Abstract
Subsurface ocean temperature indices are developed to identify two distinct types of tropical Pacific warming (El Nino) and cooling (La Nina) 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 Nino and La Nina events identified by the Nino-3.4 SST index are classified as the EP or CP types for the analysis period. It is found that most strong El Nino events are of the EP type while most strong La Nina events are of the CP type. It is also found that strong CP-type La Nina events tend to occur after strong EP-type El Nino events. The reversed subsequence (i.e., strong EP El Nino events follow strong CP La Nina 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 Nino and La Nina events.
Highlights
Recent studies have begun to suggest that there are two distinct types of interannual tropical Pacific warming and cooling events that are often referred to as El Niño and La Niña, respectively
The conventional type of El Niño/La Niña events is similar to tropical Pacific sea surface temperature (SST) variations described by Rasmusson and Carpenter (1982) which tend to have their anomaly center located in the eastern Pacific
We explored the possibility of using subsurface ocean temperature indices to identify the EP
Summary
Recent studies have begun to suggest that there are two distinct types of interannual tropical Pacific warming and cooling events that are often referred to as El Niño and La Niña, respectively. Yu et al (2010) further conducted surface ocean heat budget analysis to show that forcing from the northeastern subtropical sea level pressure (SLP) variations can excite the CP-type events via surface heat flux and wind-induced ocean advections This series of modeling and observational studies have lead to the suggestion that the CP and EP types of El Niño/La Niña should be treated and monitored separately. They, used a combined regression–empirical orthogonal function (EOF) method to identify the EP and CP events They first removed the tropical Pacific SST anomalies that were regressed with the Niño-4 SST index and applied the EOF to the residual SST anomalies to obtain the SST anomaly pattern for the EP type. Anomalous quantities are computed by removing the monthly mean climatology and the trend
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