Weather Regimes in Northern Eurasia: Statistics, Predictability and Associated Weather Anomalies

Abstract

A weather regimes approach is applied to study large-scale circulation patterns over the North Eurasian sector (NE, defined as 0–180° E and 40–80° N domain) during the 1940–2022 period using ERA5 reanalysis data. We identified four NE weather regimes (WRs) both for boreal winter and summer seasons using a k-means standard cluster analysis method for daily geopotential height fields (z500). Whereas the winter NE WRs do not have any significant long-term changes in occurrence, three summer NE WRs are found to have statistically significant linear trends of occurrence over the studied period, the most notable of which is a regime associated with anticyclonic activity centered over the Ural Mountains called Summer Ural High, which has a statistically significant trend of +2.4 days of seasonal occurrence per decade. Loops of statistically significant likely transitions are found for the NE WRs in both seasons, showing that the NE WRs evolution follows preferred paths and is not purely random. NE WRs’ seasonal occurrence is found to change depending on the phase of the El Nino oscillation and Northern Hemisphere sea-ice area anomalies in preceding seasons. El Nino events in autumn are associated with an increased occurrence of the Winter Scandinavian Blocking regime and a decreased occurrence of the Winter North Eastern High regime. Negative sea-ice area anomalies in autumn are associated with an increased occurrence of the Winter Ural High and a decreased occurrence of the Winter Scandinavian Low regime. NE WR’s are associated with considerable, up to several times, changes in probability of temperature and precipitation extremes over the NE region.

Keywords

• weather regimes
• northern Eurasia
• k-means
• cluster analysis
• circulation patterns
• weather extremes