Seasonal variation of hydrographic and nutrient fields during the US JGOFS Arabian 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 northern Arabian Sea. An analysis of this unique data suite suggests the presence of many features that are described in the canonical literature, but these new data provided the following insights.1. gh the seasonal evolution of mixed-layer depths was in general agreement with previous descriptions, the deepest mixed-layer depths in our data occurred during the late NE Monsoon instead of the SW Monsoon. gion exhibits considerable mesoscale variability resulting in extremely variable temperature-salinity (TS) distributions in the upper 1000 db. This mesoscale variability is readily observed in satellite imaging, in the high resolution data taken by a companion ONR funded project, and in underway ADCP data. nsest water reaching the sea surface during coastal upwelling appeared to have maximum offshore depths of ∼150 m and σθ’s close to the core value (∼25) for the saline Arabian Sea Water (ASW), but salinities in these upwelling waters were relatively low. The densest water found at the sea surface during late NE Monsoon conditions has σθ’s>24.8 and relatively high salinities, suggesting that they are a source for the ASW salinity maximum. n Gulf Water (PGW) with a core σθ of 26.6 forms a widespread salinity maximum. Despite the considerable extent of this feature, Persian Gulf outflow water, with a salinity (4) of ∼39 at its source, can only be a minor contributor. Within the standard US JGOFS sampling grid, maximum salinities on this surface are ∼36.8 at stations near the Gulf, falling to values as low as ∼35.3 at the stations farthest 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 maximum is associated with the suboxic portions of the Arabian Sea’s oxygen minimum zone. linity maximum associated with Red Sea Water (RSW, core σθ=27.2) in the JGOFS study region is clearly evident at the southermost sampling site at 10′N (S-15). Elsewhere, this signal is weak or absent and salinity on the 27.2 σθ surface tends to increase towards the Persian Gulf, suggesting that the disappearance of this salinity maximum is due, at least in part, to the influence of the Persian Gulf outflow. nic nitrogen-to-phosphate ratios were lower (frequently much lower) than the standard Redfield ratio of 15/1–16/1 (by atoms) at all times and all depths suggesting that inorganic nitrogen was more important than phosphate as a limiting nutrient for phytoplankton growth, and that the effects of denitrification dominated the effects of nitrogen fixation. ter upwelling off the Omani coast during the SW Monsoon has inorganic nitrogen to silicate ratios that were higher (∼2/1) than the ∼1/1 ratio often assumed as the ratio of uptake during diatom growth. mporal evolution of inorganic nitrogen-to-silicate ratios suggests major alteration by diatom uptake only during the late SW Monsoon cruise (TN050) in August–September 1995. read moderate surface layer nutrient concentrations occurred during the late NE Monsoon. 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 influence of an eddy, or both. gh our data do not contradict previous suggestions that the volume of subtoxic water may be reduced the SW Monsoon, they suggest a weaker re-oxygenation than indicated by some previous work. Similarly, they do not confirm results suggesting that secondary nitrite maxima may be common in waters with oxygen concentrations >5 μM.