Air–sea carbon flux from high-temporal-resolution data of in situ CO₂ measurements in the southern North Sea

Abstract. An important element to keep track of global change is the atmosphere–water exchange of carbon dioxide (CO₂) in the ocean as it provides insight in how much CO₂ is incorporated in the ocean (i.e. the ocean as a sink for CO₂) or emitted to the atmosphere (i.e. the ocean as a source). To date, only few high-resolution observation sets are available to quantify the spatiotemporal variability of air–sea CO₂ fluxes. In this study, we used observations of pCO₂ collected daily at the ICOS station Thornton Buoy in the southern North Sea from February until December 2018 to calculate air–sea CO₂ fluxes. Our results show a seasonal variability of the air–sea carbon flux, with the sea being a carbon sink from February until June switching to a carbon source in July and August, before switching back to a sink until December. We calculated that the sink was largest in April (−0.95 ± 0.90 mmol C m⁻² d⁻¹), while in August, the source was at its maximum (0.08 ± 0.13 mmol C m⁻² d⁻¹). On an annual basis, we found a sink for atmospheric CO₂ of 130.19 ± 149.93 mmol C m⁻² y⁻¹. Apart from region- and basin-scale estimates of the air–sea CO₂ flux, also local measurements are important to grasp local dynamics of the flux and its interactions with biogeochemical processes.