Downstream weather impacts associated with atmospheric blocking over the northeast Pacific

Relationships between atmospheric blocking over the Alaskan region of the northeast Pacific, referred to as Alaskan blocking, and weather extremes over North America during boreal winter 1979-2000 are examined. A total of 37 atmospheric blocking events are identified with durations ranging from 8 to 25 days and a mean duration of 11.3 days. A total of 15.6% of the days during the boreal winter belonged to an Alaskan blocking event. The number of blocked days over the Alaskan region was found to be sensitive to the phase of the ENSO cycle with a reduced ( increased) number of blocked days during El Nino (LaNina/neutral) winters. The average number of blocked days during El Nino winters was 12, compared with 31.2 and 27 for neutral and La Nina winters, respectively. The mature Alaskan block possesses characteristics, which are typical of blocking episodes, including the equivalent barotropic structure of the blocking anticyclone, the meridional flow both upstream and downstream of the block, the equatorward shift of the Pacific storm track, downstream development of 500-hPa geopotential height, and sea level pressure anomalies over North America. The surface temperature analysis revealed a significant shift in the daily mean surface temperature distribution during Alaskan blocking toward colder temperatures in the region extending from the Yukon southeastward to the southern plains of the United States, associated with a reduced variance of surface temperatures. Over extreme western Alaska there is a shift in the daily mean surface temperature distribution toward warmer temperatures. The shift toward colder ( warmer) daily mean surface temperatures is also accompanied by a shift in the tails of the distribution toward more extreme cold ( warm) days in these two regions. During Alaskan blocking, the regions of southern California, the Southwest, and the Intermountain West all possess a higher frequency of heavy precipitation days when compared with the long-term winter climatology. Over the eastern half of the United States, the Ohio Valley and the southeast regions experience a greater percentage of heavy precipitation days during Alaskan blocking.