Extreme storms impact on coastal sea-air CO2 fluxes: Black Sea shelf dynamics

With increasing air and water temperatures, storms are likely to increase in frequency and magnitude, especially in semi-enclosed basins like the Black Sea, where recent estimates show that surface sea water temperatures have increased between 0.06-0.1 degrees per year over the past two decades. During the fall season of 2023, the Black Sea and especially the northwest shelf region experienced several storms, like Storm Daniel, which caused major oxygen deficit in surface waters in September 2023, and two major November storms, Frederico and Bettina, associated with the highest winds (>20 m s^-1) over the past 5 years. Using a continuous pCO₂ record from a coastal station established in summer 2022, supported by the Helmholtz Association European Partnership project SeaReCap, we identify large-scale sea-air CO₂ fluxes (>800 mmol C m^-2 month^-1) to the atmosphere (3 times more than other months with positive fluxes) in November 2023. Compared to 2024 when sea-air flux is negative, both November and December of 2023 are large net sources of CO₂ to the atmosphere. This is due to both upwelling favorable conditions at the end of October, bringing bottom water enriched in carbon to the surface after a productive summer, and to increased mixing in November from the two storms. With increasing temperatures, these large and extreme storms are likely to continue to have an impact on the Black Sea sea-air CO₂ flux.