Deep-Sea Research II 45 (1998) 2053 — 2101 Seasonal variation of hydrographic and nutrient fields during the US JGOFS Arabian Sea Process Study J.M. Morrison*, L.A. Codispoti, S. Gaurin, B. Jones, V. Manghnani, Z. Zheng Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC 28695-8208, USA Center for Coastal Physical Oceanography, Crittenton Hall, Old Dominion University, Norfolk, VA 23529, USA Department of Biological Sciences, Allen Hancock Foundation, University of Southern California, Los Angeles, CA 90089, USA Received 5 September 1997; received in revised form 14 May 1998; accepted 18 May 1998 Abstract Between September 1994 and December 1995, the US JGOFS Arabian Sea Process Ex- periment 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   ’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   ’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   of 26.6 forms a widespread salinity maximum. Despite the considerable extent of this feature, Persian Gulf outflow water, with a salinity *Corresponding author. Fax: 001 99 515 7802; e-mail: john — morrison@ncsu.edu. 0967-0645/98/$ — see front matter  1998 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 9 8 ) 0 0 0 6 3 - 0