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 th e phenomenon El Nino. To initialize models, measurements are available from an impressive array of instruments that monitor the tropical Pacific conti nually; coupled general circulation models of the ocean and atmosphere are already capable of reproducing many aspects of the earth's climate, its sea sonal cycle, and the Southern Oscillation. These achievements crown the stu dies, over the past few decades, that describe, explain and simulate the at mospheric response to sea-surface temperature variations, the oceanic respo nse to different types of wind fluctuations, and the broad spectrum of coup led ocean-atmosphere modes that results from interactions between the two m edia. Those modes, which are involved, not only in the Southern Oscillation but also in the seasonal cycle and the climatology, differ primarily as re gards 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 o r, in the case of the delayed oscillator mode, by non-local winds. The obse rved Southern Oscillation appears to be a hybrid mode that changes from one episode to the next so that El Nino can evolve in a variety of ways - adve ction and nonlocally generated thermocline displacements are important to d ifferent degrees on different occassions. The extent to which random distur bances, such as westerly wind bursts over the western equatorial Pacific, i nfluence El Nino depends on whether the southern oscillation is self-sustai ning 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 in teractions between the tropics and extratropics that affect the mean depth of the thermocline, and the intensity of the climatological trade winds.