The added value of Med-CORDEX coupled high-resolution regional climate models in representing sea surface temperature and marine heatwaves in the Mediterranean Sea

Marine heatwaves (MHWs) pose significant threats to Mediterranean marine ecosystems and coastal economies, with frequency and severity projected to increase under future climate change. While Med-CORDEX fully-coupled Regional Climate System Models (RCSMs) offer enhanced resolution and improved representation of local processes relative to their parent Global Climate Models (GCMs), a systematic assessment of their added value for sea surface temperature (SST) and MHW properties has been lacking. This study quantifies the added value of Med-CORDEX RCSMs over the Mediterranean basin, evaluating their capacity to correct GCM biases and improve the representation of SST and MHW probability distributions. Results show that added value is scale-dependent and metric-specific. RCSMs generally improve the SST spatial pattern and the shape and upper tail of its temporal distribution, but mostly fail to correct Mediterranean basin-averaged errors in the mean, standard deviation, 90th percentile and linear trend. SST improvements are most consistent along coastlines and in semi-enclosed areas, where fine-scale ocean-atmosphere interactions and topographic constraints are best resolved by RCSMs. For MHW duration, downscaling provides consistent and spatially widespread improvements across nearly all models, driven by a better representation of short-lived events. For MHW intensity, added value is model-dependent and not systematic: while the majority of RCSMs improve this metric, some models exhibit deterioration linked to model-specific features. These results demonstrate that higher horizontal resolution is a necessary but not sufficient condition for improved MHW representation, and that simultaneous advances in other model components are required to fully exploit the potential of regional downscaling. In particular, within the Med-CORDEX ensemble, inter-model differences in MHW added value appear to be correlated with improvements in the thickness of the first layer.