Seasonal variability of bio-optical and physical properties in the Arabian Sea: October 1994-October 1995

A mooring instrumented with optical and physical sensors within the upper 300 m was deployed for two consecutive 6-month periods (October 15, 1994 through October 20, 1995; sampling intervals of a few minutes) in the central Arabian Sea (15 degrees 30'N, 61 degrees 30'E). Both the Northeast Monsoon (November 1, 1994-February 15, 1995) and Southwest Monsoon (June 1-September 15, 1995) were observed. During the NE Monsoon, wind speeds averaged 6 ms(-1) and reached up to 15 ms(-1) during the SW Monsoon. Intermonsoon periods (Spring February 16-May 31, 1995; and Fall September 16-October 15, 1995) were characterized by weak and variable winds. Shortwave radiation and photosynthetically available radiation (PAR) displayed half-yearly cycles, peaking during the Intermonsoon periods. Two mixed-layer depth definitions have been used to describe our results. The first is based on a temperature difference of 0.1 degrees C of the surface temperature, MLD0.1 degrees C, and the second is based on a difference of 1.0 degrees C, MLD1.0 degrees C. The maximum Winter mixed layer depth (MLD1.0 degrees C similar to 110 m) was deeper than the Summer mixed layer (MLD1.0 degrees C similar to 80 m), primarily because of surface cooling and convection. A half-yearly cycle in chlorophyll a was evident with greater values occurring during each Monsoon and into the Intermonsoon periods. High chlorophyll a values associated with cool mesoscale features were also apparent during each Monsoon. These mesoscale features and others have been identified using remotely sensed sea-surface height anomaly maps.