Influence of the freshwater plume dynamics and the barrier layer thickness on the CO2 source and sink characteristics of the Bay of Bengal

Abstract

The high influx of freshwater in the Bay of Bengal (BoB) region forms a freshwater plume at the surface. This plume's dispersal spreads a low-density layer of water on the surface, forming a barrier layer between the mixed layer depth and the isothermal layer. The low saline water reduces the sea surface partial pressure of carbon dioxide (pCO2), and the barrier layer thickness (BLT) inhibits the interaction between the subsurface and surface layers of BoB. In this study, we run a high resolution coupled physical-biogeochemical model to understand the effect of variability of freshwater plume spread (FPS) and the BLT on the seasonality of sea-surface pCO2. By comparing with observation data, we find that the model satisfactorily simulates the sea-surface pCO2 and the FPS for the BoB region. As the FPS begins to develop in the southwest monsoon season, the CO2 sink region also begins to form in the same manner. The region with sea surface salinity (SSS) less than 31 PSU was recognized as a freshwater plume. The region with SSS lower than 31 PSU showed under-saturated levels of pCO2. The model analysis showed that the CO2 sink spread exceeded the FPS in the winter monsoon season, covering almost 50% of the BoB region. This spread was attributed to the BLT, which is highest during the winter monsoon season. Based on the spatial CO2 flux, we find the Northern BoB a sink of atmospheric CO2 (−0.35±0.75 mol C m-2 yr-1). The Southwestern BoB acts as a source of CO2 (0.72±0.61 mol C m-2 yr-1) throughout the year and has the highest magnitude of flux, whereas the Southeastern BoB also acts as a weak source of CO2 (0.214±0.52 mol C m-2 yr-1).