The Sensitivity of a Coupled Climate Model to Numerical Mixing in Its Ocean Component

An ensemble of seven integrations of the Hadley Centre Global Environment Model, version 3, Global Coupled 5 configuration (HadGEM3-GC5) coupled model is used to investigate the sensitivity of the simulated climate system to settings in the ocean component that have been shown to affect the level of numerical mixing in forced simulations. This configuration is closely related to the U.K. contribution to CMIP7. The ensemble is integrated for 60 years with constant year 2000 greenhouse forcing. The ocean surface temperature has a strong and consistent 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 very similar to that of the SST but is seasonally amplified at high latitudes in the respective winter. Robust and strong sensitivities are also found for sea ice cover, rainfall, and the surface shortwave and latent heat fluxes, the latter two showing opposing changes in their global means of over 2 W m −2 across the ensemble. We present large-scale ocean and atmospheric metrics and discuss mechanisms for the sensitivity of surface temperatures in these simulations to numerical mixing in the ocean, in which more mixing warms the surface and vice versa. The magnitude of this sensitivity of the surface temperature is significant, since it is comparable with the changes found in CMIP simulations with historical or future greenhouse scenario forcings, and we speculate on the implications for modeling future climates.
Significance Statement

This study demonstrates that spurious numerical mixing in the ocean component of an IPCC-class coupled climate model can significantly affect the simulated ocean and atmospheric surface temperatures, leading to a bias in the coupled system with a magnitude comparable to that from anthropogenic greenhouse warming over the past few decades. Increasing numerical mixing leads to a warming of the surface and vice versa, with a surface temperature response of order 0.5°C in the global mean, with seasonal amplification of the surface air temperature changes at high latitudes.