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

Abstract 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 realis…