Extreme cold air outbreaks over the United States, the polar vortex, and the large‐scale circulation

Abstract

Extreme cold air outbreaks (CAOs) affect a large region of the midlatitudes during the winter months. An objective criterion was developed for identifying and ranking the 30 most extreme CAOs over the eastern United States using observational surface temperatures for November–March 1948–2002. A composite of these events shows that the tropospheric polar vortex is weaker than average prior to CAO onset, while the vortex strengthens following CAO onset. There is also very little correlation between the strength of the polar vortex and the intensity of CAOs. These results suggest that the polar vortex is generally not a useful forecasting tool by itself in predicting the timing and severity of a CAO event. Additionally, other large‐scale circulation features were examined to see whether CAO events occur in response or subsequent to teleconnection anomalies over the North Pacific and North Atlantic. It was found that the North Atlantic Oscillation (NAO) is negative prior to onset of the composite CAO but becomes positive approximately 5 days after onset. This suggests that there is some predictability in the Atlantic and European weather during CAOs on timescales of 3 to 6 days. The Pacific–North American (PNA) teleconnection pattern becomes increasingly positive during onset of the composite CAO. Even though the largest 500 mb height anomalies in the composite CAO do not align closely with the nodes of the PNA pattern, the deepening of the 500 mb trough over the eastern United States following CAO onset appears to be directly responsible for the increase in the composite PNA.