The sensitivity of a climate model to numerical mixing in its ocean component

We describe the sensitivity of an ensemble of integrations of the HadGEM3-GC5 coupled model, with a ¼° ocean and N96 (~130km) atmosphere, to settings in the ocean component that have been demonstrated to affect the numerical mixing in forced simulations. This configuration is closely related to the UK contribution to CMIP7. The ensemble is integrated for 60 years with constant present-day greenhouse forcing. The ocean surface temperature has a robust response to numerical mixing, with increased mixing leading consistently to warming over the global ocean by up to 0.5°C, while reducing mixing cools the surface by a similar degree. The response of the surface air temperature is closely similar to that of the SST, and is seasonally amplified at high latitudes in the respective winter. We present large-scale ocean and atmospheric metrics, and discuss mechanisms for the counter-intuitive sign of the sensitivity of surface temperatures in these simulations to numerical mixing in the ocean, in which more mixing warms the surface and vice versa. This sensitivity is significant, since it is comparable with the surface temperature changes expected in a simulation with historical or future greenhouse scenario forcings, and we shall speculate on the implications for modelling future climates.