A Comparison of the Winters of 2013-2014 and 2014-2015

Observations since the 1940's have shown that the preferential formation zones for the coldest air masses in the Northern Hemisphere are generally located near the Northwest Territories and Yukon of Canada as well as the Siberian regions of Russia. However, considerable inter-annual and decadal variability exists with recent decades (the past 30 years) indicating that the coldest air masses have been more commonly found over northwestern Canada and/or Greenland. The winter of 2013-2014 however deviated significantly from climatology in that the coldest air masses consistently formed over Canadian provinces that are east of the usual formation zones, in association with a polar vortex that generally resided in the vicinity of Hudson Bay. In fact, the persistence of the cold pool in the vicinity of Hudson Bay was unprecedented in the 66 year record of the reanalysis. Meanwhile, most of the rest of the Northern Hemisphere was experiencing near record warmth associated with similarly persistent flow regimes. The persistence of the flow regime over North America was seemingly related to an extreme warm pool over the east portion of the North Pacific which served as an anchor for a strongly diffluent jet exit region at the end of an abnormally retracted Pacific basin jet. The warm pool in conjunction with extremely favorable dynamics for ascent in the poleward jet region resulted in persistent storminess and diabatic heating that in turn was responsible for developing and anchoring a strong ridge along the western seaboard of North America. As this warm pool has propagated eastward toward the North American coast-line for this winter, we will investigate whether there is a corresponding phase shift in the pattern relative to 2013-2014. We will also investigate an apparent warm bias in medium range weather prediction in the vicinity of Hudson Bay for the winter of 2014-2015.