Multi-model comparison of trends and controls of near-bed oxygen concentration on the northwest European continental shelf under climate change

Abstract

Abstract. We present an analysis of the evolution of near-bed oxygen in the next century in the northwest European continental shelf in a three-member ensemble of coupled physics–biogeochemistry models. The comparison between model results helps highlight the biogeochemical mechanisms responsible for the observed deoxygenation trends and their response to climate drivers. While all models predict a decrease in near-bed oxygen proportional to climate change intensity, the response is spatially heterogeneous, with hotspots of oxygen decline (up to −1 mg L−1) developing along the Norwegian trench in the members with the most intense change, as well as areas where compensating mechanisms mitigate change. We separate the components of oxygen change associated with the warming effect on oxygen solubility from those due to the effects of changes in transport and biological processes. We find that while warming is responsible for a mostly uniform decline throughout the shelf (−0.30 mg L−1 averaged across ensemble members), changes in transport and biological processes account for the detected heterogeneity. Hotspots of deoxygenation are associated with enhanced stratification that greatly reduces vertical transport. A major change in circulation in the North Sea is responsible for the onset of one such hotspot that develops along the Norwegian trench and adjacent areas in the members characterised by intense climate change. Conversely, relatively shallow and well-mixed coastal areas like the southern North Sea, Irish Sea and English Channel experience an increase in net primary production that partially mitigates oxygen decline in all members. This work represents the first multi-model comparison addressing deoxygenation in the northwest European shelf and contributes to characterising the possible trajectories of near-bed oxygen and the processes that drive deoxygenation in this region. As our downscaled members factor in riverine inputs and small- and medium-scale circulation, which are not usually well represented in earth system models, results are relevant for the understanding of deoxygenation in coastal and shelf systems.