The Role of Sea-surface conditions in Southern-Hemisphere Polar Vortex Strength and Associated Wave Forcing Revealed by a Multi-member Ensemble Simulation with the Chemistry–Climate Model

The dynamical response of the southern hemisphere stratosphere to the ocean-surface conditions in 2002 and 2019, when exceptional sudden stratospheric warming (SSW) events occurred, was examined through the chemistry–climate model and experiments with 1,000 ensemble members using the sea-surface temperature (SST) and sea-ice conditions. Planetary waves propagating from the troposphere to the stratosphere in experiments using the ocean-surface conditions in 2002 and 2019 were markedly enhanced compared to those in experiments using climatological ocean conditions, owing to the enhancement of the zonal wavenumber-2 component in August 2002 and the wavenumber-1 component from August to November 2019. The distribution function from the ensemble members of the Antarctic polar-vortex intensity shifted to a weaker side in the 2002 and 2019 experiments relative to that of the climatological ocean conditions. The planetary wave propagation to the stratosphere was more enhanced in 2019 than in 2002 from austral winter to spring. This result is consistent with the weakening of the Antarctic polar-vortex intensity in the 2019 experiment relative to the 2002 experiment. These results suggest that the SSWs in 2002 and 2019 are closely related to the ocean surface conditions in these years through wave propagation in the troposphere and stratosphere.