An asymmetric change in circulation and nitrate transports in the Bay of Bengal

The Bay of Bengal is a dynamic region that experiences intense freshwater runoff, extreme meteorological events, and seasonally reversing surface currents. The region is particularly susceptible to anthropogenic climate change, driven in part by large air-sea fluxes, persistent freshwater stratification, and low overturning rates. Predicting how this system is likely to change in the future is paramount for planning effective adaption and mitigation strategies. Using a relocatable, coupled physics-ecosystem regional coastal ocean model (NEMO-ERSEM), we investigate potential future changes in surface circulation and coastal nitrate pathways around the coast of the Bay of Bengal from 1980 to 2060, using a “business-as-usual” climate change scenario. We find that future surface currents are reduced in the northern Bay of Bengal(summer) and strengthened in the southern Bay of Bengal (fall). Coastal nitrate transports mirror this asymmetric change and decrease by as much as 14% in the northern Bay of Bengal, perpetuating a positive feedback loop whereby the northern Bay of Bengal becomes progressively fresher and more nutrient-rich, strengthening surface stratification and increasing the risk of toxic algal blooms and eutrophication events. Conversely, in the southern Bay of Bengal, coastal nitrate transports increase by 52% that promotes localized diatom blooms despite reduced regional river runoff. This work highlights the need for more rigorous scenario testing in the region and presents new challenges for mitigating the impact of anthropogenic climate change across South Asia.