THE Arabian Sea is one of the most biologically productive ocean regions’, mainly due to the upwelling of nutrients during the summer (southwest) monsoon. But the northern Arabian Sea continues to sustain fairly high biological production after the upwelling season and during much of the winter (northeast) monsoon(2-4). The processes that enable this high winter productivity have hitherto been poorly understood, being variously attributed to surface cooling effects(2,3) or wind-driven changes in ocean circulation(4). Here we present physical, chemical and biological data that indicate that sea surface cooling drives convection processes that lead to the injection of nutrients up into the surface waters of the northeastern Arabian Sea during winter, and that this mechanism of nutrient supply is a dominant control on winter productivity. Observed seasonal changes in bacterial and microzooplankton populations may provide an explanation for the Arabian Sea ’paradox’(5-8) that mesozooplankton biomass remains more or less invariable throughout the year.