The land mass that blocks the Indian Ocean in the north at low latitudes profoundly influences the climate, circulation and biogeochemical processes of its northern basins. The asymmetrical distribution of freshwater and energy arising from the continental topography and monsoon dynamics produces contrasting hydrographic regimes in the NWIO and NEIO. The NWIO experiences a large excess of evaporation over precipitation and receives little river runoff. It is also affected by intense upwelling during the summer and convective mixing during the winter that enrich surface waters with essential nutrients, sustaining high rates of PP. In contrast, the NEIO is characterized by an excess of precipitation over evaporation, and since most of the South Asian rivers drain into this region, it is distinguished by a very strong thermohaline stratification of the upper water column. As a result, despite large terrestrial inputs of nutrients, PP is relatively low in the NEIO.