Processes explaining increased ocean dynamic sea level in the North Sea in CMIP6

Ocean dynamic sea level (ODSL) is expected to be one of the major contributors to sea level rise in the North Sea during the 21st century. This component is defined as the spatial sea level anomaly due to ocean currents, wind stresses and local thermosteric and halosteric effects. Climate models from CMIP5 and CMIP6 show a large spread, as well as an increase between CMIP5 and CMIP6 North Sea ODSL projections. In this study, we apply linear regression models on CMIP5 and CMIP6 data to get a better understanding of the processes that influence ODSL change in the North Sea. We find that neither global surface air temperature nor global mean thermosteric sea level can reproduce ODSL projections based on a linear relation in CMIP6, whereas this was the case for CMIP5. Including the strength of the Atlantic meridional overturning circulation (AMOC) as an additional predictor enables us to reproduce long-term changes in ODSL for both ensembles. The sensitivity to the AMOC increased in CMIP6, which points to a difference in model dynamics between CMIP5 and CMIP6, and a more important role of the deep ocean. To investigate this further, we analyse mixed layer depth data in the North Atlantic. We find that models with a relatively deep mixed layer in the Greenland Sea over the period 1985-2004, project larger rise in ODSL in the North Sea for both CMIP5 and CMIP6. This implies that the location of deep water formation in the North Atlantic potentially influences ODSL in the North Sea. The number of these models increased from CMIP5 to CMIP6, again pointing to a different sensitivity to larger scale processes, potentially explaining the difference between the two ensembles.