Extreme weather over parts of the Northern Hemisphere during winter 2009–2010

The large-scale circulation over the Northern Hemisphere (NH) during winter 2009-2010 was dominated by a strongly negative (3-5 standardized anomalies) Arctic Oscillation (AO). Blocking that began over the North Pacific in early December in conjunction with “ridge-building” ahead of recurving and transitioning tropical cyclones over the western North Pacific culminated in the formation of a strong high-latitude ridge over Alaska. High-latitude blocking subsequently expanded to the North Atlantic where it continued intermittently into February. The winter of 2009-2010 was noteworthy for extreme weather over North America (storminess over the southern U.S., epic snowstorms in parts of the eastern U.S, damaging cold in Florida), western Europe (persistent snow cover and cold weather), and eastern Asia (extreme cold) that occurred in conjunction with the strongly negative AO teleconnection pattern, the associated high-latitude blocking events, and a moderate El Nino event. The purpose of this presentation will be to examine how intraseasonal variaibility arising from the strongly negative AO pattern and associated persistent high-latitude blocking contributed to episodic extreme weather events that helped to determine the location, sign, and magnitude of the seasonal temperature and precipitation anomaly patterns during winter 2009-2010.

Several noteworthy extratropical cyclones (ECs) contributed significantly to the observed atmospheric flow variability on intraseasonal time scales during winter 2009-2010. One example was the EC known as the “sea-to-shining-sea” storm. This storm formed in a trough that deepened strongly over western North America in response to downstream development to the east of the aforementioned blocking ridge over Alaska. It subsequently traversed the U.S. from 7-9 December, creating extreme weather all along its path, and setting all-time minimum sea level pressure records over parts of the western U.S. As this storm exited the east coast of North America, it and a predecessor EC, triggered the onset of high-latitude blocking ridge development over the North Atlantic that was followed by an impressive downstream cold-air outbreak over western Europe beginning in mid-December.

The strong North Atlantic blocking ridge that formed by mid-December was reinforced multiple times and forced to retrograde to Greenland in early January in conjunction with repeated EC events over the western and central North Atlantic with continuing disruptive snow and cold over much of western Europe. Over eastern North America, cold air masses were forced unusually far south with prolonged and disruptive freezes occurring in Florida. The strongly negative AO pattern collapsed abruptly after mid-January as a strong 100+ m/s jet stream developed over the North Pacific and extended eastward toward California in what appeared to be a classic El Nino-onset signal. Subsequently, the flow amplified downstream across the North Pacific and set the stage for a strong trough with an embedded 100 m/s jet streak to traverse the eastern North Pacific and western North America from 20-22 January. This strong trough was responsible for the occurrence of widespread all-time minimum sea level pressure records (previously broken in December) and damaging high winds over the Southwest, consistent with an El Nino-onset signal. However, surprisingly, the AO reverted to a strongly negative pattern again by very late January. The subsequent combination of a strong North Atlantic blocking pattern with an active southern stream jet across the U.S. was followed by a month-long period of record-breaking snowstorm events in parts of the eastern U.S. in late January and the much of February.