The winter of 2009/10 had anomalously large snowfall in the central parts of the US and in northwestern Europe, which some of you may remember. December’s snowcover in the contiguous US was the greatest ever for that month and Washington D.C. had to be closed down for a weak.
Washington D.C., December 19, 2009. Here
As mentioned in the beginning of this blog, the NAO exerts a strong influence on wintertime climate across the North Atlantic basin, and a negative index NAO give cold temperature anomalies in the eastern US and northern Europe. ENSO, with its warm phase El Niño, also influences wintertime climate over the US by a southward displaced stormtrack. A paper by Seageret al. (2010) takes a closer look at the winter of 2009/10 and finds that a combination of a negative NAO and an El Niño event was causing the cold temperatures and the anomalous amounts of snow experienced in parts of the US and Europe that winter. The changes in storminess associated with El Niño explained snow anomalies in western, central and southern US, while the negative NAO provided sufficient cold air for the precipitation to fall as snow in the northeastern US and northern Europe.In a study published in Science in 2001, Thomson and Wallace find that the different phases of the NAO (NAM, see link) are marked by distinct differences in the frequency distribution of significant weather events in the Northern Hemisphere that impact human activities. They report that cold events (daily minimum temperatures dropping below a specified threshold, frozen precipitation) occur with much greater frequency over North America and Europe (and other places, see table below) during negative index days, increasing the risk of frost damage and the frequency of snowfall.
Significant weather events associated with high and low NAM index days. As you can see most of the events occur with much higher frequency during NAM- (negative index).