Coupled GCM Simulation of Atmosphere–Ocean Variability Associated with Zonally Asymmetric SST Changes in the Tropical Indian Ocean

Abstract

The nature of a recurrent pattern of variability in the tropical Indian Ocean (IO) during the boreal autumn has been investigated using a 900-yr experiment with a coupled atmosphere?ocean general circulation model. This Indian Ocean Pattern (IOP) is characterized by zonal surface wind perturbations along the equator, as well as east?west contrasts in the anomalous sea surface temperature (SST), surface pressure, and precipitation fields. The IOP is seen to be linked to the El Niño?Southern Oscillation (ENSO) phenomenon in the tropical Pacific. By constructing composite charts and analyzing the heat budget for the top ocean layer, it is illustrated that the ENSO-related changes in the surface wind modify the intensity of oceanic upwelling, horizontal temperature advection, and surface heat fluxes in various parts of the IO basin. These processes lead to SST perturbations with opposite signs in the eastern and western equatorial IO. Further diagnosis of the model output reveals that some strong IOP episodes occur even in the near absence of ENSO influences. In such IOP events that do not coincide with prominent ENSO development, the most noteworthy signal is a zonally elongated sea level pressure anomaly situated south of Australia during the southern winter. The anomalous atmospheric circulation on the equatorward flank of this feature contributes to the initiation of IOP-like events when the ENSO forcing is weak. Both simulated and observational data show that the pressure anomaly south of Australia is part of a hemisphere-wide pattern bearing a considerable resemblance to the Antarctic Oscillation. This annular mode of variability is characterized by opposite pressure changes in the midlatitude and polar zones, and is only weakly correlated with ENSO. The findings reported here indicate that the IOP is attributable to multiple factors, including remote influences due to ENSO and extratropical changes, as well as internal air?sea feedbacks occurring within the IO basin.