LAUSR

Siberia saw a heat wave of extreme monthly temperatures of +6°C anomalies from January through May 2020, culminating with near daily temperature records at the Arctic station of Verhojansk in mid‐June. This was a major Arctic event. The proximate cause for the warm extremes from January through April was the record strength of the stratospheric polar vortex (SPV) and tropospheric jet stream. The SPV and high geopotential heights to the south combined to provide strong zonal winds from the west that reduced the potential penetration of cold air from the north. An index of vortex strength is the Arctic Oscillation (AO); averaged over January–April, the AO set extreme positive records in 1989, 1990, and 2020 (baseline starting in 1950). The strength and stability of the SPV over the central Arctic contributed to the winter–spring persistence of the heat wave in Siberia. May–June temperatures were related to high tropospheric geopotential heights over Asia. An open question is whether these dynamic events are becoming more persistent. Such record events will not occur every year but one can expect that they will occasionally naturally reoccur over the next decades due to internal atmospheric variability in addition to a continued global warming background.