Drivers of the air-sea CO2 flux variability in the North Sea

Identifying the mechanisms driving the variability of the air-sea exchange of carbon dioxide (CO₂) in the North Sea is necessary to evaluate the consequences of human interventions such as coastal alkalinity enhancement (OAE) on this societally important ecosystem. For this purpose, the three-dimensional coupled physical-biological model SCHISM-ECOSMO, encompassing a carbonate chemistry module, is employed to present the local physical-biogeochemical processes as well as the exchange processes across scales and compartments. Here we present model results for a 5-year simulation (2002-2004), which are shown to agree well with the observations, indicating a net CO₂ uptake in the northern North Sea (NNS) over the year while a net source of CO₂ to the air in summer in the southern North Sea (SNS). In the NNS, the ‘Continental Shelf Pump’ mechanism, attributing to the seasonal stratification and efficient carbon export, determines the CO₂ exchange, making the ocean a net sink despite the high temperature in summer that contributes to an enhancement of the CO₂ release. In contrast, the temperature-driven release of CO₂ outcompetes the biological CO₂ drawdown in the shallower SNS. In this region, the tidal mixing prevents seasonal stratification. As a result, the CO₂ generated via remineralization gets quickly in contact with the atmosphere. In addition, the interannual variability of the CO₂ flux is assessed based on the 5-year simulation, which is mainly associated with the variations of the hydrodynamic conditions and productions induced by changes of meteorological conditions.