The transient climate response (TCR) is 20% higher in the Alfred Wegener Institute Climate Model (AWI-CM) compared to the Max Planck Institute Earth System Model (MPI-ESM) whereas the equilibrium climate sensitivity (ECS) is only by less than 10% higher in AWI-CM. These results are largely independent of the two considered model resolutions for each model. The two coupled CMIP6 models share the same atmosphere-land component ECHAM6.3 developed at the Max Planck Institute for Meteorology (MPI-M). However, ECHAM6.3 is coupled to two different ocean models, namely the MPIOM sea ice-ocean model developed at MPI-M and the FESOM sea ice-ocean model developed at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). A reason for the different TCR is related to ocean heat uptake in response to greenhouse gas forcing. Specifically, AWI-CM simulations show stronger surface heating than MPI-ESM simulations while the latter accumulate more heat in the deeper ocean. The vertically integrated ocean heat content is increasing slower in AWI-CM model configurations compared to MPI-ESM model configurations in the high latitudes. Weaker vertical mixing in AWI-CM model configurations compared to MPI-ESM model configurations seems to be key for these differences. The strongest difference in vertical ocean mixing occurs inside the Weddell Gyre and the northern North Atlantic. Over the North Atlantic, these differences materialize in a lack of a warming hole in AWI-CM model configurations and the presence of a warming hole in MPI-ESM model configurations. All these differences occur largely independent of the considered model resolutions.

Oceanic mesoscale eddies play an important role in preconditioning and restratifying the water column before and after mixing events, thereby affecting deep water formation variability. In the Labrador Sea, where deep convection occurs regularly, observations and models indicate a complex interplay of turbulence and associated tracer fluxes. Results from a realistic eddy-resolving (~5 km local horizontal resolution) ocean model in quasi-equilibrium (~300 years integration) suggest that small-scale temperature fluxes due to turbulent potential to kinetic energy conversion are the main driver of mixed layer restratification during deep convection triggered through atmospheric forcing. In addition to these baroclinic instabilities, buoyant water masses must be provided by the boundary current, where barotropic turbulence is equally important. Only acting together, the destabilizing forcing can be balanced. In a low-resolution control simulation (~20 km) the modeled turbulence is strongly reduced and the associated modeled and parameterized heat fluxes too weak to increase stratification.