Formation, maintenance and diurnal variability of subsurface chlorophyll maximum during the summer monsoon in the southern Bay of Bengal

Abstract

The vertical structure of chlorophyll in stratified oceanic waters exhibits a maximum substantially below the surface, known as the subsurface chlorophyll maximum (SCM). In the southern Bay of Bengal (BoB), a 12-day high-resolution time-series observation at 89°E and 8°N, during the 2016 summer monsoon showed that the SCM was located within the barrier layer (BL) and the euphotic layer. The strong BL (N2=3×10−4 s−2) located between 30 to 70 m, associated with a low salinity (<33.8 psu) water near the surface, maintained the stratification and the SCM. This SCM oscillated diurnally with an amplitude of 0.1 to 0.2 mg m−3. The lowest values of average chlorophyll in the SCM occurred around 06:00 hours (local time) and reached a maximum around 16:00 hours, four hours after the maximum in surface shortwave radiation. A coupled physical–biogeochemical one-dimensional model was used to examine the formation, maintenance and diurnal variability of the SCM in the southern BoB. The wind speed (<10 m s−1) that prevailed in the region during the weak phase of the monsoon was insufficient to break the BL and cause a surface bloom. The model results revealed that the surface shortwave radiation drove the diurnal cycle of the growth rate of phytoplankton at the SCM, hence the diurnal cycle of phytoplankton biomass and chlorophyll. The model’s control simulation showed the dominance of small phytoplankton, and its growth rate was limited by light and nutrients at the SCM. Whereas, in the ML, the phytoplankton growth rate was limited by nutrients. In the model, the regenerated production supported around 60% of the chl-a at the location of SCM. The subsurface high-salinity core, a prominent feature in the southern BoB, did not influence the SCM. This paper offers new insights into understanding the SCM formation, maintenance and diurnal variability in stratified tropical oceans.