The General Nature of ENSO and Its Climatic Effects

ENSO is the acronym for El Niño-Southern Oscillation despite the fact that the Southern Oscillation is composed of swings between El Niño and the almost opposite La Niña events. El Niño is a warming of the Pacific Ocean between South America and the international date line, centered on the equator, and typically extending several degrees of latitude to either side of the equator. La Niña exists when cooler than usual ocean temperatures occur in the same area (Trenberth 1997; Kelly Redmond, pers. comm., 2000). Both El Niño and La Niña affect the atmosphere as well as the ocean. There are many ways of measuring ENSO variability. One of the most common is the use of the Southern Oscillation Index (SOI), which represents the standardized Tahiti-Darwin sea level pressure (SLP) anomaly. Data on this and other climatic variables may be found at http://www.cpc.ncep.noaa.gov/ products/.

The general climatological effects of El Niños and La Niñas worldwide have been well established. El Niños in North America are generally associated with higher than average precipitation in the Southwest and Southeast of the country and lower than average precipitation in the Pacific Northwest (Ropelewski and Halpert 1986; Kiladis and Diaz 1989,1992). Trenberth and Caron (2000) have updated the climatology and provide new global maps of ENSO effects on climate. Among other things, such maps and high resolution data emphasize the fact that the LTER sites by no means provide a high-resolution spatial climatology. Rather, the role of the LTER sites is to provide detailed ecological responses. There are many internet web sites that describe the ENSO phenomenon and give almost worldwide coverage of its climatic results (e.g., http://www.pmel.noaa.gov/tao/elnino/nino-home.html). Occasionally, very intense super El Niños occur as in 1982-1983 and 1997-1998. El Niños and La Niñas can be forecasted a season or two in advance; because they tend to have similar climate effects in known parts of the world, this permits climate forecasts to be made. Both the National Oceanographic and Atmospheric Administration's (NOAA) Climate Prediction Center (CPC) and the International Research Institute for Climate Prediction (IRI) provide these forecasts. Model nested approaches can now provide accurate seasonal climate and stream-

flow forecasts for some areas that include LTER sites. Model nested results have been given for the Pacific Northwest (PNW) (Leung et al. 1999). General forecasts are also available during intense El Niño events for the coterminous United States (Barnston et al. 1999) and globally (Mason et al. 1999). These forecasts will be very useful for the design of future ecological experiments at LTER sites and are already being used in many areas of human societal systems (Buizer et al. 2000). However, K. T. Redmond (pers. comm., 2000) makes the following points with respect to the predictability of ENSO-related climate. Not every El Niño produces the same climatic effect. La Niñas have a more consistent signal, in general, than El Niños. The relations between an ENSO "cause" and a climatic "effect" are not perfect, in part because other things are happening in the climate system. Super El Niños sometimes do not show the expected resulting climatic patterns in some locations. Resulting climatic patterns for large El Niños may differ in some ways from those of typical El Niño patterns. The relationship between extreme ENSO events and climatic results is lagged. In general, the best associations are between the summer/autumn SOI and the following winter climate.

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