Sorting sudden stratospheric warmings with the downward tropospheric influence using ERA5 and CESM2-WACCM

Abstract

Sudden Stratospheric Warming events (SSWs) can have a downward impact on the troposphere, but the mechanism remains uncertain. This study focuses on classifying SSWs based on their tropospheric responses and documenting associated dynamical characteristics. Using the ERA5 data and CESM2-WACCM outputs, 52 SSWs are identified in ERA5 and 273 in CESM2-WACCM, with 33 and 119 downward-propagating SSWs (DWs), respectively. The DWs are classified into three types based on cold surges over Eurasia (EA), North America (NA), and both (BOTH), respectively. Both DWs and non-downward-propagating SSWs (NDWs) weaken and deform the polar vortex, but DWs induce stronger negative Northern Annular Mode (NAM) and North Atlantic Oscillation (NAO) responses. For DWs, the anomalous high develops in the polar region, which deflects to lower latitudes, consistent with the frequent appearance of the polar high and the midlatitude blockings. The shape of the anomalous polar high varies with the DW type, and the extension and shift of the anomalous high lead to different surface responses. The DWs are also accompanied by a southward shift of the precipitation belt, especially over the oceanic and coastal regions. The relatively weaker tropospheric impact of NDWs may be partly explained by their weaker stratospheric disturbance amplitude. The three types of DWs differ in spatiotemporal evolutions of the NAM and NAO pattern, different forcing by planetary waves, and varying ratios between displacement and split. This study reveals the diversity of the DWs and distinguishes their potential impacts on both continents in the Northern Hemisphere.