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

A mooring instrumented with optical and physical sensors within the upper 3 00 m was deployed for two consecutive 6-month periods (October 15, 1994 thr ough 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-Septembe r 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 period s (Spring February 16-May 31, 1995; and Fall September 16-October 15, 1995) were characterized by weak and variable winds. Shortwave radiation and pho tosynthetically available radiation (PAR) displayed half-yearly cycles, pea king during the Intermonsoon periods. Two mixed-layer depth definitions hav e been used to describe our results. The first is based on a temperature di fference 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. Th e 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), prim arily because of surface cooling and convection. A half-yearly cycle in chl orophyll a was evident with greater values occurring during each Monsoon an d into the Intermonsoon periods. High chlorophyll a values associated with cool mesoscale features were also apparent during each Monsoon. These mesos cale features and others have been identified using remotely sensed sea-sur face height anomaly maps. Time-series of the 1% light level depth, h(1%), tracked the depth-integrate d chlorophyll a. In general, h(1%) was deeper than MLD1.0 degrees C during the latter half of the Spring Intermonsoon (SIM) (with low chlorophyll a pe riods) and shallower than MLD1.0 degrees C during the latter portions of th e Monsoons (high chlorophyll a periods). During the SIM, the penetrative co mponents of net solar radiation at the base of the mixed layers, E-n(MLD1.0 degrees C) and E-c(MLD0.1 degrees C), reached values of similar to 40 and 75 W m(-2), respectively, when the net surface heat flux was 120 W m-2. The highest mixed layer radiant heating rates occurred during the Intermonsoon periods with peak values greater than 0.2 and 0.1 degrees C d(-1) for MLD0 .1 degrees C and MLD1(.0 degrees C) respectively. Our results indicate that biological variability is significant for the upper ocean heat budget of t he central Arabian Sea during the SIM. The present results, in conjunction with those presented in Marra et al. (1 998) and Honjo et al. (1998), demonstrate strong coupling of upper ocean bi ological processes with deep ocean particulate organic carbon fluxes. These collective results suggest that the timing and amplitudes of phytoplankton blooms associated with both seasonal stratification and eddies are quite w ell-correlated with relatively rapid export flux of organic carbon to the d eep ocean. (C) 1998 Elsevier Science Ltd. All rights reserved.