The different stratospheric influence on cold-extremes in Eurasia and North America

Marlene Kretschmer,Vivien Matthias,Jakob Runge,Dim Coumou,Judah Cohen

doi:10.1038/s41612-018-0054-4

Abstract

The stratospheric polar vortex can influence the tropospheric circulation and thereby winter weather in the mid-latitudes. Weak vortex states, often associated with sudden stratospheric warmings (SSW), have been shown to increase the risk of cold-spells especially over Eurasia, but its role for North American winters is less clear. Using cluster analysis, we show that there are two dominant patterns of increased polar cap heights in the lower stratosphere. Both patterns represent a weak polar vortex but they are associated with different wave mechanisms and different regional tropospheric impacts. The first pattern is zonally symmetric and associated with absorbed upward-propagating wave activity, leading to a negative phase of the North Atlantic Oscillation (NAO) and cold-air outbreaks over northern Eurasia. This coupling mechanism is well-documented in the literature and is consistent with the downward migration of the northern annular mode (NAM). The second pattern is zonally asymmetric and linked to downward reflected planetary waves over Canada followed by a negative phase of the Western Pacific Oscillation (WPO) and cold-spells in Central Canada and the Great Lakes region. Causal effect network (CEN) analyses confirm the atmospheric pathways associated with this asymmetric pattern. Moreover, our findings suggest the reflective mechanism to be sensitive to the exact region of upward wave-activity fluxes and to be state-dependent on the strength of the vortex. Identifying the causal pathways that operate on weekly to monthly timescales can pave the way for improved sub-seasonal to seasonal forecasting of cold spells in the mid-latitudes.

Highlights

Variability in the stratospheric polar vortex in boreal winter can influence the tropospheric circulation and is an important source of predictability on sub-seasonal to seasonal (S2S) timescales.[1,2]extremely weak polar vortex states, such as sudden stratospheric warmings (SSW), can be impactful to societies as they are often associated with large-scale cold-air outbreaks in the densely populated mid-latitudes.[3]
We identified (1) a pattern of extremely weak polar vortex states linked to cold-spells over northern Eurasia and (2) a pattern with increased geopotential height over eastern Siberia associated with coldextremes over the Northeastern US. The former is in agreement with the well-documented relationship between a strongly disrupted polar vortex, a downward propagating negative NAM3,9 and Eurasian cold spells
Combining lagged composites and Causal effect network (CEN), we demonstrate that cluster 4-type events are linked to a negative Western Pacific Oscillation (WPO) and North American cold-spells via reflected upward-propagating waves over eastern Siberia, as was hypothesized in previous studies.[8,16,17,18,19]

Summary

Introduction

Variability in the stratospheric polar vortex in boreal winter can influence the tropospheric circulation and is an important source of predictability on sub-seasonal to seasonal (S2S) timescales.[1,2]extremely weak polar vortex states, such as sudden stratospheric warmings (SSW), can be impactful to societies as they are often associated with large-scale cold-air outbreaks in the densely populated mid-latitudes.[3]. SSWs have been extensively studied and have been classified by their spatial properties[4] (split versus displaced events), by the dominant type of wave forcing[5] (wave 1 versus wave 2) or by their intensity[6] (major versus minor warmings). These metrics describe the stratospheric extreme events itself but do not necessarily capture differences in the tropospheric response.[7,8]. The circulation anomalies, characterized by a negative phase of the Northern

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Conclusion