Improving the representation of freshwater input to shelf seas in an intermediate-complexity global climate model

Whilst accounting for only 7% of the global ocean surface area, shelf seas are important links between multiple environments (including terrestrial, deep marine, and atmospheric) and modulate the freshwater influx from rivers before it reaches the open ocean. This freshwater acts as a buoyancy forcing, and, together with solar heating and tidal mixing, affects the seasonal stratification of shelf seas. As stratification impacts numerous processes within the shelf seas, such as heat uptake, ocean currents and biogeochemistry which may further be of global importance, it is important that freshwater fluxes are accurately simulated within models.
Despite their importance, due to coarse model resolution, shelf seas are generally poorly represented in intermediate-complexity global climate models. Here, we examine the accuracy of shelf sea representation in the intermediate-complexity UVic Earth System Climate Model, with a primary focus on the North Sea. Using observational data, we show that the river basin configuration and freshwater discharge in the control model set up has large errors. As a result, the North Sea receives almost double the expected freshwater discharge on an annual scale, impacting the flushing time, seasonal stratification and biogeochemistry of the region. Through a series of simulations rerouting freshwater through more realistic drainage basins, and removing excess freshwater, we improve simulation results, with variations in freshwater fluxes having a significant impact on shelf sea processes. Our results indicate that the over-freshening of shelf seas may not solely be restricted to the UVic model but may be an issue in other global Earth system models due to their low spatial resolution.