Subsurface ocean temperature indices for Central-Pacific and Eastern-Pacific types of El Niño and La Niña events

Subsurface ocean temperature indices are developed to identify two distinct types of tropical Pacific warming (El Niño) and cooling (La Niña) events: the Eastern-Pacific (EP) type and the Central-Pacific (CP) type. Ocean temperature anomalies in the upper 100 m are averaged over the eastern (80°W–90°W, 5°S–5°N) and central (160°E–150°W, 5°S–5°N) equatorial Pacific to construct the EP and CP subsurface indices, respectively. The analysis is performed for the period of 1958–2001 using an ocean data assimilation product. It is found that the EP/CP subsurface indices are less correlated and show stronger skewness than the sea surface temperature (SST)based indices. In addition, while both quasi-biennial (∼2 years) and quasi-quadrennial (∼4 years) periodicities appear in the SST-based indices for these two types, the subsurface indices are dominated only by the quasi-biennial periodicity for the CP type and by the quasi-quadrennial (∼4 years) periodicity for the EP type. Low correlation, high skewness, and single leading periodicity are desirable properties for defining indices to separate the EP and CP types. Using the subsurface indices, major El Niño and La Niña events identified by the Niño-3.4 SST index are classified as the EP or CP types for the analysis period. It is found that most strong El Niño events are of the EP type while most strong La Niña events are of the CP type. It is also found that strong CP-type La Niña events tend to occur after strong EP-type El Niño events. The reversed subsequence (i.e., strong EP El Niño events follow strong CP La Niña events) does not appear to be typical. This study shows that subsurface ocean indices are an effective way to identify the EP and CP types of Pacific El Niño and La Niña events.