Ocean Mixed Layer Temperature Variations Induced by Intraseasonal Convective Perturbations over the Indian Ocean

Abstract

In situ and satellite observations reveal that the tropical intraseasonal oscillation is occasionally associated with large variations in sea surface temperature (SST). The purpose of this paper is to find the physical origin of such strong SST perturbations (up to 3 K) over the Indian Ocean by examining two intraseasonal events in January and March 1999. Analysis of SST data from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and from drifting buoys reveals that these two intraseasonal events deeply modify the SST field between the equator and 10°S, while the surface flux perturbation extends over a wide area of the tropical Indian Ocean. Forced ocean general circulation model (OGCM) simulations are successful in reproducing the spatial patterns of this intraseasonal SST variability albeit with a weaker amplitude. The weaker amplitude given by the OGCM is partly related to the absence of warm-layer formation in the model. The model simulation reveals that the background oceanic subsurface structure explains the observed latitudinal distribution of the SST perturbations. For the Indian Ocean, the Ekman pumping (reinforced in 1999 due to La Niña conditions) gives a thermocline close to the surface between 5° and 10°S that inhibits the deepening of the mixed layer during strong wind episodes and thus gives a mixed layer temperature more reactive to surface forcing. Other factors like the Ekman dynamics associated with the wind burst and the precipitation perturbation south of the equator also contribute toward preventing the deepening of the mixed layer. For these regions, as is found over the western Pacific, the intraseasonal variability of the SST is mainly driven by the surface fluxes perturbation, and not by advection or exchanges with the subsurface. As a consequence, the phasing and the magnitude of convective and large-scale dynamical perturbations of the surface fluxes, which are regionally dependent, are also determinant factors for the local amplitude of the SST perturbation. Finally, results show a relation at interannual time scales between the thermocline structure and the mixed layer depth south of the equator that may have consequences on interannual changes in the intraseasonal activity over the Indian Ocean.