(Blog Written by Donald Sutherland)
A thought concerning the prospects for winter blocking…
The June 2012 AO averaged -0.672 and the July AO has averaged +0.09 through July 30. Assuming the AO averages < 0 for the summer (June-August) timeframe, odds would favor the AO’s averaging < 0 for the upcoming winter (December-February).
Since 1950, there were 38 cases during which the summer AO averaged < 0. 26/38 (68.4%) saw the winter AO also average < 0.
In cases when the summer AO averaged < 0, a winter AO < 0 was 1.4 times more likely than an AO > 0 during El Niño events. Winter 2009-10 was the most recent El Niño case that saw a negative winter AO.
In contrast, during those 38 cases, a winter AO > 0 was 2.9 times more likely than an AO < 0 during a La Niña. Winter 2011-12 was the most recent La Niña case that saw a positive winter AO.
In sum, if the summer 2012 AO averages < 0 and the upcoming winter experiences an El Niño (or even neutral ENSO conditions), there should be more blocking than last winter. Moreover, a blocky winter would appear more likely than not. Nevertheless, things are not yet cast in stone. Such developments as early-season Siberian snow cover expansion could provide a hint as to how things will turn out during Winter 2012-13.