From the American Geophysical Union's EOS, November 20:
An exceptionally strong stratospheric polar vortex coincided with a record-breaking Arctic Oscillation pattern and ozone destruction during the 2019–2020 winter season.
The winter of 2019–2020 in the Northern Hemisphere was one of extremes. The massive region of cold polar air encircled by stratospheric winds, known as the stratospheric polar vortex, was particularly strong, keeping the frigid air whirling above the polar region and leading to a very mild winter in many regions farther south. The strong polar vortex coincided with a record-breaking positive Arctic Oscillation circulation pattern and record low ozone levels in the Arctic that lasted into spring. In a review, published as part of an AGU special collection, Lawrence et al. outline the unique conditions that allowed this “truly extraordinary” winter season to arise.
The researchers compared the unusual 2019–2020 winter season with historical data and found that zonal (east-to-west) wind measures suggest it was the strongest polar vortex since the satellite era began roughly 4 decades ago. The strength of the polar vortex varies from year to year and depends on many factors. Atmospheric waves originating in the troposphere often propagate into the stratosphere, breaking up and weakening the westerly circulation of the polar vortex.
But atmospheric wave activity was relatively weak this year. The authors suggest that the combination of this weak activity and multiple downward wave-coupling events, in which atmospheric waves propagating upward were reflected back into the troposphere, helped to cool and strengthen the polar vortex....MORE
A related paper, "Seasonal Forecasts of the Exceptional Northern Hemisphere Winter of 2020" is open access and worth a look should the reader's interests include forecasting.
