Indo-Pacific Sea Surface Temperature Perturbations Associated with Intraseasonal Oscillations of the Tropical Convection

The intraseasonal variability (ISV; 20-90 days) of the SST is examined using 7 years of data from the Tropical Rainfall Measuring Mission's (TRMM) Microwave Imager (TMI). The ISV of the SST is maximal in the summer hemisphere where the average mixed layer depth (MLD) is relatively small. For most regions of this summer hemisphere, additional indices (like the reddening of the SST signal relative to the surface flux variability) show that the ISV of the SST is statistically controlled by the integration of local surface forcing (heat flux and wind-driven subsurface cooling) by a nearly passive mixed layer. The ISV of the SST is then studied in more detail using an adaptation of the Local Mode Analysis (LMA) that extracts SST and surface wind perturbations associated specifically with large-scale organized intraseasonal convective events. On the average, the perturbation of the surface wind tends to be maximal to the west of the convective perturbation (like for a Gill-type dynamical response), giving a primary role of the wind in the surface fluxes perturbation for region located to the west of the basin (northwest Arabian Sea and northwest Pacific Ocean). Locally, the wind perturbation (i.e. turbulent fluxes perturbation) generally lags the local convection (i.e. perturbation of surface solar flux) by less than 1/4 of period.

During boreal winter, the SST response to these large-scale organized intraseasonal convective events is large and recurrent over thin mixed layer regions of the Indian Ocean between 5°S and 10°S and North of Australia. By contrast, there is little variability of the SST linked to these organized events in the western Pacific. The eastward propagation of the convective perturbation is evident only over the Indian Basin. During boreal summer the SST response is maximal over regions of thin mixed layer located north of the Bay of Bengal, in the Arabian Sea and in the China Sea. Over the Bay of Bengal there is a northward propagation of the convective perturbation but more a standing oscillation between the equator and the north part of the Bay for the SST and the surface wind. The SST minimum generally occurs around 1/4 of period after the convective maximum and it lags the surface wind maximum by less than 1/8 of period. An intriguing result is a frequently small delay between the maximum surface wind and the minimum SST. Different explanations are invoked, like a rapid cooling due to the vanishing of warm layers as soon as the wind reinforces or abrupt wind strengthening (in regard to the time delay between two intraseasonal events) giving faster than expected cooling due to vertical mixing or surface heat fluxes.