EVIDENCE FOR THE INFLUENCE OF REMOTE FORCING ON INTERDECADAL VARIABILITY IN THE SOUTHERN INDIAN-OCEAN

Previous analysis of the Comprehensive Ocean-Atmosphere Data Set and U nited Kingdom Meteorological Office Globally Integrated Sea-Ice and Se a Surface Temperature data, when studied in terms of the four epochs 1 900-1920, 1921-1931, 1942-1962, and 1963-1983, has indicated significa nt interdecadal variability in austral summer sea surface temperature (SST) and atmospheric circulation patterns over the wider Indian Ocean region. A global ocean general circulation model is used to investiga te the dynamical response of the ocean to the observed interdecadal va riability in the local winds and to various remote wind forcings. The potential thermodynamic effect of the winds on the surface heat flux i s specifically excluded, Attention is focused on SST anomalies in the southern Indian Ocean, particularly the Agulhas Current, retroflection , and outflow zones, because this is where the observations exhibit th e most prominent SST variability. When the observed Indian Ocean epoch winds are imposed, the dynamical response of the model leads to SST a nomalies of the right sign as the observations but smaller in both mag nitude and areal extent. When the magnitude of the mean Pacific winds is increased (decreased), the model responds dynamically by strengthen ing (weakening) the Indonesian throughflow, which then modulates the s outhern Indian gyre accordingly. The resulting SST anomalies in the so uthern Indian Ocean are larger and more widely distributed than those obtained with the local winds and bear greater resemblance to the obse rved patterns in this region. The results of this study suggest that m odulations to the Indonesian throughflow can impact significantly on i nterdecadal variability in the southern Indian Ocean. Changes in the m agnitude of the mean winds over the Pacific is one remote forcing mech anism demonstrated to modulate the Indonesian throughflow and hence th e Indian Ocean.