Surface flux contributions to CMIP6 spread of dynamic sea-level change vary across regions: insights from an ocean-only perturbed forcing ensemble

Dynamic sea-level change (ΔDSL) is a key process in shaping the pattern of future sea-level rise. CMIP6 models predict a range of ΔDSL under 1% increase of CO2 per year. We analyse this CMIP6 spread into contributions from: 1) surface flux change (dF) and 2) model sensitivity to it (Φ). Specifically, we perturb the pre-industrial simulation of an ocean model with space- and time-varying dF diagnosed from different CMIP6 models (one at a time). The CMIP6 spread is thus decomposed into a flux-driven spread and a residual; the latter is linked to model spread of Φ. We improve upon previous studies by: (a) deriving the perturbed forcing ensemble using an ocean-only setup and (b) comparing it with the CMIP6 ensemble for both variance and correlation. This reveals distinct roles of surface forcing in driving the CMIP6 spread in different regions. In the North Pacific, differences in windstress forcing primarily explain the CMIP6 spread, while in the North Atlantic, differences in model sensitivity are more important. For the latter region, although buoyancy forcing drives a ΔDSL spread there, it correlates poorly with the CMIP6 spread. In the Southern Ocean, differences in forcing and sensitivity are both important for explaining the CMIP6 spread. The surface forcing affects the spread along 40°S via windstress and the spread around the Antarctic via buoyancy flux. In addition to ΔDSL analysed here, the perturbed forcing ensemble can be used to analyse future changes in other ocean variables, such as temperature, salinity and the Atlantic meridional overturning circulation. The full ensemble data is openly available online and can be freely used for future studies.