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 near 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 [shared socioeconomic pathway (SSP) 5-8.5]. 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 twenty-first century. Therefore, model deficiencies in simulating the correct position of the SH winter stratospheric jet near 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. SIGNIFICANCE STATEMENT: The Southern Annular Mode (SAM) is the leading mode of extratropical circulation variability in the Southern Hemisphere (SH) and significantly influences SH regional climate, ocean circulations, and Antarctic sea ice variations. Austral spring SAM variability is substantially driven by the SH springtime stratospheric polar vortex variability, which often follows anomalous meridional shifts of the preceding winter stratospheric jet and associated wave–mean flow interactions. We show that many climate models do not accurately simulate these stratospheric processes largely due to a systematic poleward bias in the mean position of the SH winter stratospheric jet maximum. These model deficiencies appear to be a key source of uncertainty in future projections of the SAM and associated SH climate changes in austral spring.