Relationships between ENSO and Daily Precipitation in the United States

A classification of cold (La Nina) and warm (El Nino) episodes was compiled by CPC to provide a season-by-season breakdown of conditions in the tropical Pacific (Table 1); hereafter this classification will be referred to as CPC's official list. The process of classification was primarily subjective using reanalyzed sea surface temperature analyses produced at the National Centers for Environmental Prediction / Climate Prediction Center and at the United Kingdom Meteorological Office. CPC attempted to classify the intensity of each event by focusing on a key region of the tropical Pacific (along the equator from 150o W to the date line). In the Table weak periods are designated as C- or W-, moderate strength periods as C or W, and strong periods as C+ or W+. In this classification the intensity of El Nino/La Nina episodes changes from season to season. This is because the part of the year for which the central and eastern equatorial Pacific SST's differ significantly from normal varies considerably from episode to episode. Details of the CPC classification strategy are found on the CPC Web Site (http://nic.fb4.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.html).

There is little disagreement about which ENSO episodes are the moderate and strong ones. In general, however, there is more uncertainty about the weak episodes. For this reason, and for simplicity we use 3 categories for the composites presented in sections 3 and 5: moderate/strong El Nino, moderate/strong La Nina and neutral (including weak El Nino and La Nina episodes as classified in CPC's official list). Here we argue that moderate/strong El Nino and La Nina episodes have a discernable effect on United States precipitation, while neutral episodes do not. The advantage of such a classification is that it accounts for all of the years in the record. Ideally, the weak episodes would be broken out as a separate category, but the additional level of complexity appears to be unjustified.