Winter 2009/10: A case study of an extreme Arctic Oscillation event and a skillful climate prediction

Winter 2009/10 made headlines for extreme cold and snow in most of the major population centers of the industrialized countries of the Northern Hemisphere (NH). The major teleconnection patterns of the Northern Hemisphere, El Niño/Southern Oscillation (ENSO) and the Arctic Oscillation (AO) were of moderate to strong amplitude, making both potentially key players during the winter of 2009/10. The dominant NH winter circulation pattern can be shown to have originated with a two-way stratosphere-troposphere interaction forced by Eurasian land surface and lower tropospheric atmospheric conditions during autumn. This cycle occurred twice in relatively quick succession contributing to the record low values of the AO observed. Using a skillful winter temperature forecast, it is shown that the AO explained a greater variance of the observed temperature pattern across the extratropical landmasses of the NH than did ENSO. Further, and somewhat counter-intuitively, the severe cold winter weather may be attributed to boundary forcing changes consistent with an overall warming planet. A warmer atmosphere can hold more moisture and as the interior of the NH continents cool in fall, this can lead to increased snowfall. More extensive fall snow cover contributed to the extreme negative AO observed during the winter of 2009/10.