Title: Extreme Weather Events over Parts of the Northern Hemisphere during Winter 2009-2010




Abstract: 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. The winter of 2009-2010 over the NH is a textbook example of how tropical and midlatitude flow interactions govern intraseasonal variability.




Introduction by Lodovica Illari