Influences of diurnal and intraseasonal forcing on mixed‐layer and biological variability in the central Arabian Sea

Abstract

A three‐dimensional, physical‐biological model of the Indian Ocean is used to study the influences of diurnal and intraseasonal forcing on mixed‐layer and biological variability in the central Arabian Sea, where a mooring was deployed and maintained from October 1994 to October 1995 by the Woods Hole Oceanographic Institution Upper Ocean Processes group. The physical model consists of four active layers overlying an inert deep ocean, namely, a surface mixed layer of thickness h1, diurnal thermocline layer, seasonal thermocline, and main thermocline. The biological model consists of a set of advective‐diffusive equations in each layer that determine nitrogen concentrations in four compartments: nutrients, phytoplankton, zooplankton, and detritus. Both monthly climatological and “daily” fields are used to force solutions, the latter being a blend of daily‐averaged fields measured at the mooring site and other products that include intraseasonal forcing. Diurnal forcing is included by allowing the incoming solar radiation to have a daily cycle. In solutions forced by climatological fields, h1 thickens steadily throughout both monsoons. When h1 detrains at their ends, short‐lived, intense blooms develop (the model's spring and fall blooms) owing to the increase in depth‐averaged light intensity sensed by the phytoplankton in layer 1. In solutions forced by daily fields, h1 thins in a series of events associated with monsoon break periods. As a result, the spring and fall blooms are split into a series of detrainment blooms, broadening them considerably. Diurnal forcing alters the mixed‐layer and biological responses, among other things, by lengthening the time that h1 is thick during the northeast monsoon, by strengthening the spring and fall blooms and delaying them by 3 weeks, and by intensifying phytoplankton levels during intermonsoon periods. Solutions are compared with the mixed‐layer thickness, phytoplankton biomass, and phytoplankton production fields estimated from mooring observations. The solution driven by daily fields with diurnal forcing reproduces the observed fields most faithfully.