On the two types of ENSO

Abstract

From 1950 to 1988, 10 ENSO (El Niño-Southern Oscillation) events have occurred. These ENSO events are defined by using the Southern Oscillation index (SOI) and the sea surface temperature (SST) anomaly in the central equatorial Pacific. The ENSO events are classified based on the positive anomaly length of the SST time series: about a year with one northern winter, or two or more years with two northern winters. The shorter one (BO-ENSO) terminates in the following year of the occurrence, and the longer one (LF-ENSO) terminates in the two years after the occurrence. The occurrence and termination of both types have strong phase preference to the seasonal cycle of the mean field, i. e., the seasons between northern winter and northern summer. The occurrence years of each type are as follows; the years of BO-ENSO are 1951, '53, '63, '65, '72, and 1982 (6 cases) and the years of LF-ENSO are 1957, '68, '76, and 1986 (4 cases). Since the occurrence, development, and termination of the both types of ENSO have a seasonal phase preference, composite analysis is used to explain the difference between the two types of ENSO clearly. In the northern winter immediately after the BO-ENSO occurrence, the wind stress anomaly field has a specific clockwise eddy off the east coast of the Philippines. This eddy affects the ocean through Ekman pumping and plays an important role in ENSO termination. The most significant difference between a BO-ENSO and an LF-ENSO is whether this eddy appears or not. © 1993, Meteorological Society of Japan.