Drivers of biases in the CMIP6 extratropical storm tracks. Part 2: Southern Hemisphere

Abstract

The Southern Hemisphere storm tracks are commonly simulated too far equatorward in climate models for the historical period. In the latest generation of climate models from the 6th phase of the coupled model intercomparison project (CMIP6), the equatorward bias that was present in CMIP5 models still persists, although is reduced considerably. A further reduction of the equatorward bias is found in atmosphere-only simulations. Using diagnostic large-scale fields we propose that an increase in the midlatitude temperature gradients contributes to the reduced equatorward bias in CMIP6 and AMIP6 models, reducing the biases relative to ERA5. These changes increase baroclinicity in the atmosphere, and are associated with a storm track that is situated further poleward. In CMIP6 models, the poleward shift of the storm tracks is associated with an amelioration of cold midlatitude SST biases in CMIP5 and not through a reduction of the long-standing warm Southern Ocean SST bias. We propose that increases in midlatitude temperature gradients in the atmosphere and ocean are connected to changes in the cloud-radiative effect. Persistent track density biases to the south of Australia are shown to be connected to an apparent standing wave pattern originating in the tropics, which modifies the split jet structure near Australia and subsequently the paths of cyclones.