Characteristics, trend, and precursors of extreme cold events in northwestern North America

The cold extremes along with record snowfalls in North America (NA) have attracted great concern for decades. In this study, we select 23 wintertime cold extremes in northwestern NA (NWNA) from 1981 to 2018 and find a significant weakening trend of their integrated yearly intensities. Strong northerlies and cold advections over NWNA associated with the cold events are driven by the intensified and southward Beaufort anticyclone at lower troposphere and the enhanced blocking high at higher levels. A strong low-pressure anomaly over NWNA extends from lower troposphere to stratosphere centered at around 250-300hPa, where the jet stream becomes stronger and moves northwestwards. Meanwhile, the cold anomaly under the low system is deep and more prominent at the surface of NWNA. There is a significant meridional dipole structure of height and temperature anomaly in mid-high latitudes, indicative of the negative phase of Arctic Oscillation. Results further suggest that the La Nina-like sea surface temperature anomalies may serve as a precursor for the cold extremes in NWNA, triggering a Pacific North America-like wave train including an active negative height anomaly center over NWNA, which is linked with the cold anomalies extending downward to the surface. However, the sea ice concentration anomalies in Arctic region and the snow cover anomalies in NA continent during early autumn are likely to exert a minimal influence on the wintertime cold extremes in NWNA.