Increased Occurrence of Central-Pacific El Niño in ERSST Version 4

Abstract:

Interest in El Niño diversity has increased in the past decade, with much attention given to the hypothesis that El Niño comes in two distinct types: an Eastern-Pacific (EP) El Niño centered off the coast of Peru and a Central-Pacific (CP) El Niño centered near the International Date Line. It is widely believed that the CP type of El Niño has been increasing in frequency in recent decades. However, many findings regarding El Niño diversity are uncertain given the divergence of results from different classification schemes.

We test the idea that the CP type of El Niño is a recent and increasingly frequent phenomenon by using the newly released Version 4 of NOAA’s Extended Reconstructed Sea Surface Temperatures (ERSST v4) dataset with one popular central-location classification method and a new method based on observed spatial patterns of sea surface temperature anomalies. The new method selects an EP and a CP El Niño “end-member” from events that are universally classified one way or the other by preexisting methods and uses pattern correlation with these end-members to sort all El Niño years into EP-dominated, CP-dominated, or mixed influence (M) classes.

The advantage of this end-member correlation approach is its simplicity, relying only on observed data without the need for statistical regression, while taking into account a larger portion of the equatorial Pacific than the traditional Niño 3, Niño 4, and related indices. The downside of this approach is the somewhat arbitrary nature of end-member selection, although this is mitigated through the use of transparent selection criteria.

The updated ERSST dataset identifies significantly more CP El Niños than two older versions and an independent dataset when using the central-location method. The new pattern correlation method identifies more CP El Niños than EP El Niños for all datasets. The increase in CP El Niño identification in ERSST v4 is linked mostly to differences in the central Pacific in the late nineteenth and early twentieth centuries, although differences between the datasets persist even up to the past two decades. These results underscore the importance of careful selection of El Niño events when studying the difference between EP and CP types of El Niño and the necessity of confirming results with multiple methods and/or datasets.