Long-term variation of Arctic Sudden Stratospheric Warmings (SSW) and potential causes

Utilizing the European Centre for Medium-Range Weather Forecast (ECMWF) Reanalysis v5 (ERA5), for the first time, we have confirmed close links among Sudden Stratospheric Warmings (SSWs) in the Northern Hemisphere (NH), the polar vortices, and stratospheric Planetary Waves (PWs) by analyzing and comparing their trends. Interestingly, within overall increasing trends, the duration and strength of SSWs exhibit increasing and decreasing trends before and after the winter of 2002, respectively. To reveal possible physical mechanisms driving these trends, we analyzed the long-term trends of the winter (from December to February) polar vortices and of stratospheric PWs with zonal wave number 1. Notably, our results show that in all three time periods (the entire period of 41 winters, 1980 to 2020, and the two subperiods — 1980−2002 and 2002−2020) enhancing SSWs were always accompanied by weakening winter polar vortices and strengthening polar PWs like Stationary Planetary Waves (SPWs) and 16-day waves, and vice versa. This is the first proof, based on ERA5 long-term trend data, that weakening polar vortices and enhancing stratospheric PWs (especially SPWs) could cause an increase in SSWs.