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 United Kingdom Meteorological Office Globally Integrated Sea-Ice and Sea Surface Temperature data, when studied in terms of the four epochs 1900-1920, 1921-1931, 1942-1962, and 1963-1983, has indicated significant 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 investigate the dynamical response of the ocean to the observed interdecadal variability in the local winds and to various remote wind forcings. The potential thermodynamic effect of the winds on the surface heat flux is 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 the most prominent SST variability. When the observed Indian Ocean epoch winds are imposed, the dynamical response of the model leads to SST anomalies of the right sign as the observations but smaller in both magnitude and areal extent. When the magnitude of the mean Pacific winds is increased (decreased), the model responds dynamically by strengthening (weakening) the Indonesian throughflow, which then modulates the southern Indian gyre accordingly. The resulting SST anomalies in the southern Indian Ocean are larger and more widely distributed than those obtained with the local winds and bear greater resemblance to the observed patterns in this region. The results of this study suggest that modulations to the Indonesian throughflow can impact significantly on interdecadal variability in the southern Indian Ocean. Changes in the magnitude of the mean winds over the Pacific is one remote forcing mechanism demonstrated to modulate the Indonesian throughflow and hence the Indian Ocean.