Tropical cyclones enhance net CO2 sink in the Bay of Bengal

Abstract

Tropical cyclones modify the upper ocean due to significant mixing associated with increased winds that bring CO₂ and nutrient-rich subsurface waters to the surface, altering CO₂ flux and biological production. In the region where considerable river discharge occurs with strong stratification, it is hypothesized that tropical cyclones may increase the sink of CO₂ into the ocean and vice versa in the region of weak stratification. The balance between CO₂ removal through the exchange at the air–water interface and biological production on a shorter time scale may vary with the intensity of the cyclone. To examine this, severe cyclone Michaung and very severe cyclone Hudhud was studied to understand the modifications in CO₂ fluxes and net primary production (NPP) in the Bay of Bengal. Both these cyclones occurred in the Bay of Bengal during post monsoon season (October to December) and crossed the land on the east coast of India. The barrier layer thickness was > 40 m in the northern and < 10 m in the southern Bay, resulting in spatial variability in the mixing of the upper ocean. High windspeed associated with cyclones may not rupture the stratification in the northern Bay. This led to an increase in a sink of CO₂ from the atmosphere in the northern Bay, whereas the sink was decreased in the southern Bay due to an increase in pCO₂ levels through mixing with subsurface waters. In the case of severe cyclone Michaung, the decrease in flux of CO₂ due to an increase in pCO₂ levels in the surface water through mixing was compensated by an increase in CO₂ removal through NPP. In contrast, the decrease in CO₂ sink due to the very severe cyclone Hudhud was lower than the increase in NPP, resulting in a net sink of CO₂ in the Bay of Bengal. This study suggests that tropical cyclones may enhance CO₂ sink into the Bay of Bengal. An increase in the number and intensity of tropical cyclones is projected in future due to climate change that may help to achieve net zero emissions in future.