A review of tropical ocean–atmosphere interactions

Enormous strides have been made towards the goal of operational predictions of seasonal and interannual climate fluctuations, especially as regards the phenomenon El Niño. To initialize models, measurements are available from an impressive array of instruments that monitor the tropical Pacific continually; coupled general circulation models of the ocean and atmosphere are already capable of reproducing many aspects of the earth’s climate, its seasonal cycle, and the Southern Oscillation. These achievements crown the studies, over the past few decades, that describe, explain and simulate the atmospheric response to sea-surface temperature variations, the oceanic response to different types of wind fluctuations, and the broad spectrum of coupled ocean–atmosphere modes that results from interactions between the two media. Those modes, which are involved, not only in the Southern Oscillation but also in the seasonal cycle and the climatology, differ primarily as regards the main mechanisms that determine sea-surface temperature variations in the central and eastern tropical Pacific: advection by surface currents, and vertical movements of the thermocline induced by either local winds or, in the case of the delayed oscillator mode, by non-local winds. The observed Southern Oscillation appears to be a hybrid mode that changes from one episode to the next so that El Niño can evolve in a variety of ways — advection and nonlocally generated thermocline displacements are important to different degrees on different occassions. The extent to which random disturbances, such as westerly wind bursts over the western equatorial Pacific, influence El Niño depends on whether the southern oscillation is self-sustaining or damped. Attention is now turning to the factors that determine this aspect of the Southern Oscillation, its decadal modulation which causes it to be more energetic in some decades than others. Those factors include interactions between the tropics and extratropics that affect the mean depth of the thermocline, and the intensity of the climatological trade winds.