Physical forcing of biological productivity in the northern Arabian Sea during the Northeast Monsoon

Time-series observations at a nominally fixed location in the northern Arab ian Sea (21 degreesN, 64 degreesE) during the Northeast Monsoon (winter, Fe bruary) of 1997 showed the prevalence of cold sea-surface temperatures (SST ) and deep mixed layers resulting from winter cooling and convection. The c ovariation of nitrate concentrations in the surface layers and concentratio ns of chlorophyll a and primary production in the euphotic zone with mixed- layer depth (MLD) and wind suggests that carbon fixation was controlled pri marily by physical forcing. Cooler waters during winter 1997 relative to wi nter 1995 were associated with deeper MLDs, higher nitrate concentrations, elevated primary productivity, and higher chlorophyll a concentrations, lea ding to the inference that even a 1 degreesC decrease in SST could lead to significantly higher primary productivity. Satellite data on sea surface te mperature (advanced very high-resolution radiometer; AVHRR) and TOPEX/POSE1 -DON altimeter data suggest that this interannual variation is of basin-wid e spatial scale. After the termination of winter cooling and subsequent war ming during the Spring Intermonsoon, the Arabian Sea has low primary produc tion. During the latter period, micro-organisms, i.e. heterotrophic bacteri a and microzooplankton)-proliferate, a feeding mode through the microbial l oop that appears to be inherent to mesozooplankton for sustaining their bio mass throughout the year in this region. (C) 2001 Elsevier Science Ltd. All rights reserved.