the Intertropical Convergence Zone - the band of thunderstorms found close to the equator - to migrate. This causes drought.

Last year US, French and Brazilian scientists established Pirata, the Pilot Research Array in the tropical Atlantic. Moorings were placed in the Atlantic, north and south of the equator, to monitor ocean temperature changes.

Initial findings have caught the community by surprise. Antonio Busalacchi, chief of the laboratory for hydrospheric processes at the Goddard Space Flight Centre in Maryland, US, said: "The interesting part is that a large proportion of both hemispheres is warm, not just the north or south."

Attempts to understand El Nino have been restricted to observations in the tropical Pacific for the past ten years. But there is now a realisation that the ocean at higher latitudes may have an impact on El Nino's development, perhaps helping to explain its irregularity.