A Future View of Ocean-Shelf Exchange and Shelf-Scale Circulation on the NW European Continental Shelf

The potential impacts of climate change on shelf seas is highly uncertain, with many vectors of atmospheric, oceanic and terrestrial change likely to play a role. Previous studies have highlighted the importance of change in oceanic properties (e.g. nutrients) as they are advected on-shelf. Here we demonstrate how substantial changes in circulation may also occur, with far- reaching consequences for on-shelf properties. We consider a 130 year simulation of the NEMO Atlantic Margin Model (AMM7) to 2100. This is driven by the atmospheric component of HADGEM2 (a CMIP5 Earth System Model under RCP 8.5) and a global ocean model (ORCA1, driven by the same atmosphere). In this single realisation of future conditions the circulation of the northern shelves is qualitatively similar to present day conditions. However, inflows into the North Sea through the Fair Isle channel and east of Shetland are found to substantially decrease, and the Shetland shelf current largely by-passes the North Sea. This significantly reduces the cyclonic North Sea circulation and shifts the balance between oceanic and terrestrial influence in this region, seen by a decrease in salinity. Using perturbation experiments and simple geostrophic theory, we attribute the drop in North Sea inflow to a freshening of the North Sea and an increase in its sea surface height relative to the outer-shelf. This feedback process is initiated by a rapid drop in salinity in the northern boundary condition (between Iceland and Norway), causing a freshening of the Nordic seas and a rapid drop in ocean-shelf volume flux. This is sufficient to allow the density gradient between the North Sea and outer-shelf to switch sign. Shorter experiments with either no drop in boundary condition salinity or no river inflows (so a reduced oceanic inflow does not reduce the on-shelf density), both restore the North Sea circulation to close to its present day character. We compare these driving condition to those from other CMIP5 ensemble members, which suggests that rapid drops in Nordic Sea salinity are not common but do occur in at least once in one other member.