Processes governing the seasonality of vertical chlorophyll-a distribution in the central Arabian Sea: Bio-Argo observations and ecosystem model simulation

Seasonal variability of the vertical structure of chlorophyll-a (chl-a) in the central Arabian Sea is described using seasonal climatology from bio-Argo floats. A quarter-degree resolution coupled OGCM-ecosystem model is employed to explain the physical and biogeochemical processes that determine the observed seasonality of the chl-a profiles. The most prominent feature of the chl-a is that seasonal surface bloom (chl-a > 0.25 mg m−3) occurs during the winter (November–February) and summer (June–September) monsoons. A sub-surface maximum (SCM) in chl-a occurs at a depth of about 60 m during the spring (March–May) and fall (October) oligotrophy. The SCM is absent during the peak of winter and summer bloom. The model simulated the observed seasonal evolution of chl-a and physical variables. Nitrate budget analysis, using the model simulation, reveals that vertical entrainment of nutrients plays a vital role in supplying nutrients into the surface mixed layer (ML) during the summer forced by wind mixing and winter due to convective cooling at the surface. Mixing is critical than Ekman pumping during the summer and winter monsoons. Further, the horizontal and vertical advection of nitrate is weak. The model showed the dominance of small phytoplankton in the central Arabian Sea. Analysis of the growth rate equation in the model shows that during the seasons of bloom, nutrients are surplus in the ML, and the euphotic depth that is shallower than the mixed layer depth drives the vertical structure of chl-a. The limiting nutrient above the euphotic depth during the winter (summer) is iron (phosphate). During oligotrophy, the growth rate of phytoplankton in the ML (SCM region) is determined by nitrogen (iron) limitation.