21st century marine climate projections for the NW European Shelf Seas based on a Perturbed Parameter Ensemble

Abstract. The North West European Shelf Seas (NWS) are environmentally and economically important, and an understanding of how their climate may change helps with their management. However, as the NWS are poorly represented in Global Climate Models, a common approach is to dynamically downscale with an appropriate shelf seas model. We develop a set of physical marine climate projections for the NWS. We dynamically downscale 12 members of the HadGEM3-GC3.05 Perturbed Parameter Ensemble (approximately 70 km horizontal resolution over Europe), developed for UKCP18, using the shelf-seas model NEMO CO9 (7 km horizontal resolution). These are run under the high greenhouse gas emissions RCP8.5 scenario as continuous simulations over the period 1990–2098. We evaluate the simulations against observations in terms of tides, sea surface temperature, surface and near-bed temperature and salinity, and sea surface-height. These simulations represent the state-of-the-art for marine UK projections. We project a Sea-Surface Temperature (SST) rise of 3.11 °C (±2σ = 0.98 °C), and a Sea-Surface Salinity (SSS) freshening of −1.01 psu (±2σ = 0.93 psu) for 2079–2098 relative to 2000–2019, averaged over the NWS (approximately bounded by the 200 m isobar and excluding the Norwegian Trench, Skagerrak, and Kattegat). While the patterns of NWS changes are similar to our previous projections, there is a greater warming and freshening, that could reflect the change from the A1B emissions scenario to the RCP8.5 concentrations pathway or the higher climate sensitivity exhibited by HadGEM3-GC3.05. Off the shelf, south of Iceland, there is limited warming, consistent with a reduction in the Atlantic Meridional Overturning Circulations and associated northward heat transport. These projections have been publicly released, along with a consistent 200-year present day control simulation, to provide an evidence-base for climate change assessments and to facilitate climate impact studies. For example, we illustrate how the two products can be used to estimate of climate trends, unforced variability, and the Time of Emergence (ToE) of the climate signals. We calculate the average NWS SST ToE to be 2034 (with an 8-year range) and 2046 (with a 33-year range) for SSS. We also discuss how these projections can be used to describe NWS conditions under 2 °C and 4 °C global mean warming (compared to 1850–1900), as a policy relevant exemplar use-case.