An Isentropic Mass Circulation View on the Extreme Cold Events in the 2020/21 Winter

Three extreme cold events successively occurred across East Asia and North America in the 2020/21 winter. This study investigates the underlying mechanisms of these record-breaking persistent cold events from the isentropic mass circulation (IMC) perspective. Results show that the midlatitude cold surface temperature anomalies always co-occurred with the high-latitude warm anomalies, and this was closely related to the strengthening of the low-level equatorward cold air branch of the IMC, particularly along the climatological cold air routes over East Asia and North America. Specifically, the two cold surges over East Asia in early winter were results of intensification of cold air transport there, influenced by the Arctic sea ice loss in autumn. The weakened cold air transport over North America associated with warmer northeastern Pacific sea surface temperatures (SSTs) explained the concurrent anomalous warmth there. This enhanced a wavenumber-1 pattern and upward wave propagation, inducing a simultaneous and long-lasting stronger poleward warm air branch (WB) of the IMC in the stratosphere and hence a displacement-type Stratospheric Sudden Warming (SSW) event on 4 January. The WB-induced increase in the air mass transported into the polar stratosphere was followed by intensification of the equatorward cold branch, hence promoting the occurrence of two extreme cold events respectively over East Asia in the beginning of January and over North America in February. Results do not yield a robust direct linkage from La Niña to the SSW event, IMC changes, and cold events, though the extratropical warm SSTs are found to contribute to the February cold surge in North America.