Observations show that a large number of wintertime MJO is initiated in the southwestern tropical Indian Ocean, where surface mixed layer is thin, thermocline is shallow, and intraseasonal variability of sea surface temperature (SST) obtains its largest amplitude. This region is often referred to as the Thermocline Ridge of the Indian Ocean (TRIO). Because SST is a direct variable for air-sea interaction, processes that determine intraseasonal SST variability in TRIO region are investigated by analyzing available data and performing a suite of experiments using an ocean general circulation model for the period of 1998-2004. Impacts of the 30-90 day and 10-30 day atmospheric intraseasonal oscillations (ISOs) are examined separately, with the former dominated by the Madden-Julian oscillation (MJO), and the latter by convectively coupled Rossby and Kelvin waves.

The maximum variation of intraseasonal SST occurs in 10°S-2°S TRIO region, where the wintertime Intertropical Convergence Zone (ITCZ) is located. The observed maximum warming (cooling) averaged over (60°E-85°E, 10°S-3°S) is 1.13°C (-0.97°C) for the period of interest, with a standard deviation of 0.39°C in winter. This SST change is forced predominantly by the MJO. While the MJO causes a basin-wide cooling (warming) in the ITCZ region, submonthly ISOs cause a more complex SST structure that propagates southwestward in the western-central basin and southeastward in the eastern ocean. On both the MJO and submonthly time scales, winds are the deterministic factor for the SST variability. Short wave radiation generally plays a secondary role and effects of precipitation are negligible. The dominant role of winds results roughly equally from surface heat flux plus entrainment and from oceanic upwelling associated with Ekman pumping velocity and Rossby waves. Based on these results, we speculate that "oceanic processes", such as upwelling and Rossby waves, may play some role in determining intraseasonal SST variability and thus contributing to the wintertime MJO initiation.