Dynamic synergistic effects of large-scale three-dimensional circulation anomalies on the record-breaking cold wave in February 2021 in North America

In contrast to previous studies that mainly focused on the influence of the polar vortex in the horizontal circulation on the local cold air, this study uses the three-pattern decomposition of global atmospheric circulation (3P-DGAC) to reveal the dynamic synergistic effects of horizontal circulation, local meridional circulation, and local zonal circulation on cold waves, by examining the record-breaking extreme cold wave (ECW) in February 2021 across North America. In terms of horizontal circulation, this ECW is preceded by the strengthening of blocking highs over the North Pacific and North Atlantic that force the tropospheric polar vortex edge toward North America, resulting in the development of the North American trough. The strongly northwesterly wind caused by the North American trough strengthens the local meridional circulation over North America, transporting cold Arctic air to the mid-low latitudes. Meanwhile, the local meridional circulation over North America strengthens the ascending motion and subsequently promotes the development of local zonal circulation over the North Atlantic. In turn, the developed local zonal circulation reinforces the blocking highs over the two oceans, slowing the movement of the trough and ridge systems eastward and leading to a long duration for this ECW. The composite analysis further conforms that the vertical sinking motion corresponding to blocking highs over the two oceans is mainly contributed by the local zonal circulation, and the strong local zonal circulation over the North Atlantic could lengthen the duration of the cold wave by strengthening the blocking highs. This study suggests that 3P-DGAC is an effective tool for investigating the three-dimensional structural characteristics of anomalous circulations in ECWs.