The association between stratospheric weak polar vortex events and cold air outbreaks

TL;DR

This study reveals the stages of tropospheric temperature changes linked to weak stratospheric polar vortex events, demonstrating their potential to improve cold air outbreak forecasts using re-analysis data and climate models.

Contribution

It identifies the temporal and spatial evolution of cold air outbreaks associated with weak polar vortex events, providing new insights into stratosphere-troposphere coupling for weather prediction.

Findings

Cold air outbreaks increase by 40-70% during weak vortex phases.

A precursor high-pressure anomaly over Northwest Europe signals upcoming cold events.

Climate models corroborate the observed link between vortex weakness and cold outbreaks.

Abstract

Previous studies have identified an association between near-surface temperature anomalies in the Northern Hemisphere and weak stratospheric polar westerlies. Large regions in northern Asia, Europe and North America have been found to cool in the mature and late stages of stratospheric weak vortex events. A substantial part of the temperature changes are associated with changes to the tropospheric Northern Annular Mode and North Atlantic Oscillation pressure patterns. The apparent coupling between the stratosphere and the troposphere may be of relevance for weather forecasting, but only if the temporal and spatial nature of the coupling is known. Here we show, using 51 winters of re-analysis data, that the tropospheric temperature development relative to stratospheric weak polar vortex events goes through a series of well-defined stages, including geographically distinct cold air outbreaks. At the inception of weak vortex events, a precursor signal in the form of a strong high-pressure anomaly is found over Northwest Europe. At the same time, long-lived and robust cold anomalies appear over Asia and Western Europe. A few weeks later, near the mature stage of weak vortex events, a shorter-lived cold anomaly emerges off the east coast of North America.The probability of cold air outbreaks in different phases of the weak vortex life cycle increases by 40-70 % in four key regions. This shows that the stratospheric polar vortex contains information that can be used to enhance forecasts of cold air outbreaks. 300-year pre-industrial control runs of 11 state-of-the-art coupled climate models corroborate our results.