Earlier this year, many weather-watchers were theorising that unusual conditions in the Arctic atmospheric pressure patterns could be behind the deeply cold and snowy winter :-
The anti-science mob just jumped up and down and yelled at it, calling it “global cooling”, but that’s what you’d expect from a one-dimensional crowd trying to “snow job” the truth about Global Warming.
However, it has now been reliably demonstrated that a combination of a moderately strong El Nino and a very low North Atlantic Oscillation (NAO) index locked in cold conditions and higher than usual precipitation leading to general whiteout in medium latitudes :-
“Abstract : Winter 2009/10 had anomalously large snowfall in the central parts of the United States and in northwestern Europe. Connections between seasonal snow anomalies and the large scale atmospheric circulation are explored. An El Niño state is associated with positive snowfall anomalies in the southern and central United States and along the eastern seaboard and negative anomalies to the north. A negative NAO causes positive snow anomalies across eastern North America and in northern Europe. It is argued that increased snowfall in the southern U.S. is contributed to by a southward displaced storm track but further north, in the eastern U.S. and northern Europe, positive snow anomalies arise from the cold temperature anomalies of a negative NAO. These relations are used with observed values of NINO3 and the NAO to conclude that the negative NAO and El Niño event were responsible for the northern hemisphere snow anomalies of winter 2009/10.”
“Citation: Seager, R., Y. Kushnir, J. Nakamura, M. Ting, and N. Naik (2010), Northern Hemisphere winter snow anomalies: ENSO, NAO and the winter of 2009/10, Geophys. Res. Lett., 37, L14703, doi:10.1029/2010GL043830.”
Now, I must admit to a little confusion, because when I came to draw the graph at the top of this post, I found that the AO Oscillation Index (Arctic Oscillation, or Northern Annular Mode (NAM)) was the value that plunged the depths.
Before anybody shouts at me, I know the NAO and the AO are closely related, but from the data I retrieved, it looks like the AO was the strongly anomalous index.
Anyway, the headlines are bouncing around :-
Here’s another view of the NAO (“a timeseries of the winter (December to March average) of the Jones et al. NAO index”) :-
This demonstrates the very low Oscillation values for the Winter of 2009/2010 over the whole instrumental record.
[ Interesting additional note : in the graph at the top of this post, you can see the “year without winter” mild winter of 2006/2007 could be correlated with the strongly positive Arctic Oscillation / North Atlantic Oscillation. ]