High-frequency wind speed and wave variability influence the air-sea CO2 flux by modulating the gas transfer velocity. Traditional gas transfer velocity formulations scale solely with wind speed and ignore wave activity, including wave breaking and bubble-mediated transfers. In this study, we quantify the effects of wave-induced spatiotemporal variability on the CO2 flux and the ocean carbon storage using a wind-wave-dependent gas transfer velocity formulation in an ocean general circulation model (MOM6-COBALTv2). We find that wave activity introduces a hemispheric asymmetry in ocean carbon storage, with gain in the southern hemisphere where wave activity is robust year-round and loss in the northern hemisphere where continental sheltering reduces carbon uptake. Compared to a traditional wind-dependent formulation, the wind-wave-dependent formulation yields a modest global increase in ocean carbon storage of 4.3 PgC over 1959-2018 (~4%), but on average, enhances the CO2 gas transfer velocity and flux variability by 5-30% on high-frequency and seasonal timescales in the extratropics and up to 200-300% during storms (>15 m s-1 wind speed). This wave-induced spatiotemporal variability in CO2 flux is comparable to the flux expected from marine carbon dioxide removal (mCDR) techniques, such that neglecting wind-wave variability in modeled CO2 fluxes could hinder distinguishing between natural variability and human-induced changes, undermining mCDR verification and monitoring efforts.

The coastal ocean contributes to regulating atmospheric greenhouse gas concentrations by taking up carbon dioxide (CO2) and releasing nitrous oxide (N2O) and methane (CH4). Major advances have improved our understanding of the coastal air-sea exchanges of these three gasses since the first phase of the Regional Carbon Cycle Assessment and Processes (RECCAP in 2013), but a comprehensive view that integrates the three gasses at the global scale is still lacking. In this second phase (RECCAP2), we quantify global coastal ocean fluxes of CO2, N2O and CH4 using an ensemble of global gap-filled observation-based products and ocean biogeochemical models. The global coastal ocean is a net sink of CO2 in both observational products and models, but the magnitude of the median net global coastal uptake is ~60% larger in models (-0.72 vs. -0.44 PgC/yr, 1998-2018, coastal ocean area of 77 million km2). We attribute most of this model-product difference to the seasonality in sea surface CO2 partial pressure at mid- and high-latitudes, where models simulate stronger winter CO2 uptake. The global coastal ocean is a major source of N2O (+0.70 PgCO2-e /yr in observational product and +0.54 PgCO2-e /yr in model median) and of CH4 (+0.21 PgCO2-e /yr in observational product), which offsets a substantial proportion of the net radiative effect of coastal \co uptake (35-58% in CO2-equivalents). Data products and models need improvement to better resolve the spatio-temporal variability and long term trends in CO2, N2O and CH4 in the global coastal ocean.