Abstract :
The appearance of unusually warm sea surface temperatures in the eastern equatorial Pacific, the signature of El Nino, alters the location and intensity of regions of deep convection in the tropics, and thus affects the global atmospheric circulation. The increase in the temperature of the surface waters is part of the oceanic response to the altered atmospheric conditions, especially the changes in the trade winds over the Pacific. This circular argument-the warm surface waters are both the cause and consequence of the changes in atmospheric conditionsimplies that the interactions between the tropical Pacific Ocean and the atmosphere amount to a positive feedback and can result in natural modes of oscillation with timescales of the order of a few years. The Southern Oscillation, between complementary El Nine and La Nina states, corresponds to such a mode. Considerable progress has been made towards a capability to predict El Nino, La Nina, and climate fluctuations in general: an array of instruments now monitors the tropical Pacific; coupled ocean-atmosphere models capable of simulating and forecasting El Nino are growing rapidly in realism and skill. The predictability of El Nino appears to vary with time, probably because the Southern Oscillation is self-sustaining and hence highly predictable during some decades, damped and hence difficult to predict in other decades. During the latter periods, bursts of westerly winds that sporadically persist over the western equatorial Pacific for a week or two, have a spatial structure that enables them to excite the Southern Oscillation. Attention is now turning to the exchanges between the tropical and extra-tropical oceans that influence the decadal modulation of El Nino.
