An assessment of air-sea CO2 flux parameterizations during tropical cyclones in the Bay of Bengal

The exchange of air-sea CO₂ plays a significant role in regulating the Earth’s climate. The errors associated with the estimations of air-sea CO₂ fluxes during extreme transient events like tropical cyclones (TCs) are important for climate research. In this study, we assess the estimates of CO₂ gas transfer velocity and the corresponding air-sea flux derived by employing five wind-dependent and two wave-dependent parameterizations for eight TCs in the Bay of Bengal using mooring observations and reanalysis datasets. To start with, we analyze drag coefficient ($C_D$) and associated frictional velocity ($u^{*}$) derived from two globally very commonly used bulk flux algorithms, COARE 3.0 and the updated version COARE 3.6, with the estimates from wind-wave tank experiments for moderate and high wind speeds for all TCs. The analysis indicates that COARE 3.6 provides the best estimate of drag coefficient. Further, we find that the wave-dependent parameterization by Woolf (2005) provides the best estimates of CO₂ gas transfer velocity compared to existing estimates of laboratory-based wind-wave tank experiments for high winds. Among all wind-only parameterizations, the hybrid parameterization proposed by Nightingale et al. (2000) performs best for high winds. We find that for winds < 20 m/s, the resultant fluxes of CO₂ estimated using these seven parameterizations vary within 5 mmol CO₂ m^-2 d^-1. However, for winds > 20 m/s, the difference between wind- and wave-parameterized fluxes are significant (∼50 mmol CO₂ m^-2 d^-1). The percentage of variation in CO₂ flux explained by transfer velocity (difference in sea and air pCO₂) during TC conditions is nearly 78 (15)%.