Role of the ocean in formation of the Indian Monsoon low-pressure systems
Satish R. Shetye
National Institute of Oceanography Dona Paula, Goa 403 004, India
Atmospheric low-pressure systems (LPS), that include cyclones, depressions, and lows are an important component of the Indian summer monsoon (June-September). The LPS are important for precipitation over the central and northern parts of the Indian subcontinent. For their genesis, the LPS exhibit a marked preference for the Bay of Bengal, particularly its northern part. The formation of a LPS is believed to be triggered by a combined barotropic - baroclinic instability in the atmosphere. The growth of a LPS is sustained by large-scale atmospheric convection over the ocean. For the latter to occur, the SST has to exceed a critical value of about 27.5 C. The bay satisfies this condition most of the time during the summer monsoon. We examine the heat budget of the bay to understand how.
Three monthly climatologies - air-sea fluxes using monthly NCEP/NCAR data, Levitus ocean temperature, and ship-drifts - suggest that for the uppermost 50 m ocean layer north of 6 N the primary balance in the heat budget is between the net heat flux across the air-sea interface and accumulation of heat in the layer. Net radiation is the principal source of heat, and latent heat fluxes is the main sink. Such a heat balance is in striking contrast with that in the Arabian sea where entrainment at the bottom of the mixed-layer, upwelling along western boundary, and horizontal transport are significant.
It appears that in the bay, the effects of stronger near-surface stratification due to salinity variation, and weaker mean winds during summer lead to the following implications: (1) transfer of heat to deeper than approximately 50 m is not possible due to insufficient turbulent kinetic energy available to deepen the mixed layer; (2) mean winds during summer, though upwelling-favourable along the western boundary of the bay, are unable to induce much cooling because the upwelling is restricted to shallow depths; (3) ocean currents are too weak to remove any significant amounts of heat. As a result, the solar heat gained by the bay is removed primarily by transfer to the atmosphere.
Weaker mean winds in the bay are a consequence of the overall pattern of the winds during the summer monsoon. Strong vertical stratification is a consequence of the large influx of freshwater to the bay in the form of precipitation and runoff that result to a significant extent from atmospheric convection sustained by the warm SSTs. Thus there is, at least in part, an interesting positive feedback mechanism: warm SSTs lead to atmospheric convection and hence to influx of freshwater, which in turn helps sustenance of warm SSTs.
