The impact of model uncertainty in surface forcings on the properties of Southern Ocean mode waters.

The Southern Ocean plays an outsized role in the global climate system, responsible for around 75% of global ocean heat uptake. Mode waters which form there in winter are subducted and enter the deep interiors of other basins, locking away heat and carbon for hundreds to thousands of years. However, the Southern Ocean is also a data desert, especially in winter, where surface properties such as heat fluxes, winds, and sea surface temperatures are poorly observed. The lack of surface observations leads to poorly constrained ocean models. In this study we calculate the impact of uncertainties in surface forcings on the properties of Southern Ocean mode water formation regions (MWFRs) in CMIP models. We take the sensitivities of the heat content of MWFRs to surface properties (from a previous adjoint model study) and convolve them with inter-model spreads in those surface properties, calculated from CMIP models. This gives us an estimate of the impact of the uncertainty in surface forcings on the modelled mode waters, which we validate by directly looking at the relationships between modelled mode water properties and surface properties in the CMIP models. We find that inter-model uncertainty in zonal wind stress and SST have the largest impacts on MWFR heat content. This study allows for targeted improvements in surface forcings to improve the modelling of MWFRs.