Processes controlling interannual variations in wintertime (Northeast Monsoon) primary productivity in the central Arabian Sea

Abstract

Three years of ocean color observations obtained by SeaWiFS reveal significant interannual variation in surface chlorophyll a (Chl a) concentrations in the central Arabian Sea during the Northeast (winter) Monsoon (NEM). Consistent with previous findings in the literature, no obvious relation to sea-surface temperature is apparent. A strong relationship with interannual variability in thermocline depth has been established using an interannually forced ocean general circulation model (OGCM). This relationship consists of reduced Chl a concentration associated with a deeper thermocline. Deeper winter convection is generally associated with higher nutrient concentrations and therefore higher phytoplankton biomass. Both in situ observations from the US JGOFS Arabian Sea Expedition and net wintertime nutrient entrainment estimated with the OGCM simulation indicate that mixed-layer concentrations are always sufficiently high to be non-limiting for phytoplankton growth. This raises two questions. What process(es) check phytoplankton growth? What leads to the observed relationship between deeper thermocline and reduced chl a concentration? A prominent feature of the NEM is a large-amplitude diurnal cycle of the mixed layer that is evident in moored temperature time-series. We hypothesize that the night-time penetration of this diurnal mixing, which is defined by the interannually varying thermocline depth, determines the magnitude of phytoplankton biomass that will be retained in the euphotic zone for the following photoperiod. This daily dilution acts to check the accumulation of phytoplankton biomass and prevents a full phytoplankton bloom. A simple 1-D model has been developed to quantify this process. An excellent correspondence exists between model-predicted mixed-layer Chl a concentration as it varies with thermocline depth and the similarly represented SeaWiFS observations.