Tropical instability waves (TIWs) have been linked to the barotropic instability related to the shears of ocean currents and the baroclinic instability associated with the sea surface temperature (SST) front immediately north of the equator. The relative importance of these two generation mechanisms may differ for TIWs at different regions of the tropical ocean. Recent satellite observations have also revealed that tropical instability waves (TIWs) are characterized by strong coupling between the atmosphere and ocean. The mechanisms that produce this ocean-atmosphere coupling are not fully understood. Two different hypotheses have been proposed. One of the hypotheses argues that sea surface temperature (SST) coupled with sea level pressure and change the surface wind. The other hypothesis suggests that surface winds vary in response to SST modification of atmospheric boundary layer stability. The characteristics of air-sea coupling associated with TIWs may vary from season to season and can be different between warm and cold ENSO (El Nino Southern Oscillation) years.

This study uses both coupled atmosphere-ocean GCM (CGCM) simulations and satellite observations to study the instability and ocean-atmosphere coupling mechanisms of TIWs. Our results indicate that the instability mechanisms of the northern branch of the TIWs is linked to the baroclinic shear of SST front to the north of the equator and that of the northern branch of TIWs is linked to the barotropic shear between the Equatorial Undercurrent and the northern branch of the South Equatorial Current. The co-variability of TIWs in the atmosphere suggests that the air-sea coupling mechanisms is more consistent with the boundary layer mixing mechanism of Wallace et al. (1989).