Impact of a newly observed recurring eddy on the western Bay of Bengal productivity

Mesoscale eddies have a significant impact on the biological productivity in the Bay of Bengal (Eastern Indian Ocean). However, we do not currently have a complete understanding of them. The Bay of Bengal is a region that is highly dependent on fishing for its population’s survival, with surrounding countries such as Bangladesh depending on fish from the bay for approximately 60% of their consumed animal protein, and hence understanding any changes in this biological productivity is important. In this work, using 25-42 years of satellite data and the py-eddy-tracker eddy detection algorithm (Mason et al., 2014), we observe a yearly recurring mesoscale eddy (called hereafter the Odisha Eddy) on the western coast of the Bay of Bengal, which was noted before but investigated here, for the first time, in terms of physics and biological response. We also examine the impact of a confluence zone that forms between the EICC as it reverses poleward and an equatorward flowing boundary during May to September on the Odisha Eddy variability, using a new algorithm that automatically identifies this confluence zone based on satellite altimetry currents data. We further investigated the impact that the EICC confluence zone has on the biological productivity of the Odisha Eddy. We found that the eddy has an increase in the concentration of chlorophyll-a by 42% as compared to the surrounding waters, and that when the eddy occurs alongside the confluence zone, the chlorophyll-a content within the Odisha Eddy is 36% higher than when the eddy occurs alone. The Odisha Eddy also presents a larger radius (31% increase), amplitude (47% increase) and faster rotational velocity (15% increase) when it occurs alongside the confluence zone. We conclude that the EICC confluence zone amplifies the positive effect that the Odisha Eddy has on the biological productivity of this region of the Bay of Bengal. We hypothesise that this is due to the confluence zone enhancing advection of nutrients within and around the eddy. More research is needed to fully examine this mechanism and other controls (e.g., impact of ocean planetary waves, wind field) influencing the Odisha Eddy productivity in the Bay.