Pacific warm (El Niño) and cold (La Niña) episodes. Warm (ENSO) episodes sources: Rasmusson and Carpenter 1983, Monthly Weather Review; Ropelewski and Halpert 1987, Monthly Weather Review. Cold episode sources Ropelewski and Halpert 1989, Journal of Climate; Climate Diagnostics Bulletin. For episodes since 1950 the intensity is based on the pattern and magnitude of SST anomalies in the tropical Pacific. Since both warm and cold episodes tend to reach their peak during the Northern Hemisphere winter, they tend to span two different calendar years. In some cases years are listed as both cold and warm episode years (e.g., 1925, 1939 and 1976). In those cases there was a transition between the two extreme states during the course of the year.