Seasonal variability of the mixed layer in the central Arabian Sea and its implication on nutrients and primary productivity

The upper layers of the ocean play an important role in the regulation of the ocean–atmosphere as a coupled system. The Arabian Sea, situated in the western part of the northern Indian Ocean, participates in the intense air–sea exchange processes associated with the semi-annually reversing monsoons. During winter monsoon the weak northeast trade wind brings cool, dry continental air, while during the summer monsoon the strong southwest wind brings humid maritime air into the Arabian Sea. It is natural to expect that these semi-annual atmospheric forcings would modulate the thickness of the upper ocean. The seasonal variability of the mixed layer in the central Arabian Sea (equator to 25°N and 62–66°E) was studied using hydrographic data from NODC and INODC, while the atmospheric forcing responsible for the observed changes were explored using meteorological data from COADS and FSU winds. The mixed layer depth (MLD) and sea-surface temperature (SST) in the central Arabian Sea, to a large extent, were regulated by the incoming solar radiation and wind forcing. The Ekman dynamics associated with the Findlater Jet controlled the mixed layer depth during summer monsoon, while in winter the cooling and convective mixing regulated the MLD. We speculate that the deep mixed layer during February–March in the southern region is associated with that of the Rossby wave propagation. During spring and fall intermonsoons, the weak winds along with strong incoming solar radiation lead to the formation of a thin, warm and stratified mixed layer. We show that biological productivity of the Arabian Sea is rigidly coupled to the seasonal cycle of MLD, which regulates the nutrient levels in the euphotic zone.