Radiant heating rate associated with chlorophyll dynamics in upper ocean of Southern Bay of Bengal: A case study during Bay of Bengal Boundary Layer Experiment

Abstract

A 11-day time series of optical, physical and biological observations were collected from the southern Bay of Bengal during the summer monsoon of 2016 as a part Bay of Bengal Boundary Layer Experiment (BoBBLE). These observations were used to estimate Radiative Heating Rate (RHR) and to further understand its dependence on biological and physical variables. Initially, the water column was characterized by an oligotrophic condition with weak surface chlorophyll (chl-a) concentration (<0.1 mg m-3). During this period, the sky was clear, mixed layer was shallow and RHR reached up to 0.14 °C hr-1. Subsequently, the mixed layer deepened (60–70 m) and the surface chl-a concentration increased to 0.4 mg m-3. However, the sky was relatively cloudy (cloud index = 32%) and the RHR reduced to 0.01 °C hr-1 owing to reduced solar incidence. The observations suggest that the combined effect of physical and biological factors determine the RHR in the water column. A one-dimensional light penetration model simulations showed that the biological heating of 10 W m-2 would generate additional heating of 0.008 °C hr-1 in the upper ocean indicating significant impact of elevated chl-a concentration. These improved understanding of biophysical interaction in the Bay of Bengal will help to improve our knowledge about the chl-a induced feedbacks on sea surface temperature.