The Northern Hemisphere (NH) planetary-scale circulation during winter 2010-2011, although somewhat less extreme than winter 2009-2010, was characterized by an unusual combination of persistent high-latitude blocking and southward–displaced storm tracks, manifest by a strongly negative Arctic Oscillation (AO), through mid-January. The high-latitude blocking activity and southward-displaced storm tracks supported episodic cold-air outbreaks and enhanced storminess over parts of midlatitude eastern Asia, eastern North America, and western Europe, and anomalous warmth over northeastern Canada and Greenland where delayed sea ice formation and ice thickening occurred. The high-latitude blocking activity occurred during a moderate La Nina event in contrast to the moderate El Nino event that prevailed during winter 2009-2010 when similar large-scale circulation anomalies were noted over the NH. Beginning in the latter half of January 2011 and continuing into early February 2011, the phase of the AO changed from strongly negative to strongly positive while the phase of the Pacific North American (PNA) pattern switched from near neutral to moderately negative. This observed double phase change in the AO and PNA teleconnection indices, indicative of intraseasonal flow variability, occurred during a tumultuous period that was characterized by extreme weather events over parts of North America.

This presentation will document how the structure and evolution of large-scale NH circulation anomalies on intraseasonal time scales during winter 2010-2011 governed, and were in turn influenced by, extreme weather-producing synoptic-scale disturbances during the two circulation regimes described above. Emphasis will be placed on showing how individual episodic weather events (e.g., recurving and transitioning western Pacific tropical cyclones, episodic diabatically driven upper-level outflow from MJO-related deep convection, and recurrent western North Atlantic storminess) contributed to the formation of significant and persistent large-scale circulation and jet-stream anomalies. Emphasis will also be placed on examining how these large-scale circulation anomalies helped to govern the location of the storm tracks and the magnitude of regional temperature and precipitation anomalies (e.g., intense cyclones over the western U.S., persistent cold and freezes in Florida, cold outbreaks over Mexico, and frequent snowstorms in parts of the southern U.S.). Between late January and late February the phases of the AO and PNA reversed. Accompanying this double phase reversal, major snow and ice storms and extreme cold during the first part of this period over much of the U.S. gave way to a dramatic warm-up in the second half of this period. Intraseasonal variability during the late January to late February period and its linkage to the occurrence of multiple extreme weather events will be discussed.