Simulation and projection of the sudden stratospheric warming events in different scenarios by CESM2-WACCM

Using different scenario experiments by the CESM2-WACCM model, the simulation and projection of the SSW and its impact on the near surface are explored as compared with the ERA5 and NCEP/NCAR reanalyses. The SSW frequency ranges from 4 to 7 per decade in CESM2-WACCM experiments (piControl, AMIP, historical, 1pctCO2, abrupt-4xCO2, SSP245, SSP585), comparable to ERA5 (6 per decade). Projected relative change in the displacement and split SSWs is much more uncertain due to the underestimation of the SSW frequency in the model and uncertainty in the greenhouse gas emission pathways. In all CO2 increase experiments, the downward propagation of annular stratospheric signals at short lags associated with displacement SSWs likely reinforces, whereas the downward coupling is projected to change little at long lags for displacements and at all lags for splits. CESM2-WACCM also projects a weakening of the wavenumber-2 forcing for split SSW in the future. Enhanced tropospheric negative annular mode response is projected at short lags for displacements in all future scenarios, and there is no significant enhancement of the negative NAO-like response to displacements at long lags. In contrast, the projected change in the tropospheric response to splits is zonally heterogenous at short lags, resembling a wave train pattern with the East Asian trough deepening, but the NAO-like response to splits at long lags is not projected to change significantly. The cold pattern over North Eurasia following displacement SSWs might expand further equatorward in the future projections, whereas cold anomalies over North America following splits might enhance in the future.