Seasonal variation of hydrographic and nutrient fields during the US JGOFSArabian Sea Process Study

Between September 1994 and December 1995, the US JGOFS Arabian Sea Process Experiment collected extensive, high quality hydrographic data (temperature , salinity, dissolved oxygen and nutrients) during all seasons in the north ern Arabian Sea. An analysis of this unique data suite suggests the presenc e of many features that are described in the canonical literature, but thes e new data provided the following insights. (1) Although the seasonal evolution of mixed-layer depths was in general ag reement with previous descriptions, the deepest mixed-layer depths in our d ata occurred during the late NE Monsoon instead of the SW Monsoon. (2) The region exhibits considerable mesoscale variability resulting in ext remely variable temperature-salinity (TS) distributions in the upper 1000 d b. This mesoscale variability is readily observed in satellite imaging, in the high resolution data taken by a companion ONR funded project, and in un derway ADCP data. (3) The densest water reaching the sea surface during coastal upwelling app eared to have maximum offshore depths of similar to 150 m and sigma(theta)' s close to the core value( similar to 25) for the saline Arabian Sea Water (ASW), but salinities in these upwelling waters were relatively low. The de nsest water found at the sea surface during late NE Monsoon conditions has sigma(theta)'s > 24.8 and relatively high salinities, suggesting that they are a source for the ASW salinity maximum. (4) Persian Gulf Water (PGW) with a core sigma(theta) of 26.6 forms a wides pread salinity maximum. Despite the considerable extent of this feature, Pe rsian Gulf outflow water, with a salinity of similar to 39 at its source, c an only be a minor contributor. Within the standard US JGOFS sampling grid, maximum salinities on this surface are similar to 36.8 at stations near th e Gulf, falling to values as low as similar to 35.3 at the stations farthes t removed from its influence. Even at our standard stations closest to the Gulf (N-1 and N-2), the high-salinity, low-nutrient Persian Gulf water has only a modest direct effect on nutrient concentrations. This PGW salinity m aximum is associated with the suboxic portions of the Arabian Sea's oxygen minimum zone. (5) The salinity maximum associated with Red Sea Water (RSW, core sigma(the ta) = 27.2) in the JGOFS study region is clearly evident at the southermost sampling site at 10 degrees N (S-15). Elsewhere, this signal is weak or ab sent and salinity on the 27.2 sigma(theta) surface tends to increase toward s the Persian Gulf, suggesting that the disappearance of this salinity maxi mum is due, at least in part, to the influence of the Persian Gulf outflow. (6) Inorganic nitrogen-to-phosphate ratios were lower (frequently much lowe r) than the standard Redfield ratio of 15/1-16/1 (by atoms) at all times an d all depths suggesting that inorganic nitrogen was more important than pho sphate as a limiting nutrient for phytoplankton growth, and that the effect s of denitrification dominated the effects of nitrogen fixation. (7) The water upwelling off the Omani coast during the SW Monsoon has inorg anic nitrogen to silicate ratios that were higher (similar to 2/1) than the similar to 1/1 ratio often assumed as the ratio of uptake during diatom gr owth. (8) The temporal evolution of inorganic nitrogen-to-silicate ratios suggest s major alteration by diatom uptake only during the late SW Monsoon cruise (TN050) in August-September 1995. (9) Widespread moderate surface layer nutrient concentrations occurred duri ng the late NE Monsoon. (10) A zone of high offshore nutrient concentrations was encountered during the SW Monsoon, but instead of being associated with offshore upwelling it may represent offshore advection from the coastal upwelling zone, the infl uence of an eddy, or both. (11) Although our data do not contradict previous suggestions that the volu me of subtoxic water may be reduced the SW Monsoon, they suggest a weaker r e-oxygenation than indicated by some previous work. Similarly, they do not confirm results suggesting that secondary nitrite maxima may be common in w aters with oxygen concentrations >5 mu M. (C) 1998 Elsevier Science Ltd. Al l rights reserved.