Mesoscale eddy dynamics and their influence on biological productivity in the western Bay of Bengal during the fall intermonsoon.

The western Bay of Bengal (BoB) is characterized by strong upper ocean stratification resulting from substantial freshwater input and complex monsoon-driven circulation, both of which strongly influence mesoscale dynamics and biological productivity. However, the characteristics of mesoscale eddies and their role in regulating biological productivity during the fall intermonsoon (FIM) remain insufficiently understood. This study investigates mesoscale eddy variability and associated biological responses in the western BoB during FIM 2006 using in situ hydrographic observations and satellite datasets. Hydrographic sections along five meridional transects (19-11°N) identified two cyclonic eddies near 17°N and 11°N and one anticyclonic eddy near 13°N. The northern sector exhibited intense freshwater-induced haline stratification and a thick barrier layer that restricted vertical coupling, masking the surface expression of cyclonic eddies despite enhanced subsurface nutrient uplift. In contrast, weaker stratification and a thinner barrier layer promoted efficient eddy-induced nutrient transport into the euphotic zone. The observed eddies were generated through baroclinic instability enhanced by wind stress curl, basin-scale gyre persistence and Rossby wave propagation, and intrusion of negative sea level anomalies from the Sri Lanka Dome. Biological observations revealed elevated chlorophyll a concentrations and primary productivity in regions influenced by cyclonic eddies, salinity fronts, and coastal upwelling, whereas the anticyclonic eddy reduced nutrient supply through downwelling. These findings demonstrate that mesoscale eddies drive biological variability during the FIM, while freshwater-induced stratification and barrier layer thickness regulate the efficiency of physical-biological coupling in the western BoB.