Abstract/Summary

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 is largely reduced. A further reduction of the equatorward bias is found in atmosphereonly simulations. Using diagnostic large-scale fields we find that the reduced equatorward bias in CMIP6 and AMIP6 models comes about from an increase to the midlatitude temperature gradients, which increases baroclinicity in the atmosphere, and is associated with poleward shifts in the storm track. In CMIP6, the storm track poleward shift is associated with more equatorward SST biases than in CMIP5 and not through a correction of pre-existing biases. Increases in mid-latitude temperature gradients in the atmosphere and ocean are shown to be connected to errors in radiative forcing. 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.