Abstract
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.