Influence of Preconditioned Stratospheric State on the Surface Response to Displacement and Split Sudden Stratospheric Warmings

Sudden stratospheric warmings (SSWs) can affect tropospheric weather and climate, while the factors affecting the surface response to SSWs are not fully understood. This study shows the important effects of preconditioned stratospheric state on the surface response to displacement and split SSWs using ERA5 reanalysis and historical simulations from 12 Coupled Model Intercomparison Project 6 (CMIP6) models. Both ERA5 and CMIP6 models show that preconditioned stratospheric signals (e.g., positive meridional potential vorticity gradient anomaly in the upper stratosphere) can promote the upward propagation of stratospheric planetary waves by decreasing the negative refractive index frequency before SSWs. The modulation of strong stratospheric preconditions causes larger upper-stratospheric anomalies for subsequent SSWs and prominent barotropic downward impacts for split SSWs. Meanwhile, the strong preconditioned signals affect the preexisting tropospheric variability, masking the downward impacts of displacement SSWs. The surface response is thus weaker than that to displacement SSWs with weak preconditions.