CMIP6 model biases in Antarctic stratospheric vortex dynamics and their implications for Southern Hemisphere circulation projections

The springtime variability of the stratospheric polar vortex over Antarctica is an important source of predictability for the Southern Annular Mode (SAM) and associated Southern Hemisphere (SH) regional climate anomalies in austral spring-summer seasons. Variations in the Antarctic spring vortex often result from anomalous meridional shifts of the SH winter stratospheric jet maximum at the stratopause and associated wave-mean flow feedback. In this study, we evaluate 1) the ability of the models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6) under historical forcings to simulate the observed dynamical chain of processes linking wintertime upper-stratospheric vortex anomalies to springtime surface climate anomalies; and 2) the influence of this ability on future projections of the SH stratospheric vortex and SAM under the highest greenhouse gas emission scenario (SSP5-8.5). Our results show that many CMIP6 models are deficient in reproducing the observed dynamical evolution of SH vortex anomalies, mainly due to a systematic poleward bias in the mean position of the SH winter stratospheric jet maximum. These biases appear to significantly influence projected changes in the stratospheric zonal circulation and the associated SAM by the end of the 21 st century. Therefore, model deficiencies in simulating the correct position of the SH winter stratospheric jet at the stratopause and the dynamical evolution of its anomalous meridional shift to a spring polar vortex strength anomaly constitute a key source of uncertainty in projections of the SAM and associated SH climate changes in austral spring.