ENSO relationship to the rainfall of Sri Lanka

Abstract

Each year during 1881–1990 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 Pactfic sea surface temperatures SST. Several years were ENSOW, which were further subdivided into two groups namely, unambiguous ENSOW where El Niño existed and SOI minima and SST maxima were in the middle of the calendar year (May–August), and ambiguous ENSOW where El Niño existed but the SOI minima and SST maxima were in the early or late part of the calendar year, not in the middle. Other El Niño events were of the type ENSO, ENW, ENC and EN. Some years not having El Niño were of the types SOW, SOC, SO, W and C, the last one (C) containing all anti-El Niños, i.e. La Niñas. Remaining years were termed as non-events. For all these years, the normalized rainfall deviations from the mean were examined for seven rainfall series, three in India and four in Sri Lanka. For the all India summer monsoon rainfall and Indian southern peninsula summer monsoon rainfall, unambiguous ENSOW years showed a very good association with droughts. The Sri Lanka southwest monsoon rains also showed a similar tendency; but the Sri Lanka second intermonsoon season rainfall showed a strong opposite tendency (floods instead of droughts). For other types of El Niño years, results were generally obscure. For C type events, results were opposite to those of unambiguous events, as expected. It is suggested that for obtaining composite maps based on El Niño years, only the unambiguous ENSOW years may be used. © 1998 Royal Meteorological Society