A coupled, physical-biological model is used to study the processes that determine the annual cycle of biological activity in the Arabian Sea. The physical model is a 2.5-layer system with a surface mixed layer imbedded in the upper layer, and fluid is allowed to move between layers via entrainment, detrainment and mixing processes. It is forced by climatological wind-stress, air-temperature, specific-humidity and scalar-wind fields, the latter three being necessary to determine the surface heat flux. The biological model consists of a set of advective-diffusive equations in each layer that determine the nitrogen concentrations in four compartments: nutrients, phytoplankton, zooplankton and detritus. It is forced by photosynthetically active radiation, defined to be a constant fraction of the incoming shortwave radiation. The source of nitrogen for the system is upward diffusion of nutrients into the lower layer, and there is a compensating loss of nitrogen via the sinking of detritus into the deep ocean. Coupling between the two models is provided by the horizontal-velocity, layer-thickness, entrainment and detrainment fields from the physical solution. Our main-run solution compares favorably with observed physical and biological fields; in particular, the model is able to simulate the prominent phytoplankton blooms visible in the CZCS data.

Blooms develop in the model in response to three, dynamically distinct, physical processes: upwelling, entrainment and detrainment blooms. Upwelling blooms occur when nutrients are upwelled into the surface layer either by alongshore winds adjacent to coasts or by positive Ekman pumping in the interior ocean; they are long-lasting events that exist as long as the upwelling continues. Entrainment blooms result when nutrients are entrained into the mixed layer during periods of turbulence generated by wind stirring or surface cooling; they are weaker than upwelling blooms because the mixed layer thickens during entrainment, thereby decreasing the depth-averaged light intensity necessary for phytoplankton growth. Detrainment blooms occur when the depth of the mixed layer shallows due to weakened winds or surface warming, thereby increasing the depth-averaged light intensity. Since detrainment does not inject new nutrients into the mixed layer, these blooms are short-lived events that last only until the initial nutrient supply is consumed.

The dynamics of the biological model are isolated in a suite of diagnostic calculations and test solutions. Some results from these analyses are the following. Entrainment is the primary nutrient source for the bloom that takes place in the central Arabian Sea during the late summer and fall, but advection and recycling also contribute significantly. Detrital remineralization is an essential process: in a test solution without remineralization, nutrient levels drop markedly in every layer of the model and all the blooms are severely weakened. The primary causes of bloom decay are nutrient limitation for upwelling and detrainment blooms and light limitation for entrainment blooms. Zooplankton grazing limits phytoplankton concentrations both during blooms and during oligotrophic periods. In contrast, senescence has little effect on phytoplankton concentrations in the main run: in a test solution without senescence, its lack is almost completely compensated for by increased grazing.

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