AUTHOR=Kossack Jan , Mathis Moritz , Daewel Ute , Liu Feifei , Demir Kubilay Timur , Thomas Helmuth , Schrum Corinna TITLE=Tidal impacts on air-sea CO2 exchange on the North-West European shelf JOURNAL=Frontiers in Marine Science VOLUME=11 YEAR=2024 URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2024.1406896 DOI=10.3389/fmars.2024.1406896 ISSN=2296-7745 ABSTRACT=

Tidal forcing is a dominant physical forcing mechanism on the Northwest European Shelf (NWES) that regulates the mixing-stratification status of the water column and thus acts as a major control for biological productivity and air-sea CO₂ exchange. Tides further influence the marine carbon cycle on the shelf by affecting benthic-pelagic coupling, vertical mixing and the large-scale residual circulation. The cumulative tidal impact on oceanic uptake of atmospheric CO₂ on the NWES, however, remains largely unexplored. We use a coupled physical-biogeochemical ocean model to gain quantitative understanding of the tidal impacts on the air-sea CO₂ exchange of the NWES by comparing hindcast simulations with and without tidal forcing. Our results show that tidal forcing weakens the annual oceanic CO₂ uptake on the NWES by 0.15 Tmol C yr−1, corresponding to a ~13% stronger CO₂ sink in the experiment without tidal forcing. The tide-induced increase in marine primary production demonstrated in earlier studies, which primarily enhances biological carbon fixation in shallow inner-shelf regions of the NWES, does not significantly affect net air-sea CO₂ exchange. Instead, we find tidal mixing, tide-induced baroclinic circulation and the tidal impact on benthic-pelagic coupling to be dominant controls of air-sea CO₂ exchange. Tidal mixing in the permanently mixed shelf regions accounts for the majority (~40%) of the weakening effect on CO₂ uptake, while the modulation of water mass composition in the Celtic Sea by tide-induced baroclinic circulation reduces the uptake further (~33% of the difference in annual mean CO₂ uptake). In terms of the shelf carbon budget, the tidal response of air-sea CO₂ exchange is primarily mediated by changes to the pelagic DIC reservoir (~73%; −0.11 Tmol C yr−1). Tidal impacts on off-shelf carbon export to the North Atlantic only account for ~20% (−0.03 Tmol C yr−1) of the tidal impact on shelf CO₂ uptake, and changes in sedimentation of particulate organic carbon account for the remaining ~7% (−0.01 Tmol C yr−1).