A review of tropical ocean–atmosphere interactions

Abstract

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 initializemodels, measurements are available from an impressive array of instruments that monitor thetropical Pacific continually; coupled general circulation models of the ocean and atmosphereare already capable of reproducing many aspects of the earth’s climate, its seasonal cycle, andthe Southern Oscillation. These achievements crown the studies, over the past few decades, thatdescribe, 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 coupledocean–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 theclimatology, differ primarily as regards the main mechanisms that determine sea-surface temperaturevariations in the central and eastern tropical Pacific: advection by surface currents, andvertical movements of the thermocline induced by either local winds or, in the case of thedelayed oscillator mode, by non-local winds. The observed Southern Oscillation appears to bea hybrid mode that changes from one episode to the next so that El Ninño can evolve in avariety of ways–advection and nonlocally generated thermocline displacements are importantto different degrees on different occassions. The extent to which random disturbances, such aswesterly wind bursts over the western equatorial Pacific, influence El Niño depends on whetherthe southern oscillation is self-sustaining or damped. Attention is now turning to the factorsthat determine this aspect of the Southern Oscillation, its decadal modulation which causes itto be more energetic in some decades than others. Those factors include interactions betweenthe tropics and extratropics that affect the mean depth of the thermocline, and the intensity ofthe climatological trade winds.