Precursory atmospheric circulations with Rossby wave trains leading to Eurasian extreme cold events

Abstract

This work examines precursory atmospheric circulations with various wave trains contributing to extreme cold weather over central Eurasia in boreal winter from 1979 to 2019. By conducting extended empirical orthogonal function (EEOF) on the preceding propagation circulation fields 2 weeks before the onset of extreme cold event (ECE) cases, three types of ECEs with different disturbance origins are classified and analysed. Type 1 denotes the positive phase of EEOF mode 1, shows as negative phase of Arctic Oscillation-like pattern.The outbreak of this type of ECE is affected by a wave train originating from Baffin Bay, where an anomalous anticylonic system persisted under a background of weakened westerlies over middle-high latitudes. Type 2 is picked from positive phase of EEOF mode 2, manifests as a developing blocking system that forms over Scandinavia and shifts to the Barents area. It is found that the blocking system is mainly strengthened by the downstream dispersion process of wave packets that are generated at the northern exit area of the North Pacific westerly jet, where exist anomalous cyclonic zonal wind shear and precipitation. Type 3 is selected from the negative phase of EEOF mode 2, which has a similar origin to type 2 but with the North Pacific jet exit more southward. Then, the generated wave packets propagate to Europe along the northerly jet stream over the North Atlantic, which acts as a waveguide and extends the wave train to downstream. In a word, these three types of precursory atmospheric wave train patterns that bring extreme cold anomalies to Eurasia possess diverse disturbing sources and downstream development mechanisms, and the essential role of the westerly jet is further highlighted. The results, which are investigated based on a quasi-biweekly time scale, may deepen our understanding of the atmospheric genesis of extreme weather and improve extended-range weather forecast.