Winter cooling in the northern Arabian Sea

S. Prasanna Kumar and T. G. Prasad

Physical Oceanography Division, National Institute of Oceanography, Dona Paula, Goa 403 004, India

The upper thermo-haline structure and the surface meteorological parameters of the central and eastern Arabian Sea during the inter-monsoon (April-May, 1994) and winter monsoon (February-March, 1995) periods, were analysed to understand physical forcing that leads to the observed high productivity during winter in the northern Arabian Sea. The weak northerly winds and increased solar insulation during the inter-monsoon period, led to the development of a highly stratified upper layer with warm sea surface temperature (SST) (29.5^oC) and shallow mixed layer depth (MLD) (25 m). In contrast, during winter, the upper thermo-haline field showed a dramatic change, with cold SST (25^oC) and deep MLD (100 m) in the north, though the winds were weak (5 ms^-1). The atmospheric forcing that leads to the observed changes in the upper layer of the ocean is a combination of enhanced evaporation under the influence of the dry continental air from the north brought by the prevailing northeasterly trades and reduction in the solar insulation. Subsequent cooling and convective mixing injects nutrients into the surface layers from the thermocline region, which in turn triggers the primary production.