CONTROL OF TROPICAL INSTABILITY WAVES IN THE PACIFIC

Westward-propagating waves with periods of 20-30 days and wavelengths of similar to 1,100km are a prominent feature of sea-surface temperatu res (SSTs) in the equatorial Pacific and Atlantic Oceans. They have be en attributed to instabilities due to current shear. We compare SST ob servations from the spaceborne Along Track Scanning Radiometer (ATSR) and TOGA-TAO moored buoys with SSTs from a model of the tropical Pacif ic forced with observed daily windstress data. The phases of the stron gest ''Tropical Instability Waves'' (TIWs) in the model are in closer correspondence with those observed than we would expect if these waves simply developed from infinitesimal disturbances (in which case their phases would be arbitrary). If we filter out the intraseasonal compon ent of the windstress, all phase-correspondence is lost. We conclude t hat the phases of these waves are not arbitrary, but partially determi ned by the intraseasonal winds. The subsurface evolution of the model suggests a possible control mechanism is through interaction with remo tely-forced sub surface Kelvin and Rossby waves. This is supported by an experiment which shows how zonal wind bursts in the west Pacific ca n modify the TIW field, but other mechanisms, such as local feedbacks, are also possible.