Revisiting the Processes That Determine Wintertime Intraseasonal SST Variability in the Thermocline Ridge of the Tropical South Indian Ocean

Abstract:

Intraseasonal sea surface temperature (SST) variability over the Seychelles–Chagos thermocline ridge (SCTR) induced by boreal wintertime Madden–Julian oscillations (MJOs) is investigated by performing a series of OGCM experiments with improved model configuration and the recently available high quality satellite forcing fields. The impact of the ocean interannual variation of the thermocline depth –represented by the depth of 20C isotherm (D20) - in the SCTR is also assessed. The OGCM main run solution agrees well with the observations.

The results show that for the 2001-2011 period, surface shortwave radiation (SWR), turbulent heat fluxes associated with wind speed, and wind stress–driven ocean dynamical processes are all important in causing the MJO-related intraseasonal SST variability in the SCTR region. Overall, forcing by SWR contributes ~31%, and forcing by winds (via both surface turbulent heat flux and ocean dynamics) contributes ~62%. The contribution of turbulent heat flux associated with wind speed is ~39% and that of wind-stress driven ocean dynamics is ~23%. The contribution of ocean dynamics, however, is considerably larger during strong (“prime”) MJO events under “strong” thermocline condition. The overall effect of interannual variability of D20 on intraseasonal SST during 2001-2011 is significant in the eastern part of the SCTR (70E–85E), where the intraseasonal SST amplitudes are strengthened by about 20%. In general, a shallower/deeper SCTR favors larger/smaller SST responses to the MJO forcing.

In the eastern SCTR, both the heat flux forcing and entrainment are greatly amplified under the strong SCTR condition, but only slightly suppressed under the weak SCTR condition, leading to an overall strengthening effect on intraseasonal SST variability.