Dynamical Response of the Oceanic Circulation and Temperature to Interdecadal Variability in the Surface Winds over the Indian Ocean

Abstract

A global ocean general circulation model (OGCM) is used to investigate the sensitivity of the circulation and temperature fields to observed interdecadal variability in Indian Ocean winds for the austral summer (January, February, March). Focus is placed on the dynamical response of the model to the imposed winds. These comprise the observed winds from COADS for the region 46°S?30°N, 17°?152°E organized into four 21-yr epochs(1900?1920,1921?1941, 1942?1962, and 1963?1983). During the first two epochs, the southern Indian anticyclone, African monsoonal flow, and associated trades were anomalously weak, whereas during the 1963?1983 period the reverse was true. The 1942?1962 epoch appears to be a transition between the 1921?1941 and 1963?1983 periods. The model indicates an overall decrease (increase) in the transports of the southern Indian and tropical Indian gyres for the 1900?1920, 1921?1941 (1963?1983) cases in dynamical response to the variability in the surface winds over the Indian Ocean. Sea surface temperature (SST) perturbations in the southern Indian Ocean have the same sign as the observed anomalies but are smaller in magnitude (0.05°?0.1°C model, 0.1°?0.3°C observed). The model SST patterns are restricted to the southern Indian Ocean midlatitudes, occurring particularly in the region of the Agulhas retrofiection and outflow zones, whereas observations indicate anomalies throughout the Indian Ocean basin. Analysis of the streamfunction anomalies induced by the epoch winds in the model indicates that the JEBAR term is important in modulating the Indian gyre transports. While it is noted that thermodynamic effects not explicitly included in the model may contribute toward the observed SST variability in certain regions and that previous model studies have shown that SST in the southern Indian Ocean is sensitive to variations in the Indonesian throughflow and the Pacific trade winds, the results lend support to the hypothesis that changes in the basin-scale ocean circulation driven by the Indian Ocean epoch winds may contribute significantly toward the observed interdecadal variability in SST in the southern regions of this ocean.