Winter Monthly Mean Atmospheric Anomalies over the North Pacific and North America Associated with El Niño SSTs

Abstract

The characteristics of winter monthly mean extratropical circulation associated with El Niño, including precipitation and surface temperature over the United States, are examined for nine El Niño events during 1950?94. Precipitation and surface temperature over the United States, also the 500-mb geopotential height and sea level pressure over the North Pacific and North America, are significantly different between early winter (November and December) and late winter (January to March). The typical El Niño-related U.S. precipitation and surface temperatures identified in many previous studies, as well as the Pacific?North American (PNA) circulation pattern, emerge in January and persist through February and March. The PNA patterns during these late winter months are coupled both with the tropical El Niño sea surface temperature (SST) variation and with the North Pacific SST variation. In contrast, the PNA patterns in the early winter months correlate only with the North Pacific SST. The tendency for the PNA pattern to occur during El Niño years is much less in early winter months than in late winter months. An ensemble analysis of 12 45-yr (1950?94) integrations of the National Center for Atmospheric Research Community Climate Model forced by the observed time-varying SST shows that the model 500-mb heights display a PNA-like pattern in both early and late winters of El Niño. The ensemble model response to the El Niño SST is thus unable to reproduce the observed differences in the extratropical atmospheric circulation between early and late winter months.