  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.
