Role of the upper-stratospheric mean state in the 2021 sudden stratospheric warming onset

This study investigates the role of upper-stratospheric preconditioning in the 2021 sudden stratospheric warming (SSW) onset by conducting a series of numerical model experiments. A free-run simulation predicts the SSW onset with a lead time of 13 days, in agreement with operational subseasonal-to-seasonal predictions. The limited prediction skill is partly due to misrepresentation of the polar vortex near the stratopause, which results in weaker upward propagation of planetary-scale waves at high latitudes. A sensitivity experiment is performed by nudging the zonal-mean flow in the upper stratosphere toward observations. With the realistic upper-stratospheric mean flow, the model successfully predicts the SSW onset with a lead time of up to 15 days. Although the improvement is marginal, this simulation predicts the SSW onset more robustly than the free-run simulation and realistically reproduces the observed features of planetary-scale wave propagation into the polar region, guided by the poleward shift of the polar vortex in the upper stratosphere. This result provides modeling evidence for the importance of the upper-stratospheric mean state in the 2021 SSW onset and its prediction.