Kumar, S. Prasanna, Madhupratap and Dileep Kumar

National Institute of Oceanography, Dona Paula, Goa-403 004, India, Tel: 91-832-2456700 ext. 4300, Fax: 91-832-2456701, E-mail: prasanna@csnio.ren.nic.in

How tightly is physical forcing coupled to biological production in the Arabian Sea?

Arabian Sea is a tropical basin, land locked in the north and forced by seasonally reversing monsoon winds. Being a tropical basin where sunlight is usually not a limiting factor, the biological production is constrained by the availability of nutrients. Using in situ data collected under Indian JGOFS program during 1992-1997, we show how the seasonally altering physical forcing brings about a spectrum of changes in the water column nutrients and primary productivity, switching from oligotrophic-less productive regime to eutrophic-high productive regime. During summer monsoon (June-September), upwelling occurring along the continental margins of Somalia, Arabia and along the southwest coast of India makes the coastal regions biologically productive, while open ocean upwelling, wind-mixing and advection from the Somalia and Arabian upwelling fertilizes the open Arabian Sea. In winter (November-February), cooling driven by reduced solar insolation and enhanced evaporation leads to convection. This in turn injects nutrients from the upper nutricline into the surface layer and triggers biological production. During both spring and fall inter-monsoon seasons, Arabian Sea remains warm, stratified and oligotrophic with low biological production. The above sequential changes within the upper ocean demonstrate the tight coupling between the physical forcing and biological productivity in the Arabian Sea. These findings help to explain several other processes such as the low oxygen content at intermediate depths of the open Arabian Sea, the dilemma that denitrification processes which are decoupled from productive coastal waters, and that high particle fluxes are observed in central regions.