The Northeast Monsoon's impact on mixing, phytoplankton biomass and nutrient cycling in the Arabian Sea

In the northern Arabian Sea, atmospheric conditions during the Northeast (w inter) Monsoon lead to deep convective mixing. Due to the proximity of the permanent pycnocline to the sea surface, this mixing does not penetrate bel ow 125 m. However, a strong nitracline is also present and the deep convect ion results in significant nitrate flux into the surface waters. This leads to nitrate concentrations over the upper 100 m that exceed 4 mu M toward t he end of the monsoon. During the 1994/1995 US JGOFS/Arabian Sea expedition , the mean areal gross primary production over two successive Northeast Mon soons was determined to be 1.35 gC/m(2)/d. Thus, despite the deep penetrati ve convection, high rates of primary productivity were maintained. An inter disciplinary model was developed to elucidate the biogeochemical processes involved in supporting the elevated productivity. This model consisted of a 1-D mixed-layer model coupled to a set of equations that tracked phytoplan kton growth and the concentration of the two major nutrients (nitrate and a mmonium). Zooplankton grazing was parameterized by a rate constant determin ed by shipboard experiments. Model boundary conditions consist of meteorolo gical time-series measured from the surface buoy that was part of the ONR A rabian Sea Experiment's central mooring. Our numerical experiments show tha t elevated surface evaporation, and the associated salinization of the mixe d layer, strongly contributes to the frequency and penetration depth of the observed convective mixing. Cooler surface temperatures. increased nitrate entrainment, reduced water column stratification, and lower near-surface c hlorophyll a concentrations all result from this enhanced mixing. The model also captured a dependence on regenerated nitrogen observed in nutrient up take experiments pet-formed during the Northeast Monsoon. Our numerical exp eriments also indicate that variability in mean pycnocline depth causes up to a 25% reduction in areal chlorophyll a concentration. We hypothesize tha t such shifts in pycnocline depth may contribute to the interannual variati ons in primary production and surface chlorophyll a concentration that have been previously observed in this region. (C) 2000 Elsevier Science Ltd. Al l rights reserved.