The Arabian Sea experiences extremes in atmospheric forcing that lead to the greatest seasonal variability in any ocean. During 1995, 6 cruises were carried out within the northern Arabian Sea. The data collected represent the first consistent dataset covering an entire monsoonal cycle. Ocean soundings of temperature and salinity from these cruises are used to characterize variability of water masses in the surface layers associated with monsoonal forcing. This paper documents the role of advection versus local water mass modification on seasonal extremes in T-S in near-surface waters of the Arabian Sea. Seasonal changes are associated with seasonal heating, advection of waters from the Oman upwelling zone, varying of mixed-layer depth due to Ekman pumping in the central basin and mixing of high-salinity waters from Persian Gulf.¿ 1997 American Geophysical Union