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) Although 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. (2) The region 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. (3) The densest water reaching the sea surface during coastal upwelling appeared to have maximum offshore depths of ~ 150 m and δ(θ)'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 δ(θ)'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 δ(θ) of 26.6 forms a widespread salinity maximum. Despite the considerable extent of this feature, Persian Gulf outflow water, with a salinity Institution of Oceanography; and Robert Masserini, Kevin Riskowitz, and Howard Rutherford from the University of South Florida. We also thank Mindy Black and Codie Codispoti for editorial assistance on this manuscript. The cooperation of the Officers and crew of the R/V Thomas G. Thompson during this long deployment was consistently outstanding. Seldom has a team of scientists benefitted from such a superlative and sustained effort. They are the unsung heros who provided the foundation that led to the success of the US JGOFS Arabian Sea Process Study. We also express our thanks to our Omani colleagues at the Marine Science and Fisheries Centre, Oman, for their support and hard work on our behalf, without which the JGOFS Arabian Sea Process Study could never succeeded. Finally, we thank the two anonymous reviewers for their many useful and insightful suggestions.