Assessment of the Southern Ocean Sea Surface Temperature Biases in CMIP5 and CMIP6 Models

This work evaluates the performances of climate models in simulating the Southern Ocean(SO) sea surface temperature(SST) by a large ensemble from phases 5 and 6 of the Coupled Model Intercomparison Project(CMIP5 and CMIP6). By combining models from the same community sharing highly similar SO SST biases and eliminating the effect of global-mean biases on local SST biases, the results reveal that the ensemble-mean SO SST bias at 70° – 30°S decreases from 0.38℃ in CMIP5 to 0.28℃ in CMIP6, together with increased intermodel consistency. The dominant mode of the intermodel variations in the zonal-mean SST biases is characterized as a meridional uniform warm bias pattern, explaining 79.1% of the intermodel variance and exhibiting positive principal values for most models. The ocean mixed layer heat budget further demonstrates that the SST biases at 70° – 50°S primarily result from the excessive summertime heating effect from surface net heat flux. The biases in surface net heat flux south of 50°S are largely impacted by surface shortwave radiation from cloud and clear sky components at different latitudes. North of 50°S, the underestimated westerlies reduce the northward Ekman transport and hence northward cold advection in models, leading to warm SST biases year-round. In addition, the westerly biases are primarily traced back to the atmosphere-alone model simulations forced by the observed SST and sea ice. These results disclose the thermal origin at the high latitude and dynamical origin at the low latitude of the SO SST biases and underscore the significance of the deficiencies of atmospheric models in producing the SO SST biases.