Shifting of the convective heat source over the Indian Ocean region in relation to performance of monsoon: a satellite perspective

Abstract

The TRMM Microwave Imager (TMI) is relatively unaffected by the presence of clouds, so the TMI provides excellent data sets for sea surface temperature (SST), sea surface wind speed (SWS) and total precipitable water vapour (TWV) over the Indian Ocean basin during the summer monsoon regime. We show here that rainfall peak in the month of July during the Indian summer monsoon is related to the unexpected inter‐annual and intra‐seasonal variability of SST over the Arabian Sea (AS) and South Central Indian Ocean in the months of April and May. There was a shift of mean convective heat source (SST>30°C) from the Western Indian Ocean (WIO) to the Central Equatorial Indian Ocean in April during deficient rainfall years (2002 and 2004). In May this convective source moved north to the Arabian Sea in 2003, (AS)/Bay of Bengal (BB) in 1998, whereas it moved to the Bay of Bengal in 2002 and 2004. The years 2002 and 2004 were below normal monsoon years (drought) with total seasonal rainfall being 71 cm and 76 cm respectively, whereas 1998 and 2003 were above normal monsoon years with total seasonal rainfall being 92 cm and 90 cm respectively. The gradual onset of monsoon (early start, slow growth to full strength), as in the case of 2004 or multiple onset (early start, then a lull and resumption to full strength) as in the case of 2002 (Simon et al.2006), especially in the beginning of May and subsequent upwelling, led to Arabian Sea cooling. This also resulted in reducing the convection in subsequent months and influenced the overall circulation patterns over the Indian subcontinent. The vertical winds (NCEP) averaged over the Arabian Sea in May show there is convection and cooling instead of the normal subsidence (0.5 Pa/s) for the years 2002 and 2004. The wind circulation at 200 hPa in May 2002 and 2004 shows a shift of its ridge position from its normal position of 50° E to 60° E (10° longitude) in 2002 and 2004. This may be a response to the shift in the convective heat source, resulting in reduced movement of depression from the Bay of Bengal. The shifting of the convective source would have resulted in shifting of the Walker circulation pattern 10° east of its normal position.