Physical control of primary productivity on a seasonal scale in central and eastern Arabian Sea

Using in situ data collected during 1992-1997, under the Indian programme o f Joint Global Ocean Flux Study (JGOFS), we show that the biological produc tivity of the Arabian Sea is tightly coupled to the physical forcing mediat ed through nutrient availability. The Arabian Sea becomes productive in sum mer not only along the coastal regions of Somalia, Arabia and southern part s of the west coast of India due to coastal upwelling but also in the open waters of the central region. The open waters in the north are fertilized b y a combination of divergence driven by cyclonic wind stress curl to the no rth of the Findlater Jet and lateral advection of nutrient-rich upwelled wa ters from Arabia. Productivity in the southern part of the central Arabian Sea, on the other hand, is driven by advection from the Somalia upwelling. Surface cooling and convection resulting from reduced solar radiation and i ncreased evaporation make the northern region productive in winter. During both spring and fall inter-monsoons, this sea remains warm and stratified w ith low production as surface waters are oligotrophic. Inter-annual variabi lity in physical forcing during winter resulted in one-and-a-half times hig her production in 1997 than in 1995.