Abstract

We summarise a two year integrated assessment of the representation of Southern Ocean processes, from the stratosphere to the deep ocean, in HadGEM3, in the context of other IPCC models, focussing on the air-sea flux errors in atmosphere only runs. Using a novel net air-sea observational flux product we find that for HadGEM3 there is a good correspondence between atmosphere-only air-sea flux errors and coupled heat content errors in the Southern Ocean, which suggests that atmosphere errors are the primary driver of coupled heat content biases in HadGEM3. Evidence suggests that these net flux errors are (_{50%) due to radiation errors, mainly short wave due to cloud errors, and (}50%) due to turbulent flux errors, associated with the representation of storms. Observational evidence suggests that cloud errors are in part due to a lack of super-cooled cloud liquid water content, which is known to be prevalent in polar regions, in HadGEM3, and in part due to representation of aerosols. New aerosol and mixed-phase cloud schemes reduce the atmosphere only net heat flux bias by _{30% and also reduce coupled sea surface temperature biases by} 50%. Clearly ocean transport and mixing errors also contribute to the coupled heat-content and SST biases. For example, for the HadGEM3 atmosphere model, employing a coupled 1/4 degree eddy permitting ocean model roughly doubles the Southern Ocean SST bias compared to that when using 1 degree parameterised eddy ocean model, apparently due to more initial heat transport into the Southern Ocean. Ocean mixing is also poorly represented in the HadGEM3 NEMO ocean, giving rise to under-estimation of Southern Ocean mixed layer depths, which we have plans to address.