Precipitation anomalies in the United States associated with a finer classification of El Niño events

Abstract

  A finer classification of El Niño events is attempted. Each year was examined to check whether it had an El Niño (EN) and or a Southern Oscillation Index SOI minimum (SO)  and or warm (W) or cold (C) equatorial eastern Pacific sea surface temperatures (SST). Several years were ENSOW, which were further subdivided into two groups viz. Unambiguous ENSOW where SOI minima and SST maxima were observed in the middle of the calendar year (May-Aug) and Ambiguous ENSOW where these conditions were obtained in the early or late part of the calendar year. Other El Niño events were of the type ENSO; ENW, ENC, EN. Years not having El Niño were of the types SOW, SOC, SO, W, C, the last one (C) containing all Anti-El Niños or La Niñas.

For the all India summer monsoon rainfall, Unambiguous ENSOW were overwhelmingly associated with droughts and the cold (C) events with floods. For the rainfall in USA during 1900-90, several regions showed distinct ENSO relationships, but different for different regions. Generally, there were good associations between (i) western USA/Great lakes droughts and ambiguous ENSOW, (ii) midwest/central USA floods and unambiguous ENSOW, (iii) south coastal plain/Gulf coast/eastern/southeastern droughts and both ambiguous and unambiguous ENSOW. La Niñas showed effects mostly opposite to those of unambiguous ENSOW, but not always, especially in the far west. For the same region, ENSO effects were often substantial and different (opposite) for the main rainy season and the pre and or post-rainy season. Many other details are also presented.