ENSO regimes: Reinterpreting the canonical and Modoki El Niño

Abstract

We propose that the first two empirical orthogonal function (EOF) modes of tropical Pacific sea surface temperature (SST) anomalies do not describe different phenomena (i.e., El Niño‐Southern Oscillation (ENSO) and “El Niño Modoki”) but rather the nonlinear evolution of ENSO. We introduce two new uncorrelated indices (E and C), based on the leading EOFs, that respectively account for extreme warm events in the eastern and cold/moderate warm events in the central equatorial Pacific, corresponding to regimes with different evolution. Recent trends in ENSO can be described as an increase in the central Pacific (C) variability that is associated with stronger cold events, as well as a reduction in the eastern Pacific (E) variability within the cold/moderate warm regime, consistent with model projections. However, little can be said observationally with respect to the extreme warm regime.

Linear trends in sea surface temperature of the tropical Pacific Ocean and implications for the El Niño-Southern Oscillation

“…This notion is supported by the Modoki-like pattern that arises from the summation of El Niño and La Niña SST composites, which is due to larger eastern Pacific SST anomalies during El Niño relative to La Niña (Hoerling et al 1997). Recently, Takahashi et al (2011) argued that ENSO events can be described by the superposition of two independent indices representing central and eastern Pacific ENSO variability.…”

Strong and moderate nonlinear El Niño regimes

“…The monthly HadISST v1.1 dataset (1870-2010) (Rayner et al 2003) is used for calculating SST anomalies and the E and C indices, as in Takahashi et al (2011). For this, we consider the first two normalized principal components (PC) of the equatorial Pacific (10 • S − 10 • N) SST anomalies and calculate the indices as E = (PC1 − PC2)/ √ 2 and C = (PC1 + PC2)/ √ 2.…”

“…Stochastically forced linear models fitted to observations are able to capture some aspects of the ENSO diversity (e.g. Newman et al 2011b), but linear dynamics alone can not generate the nonlinear relation between the first two dominant statistical modes of equatorial Pacific SST anomalies, which has been proposed to emerge from dual EN regimes, with the 1982EN regimes, with the -1983EN regimes, with the and 1997EN regimes, with the -1998 events corresponding to different dynamics from the other EN (Takahashi et al 2011;Capotondi et al 2015).…”

“…Here we expand on previous studies (Takahashi et al 2011;Dommenget et al 2012) by investigating the possible existence and the nonlinear processes responsible for the ENSO regimes associated to strong and moderate EN events, using available observations and long-term simulations with the GFDL CM2.1 model. In this paper, we first describe the datasets and basic processing (Sect.…”

The very strong coastal El Niño in 1925 in the far-eastern Pacific

“…Neelin et al 1998), recent research is focused on understanding the diversity among the individual events, for which a popular procedure is to classify these based on whether the maximum SST anomalies are predominantly found in the central or eastern Pacific (see review by Capotondi et al 2015), although this classification is somewhat arbitrary (Takahashi et al 2011). One possibly "true" distinct type of EN could consist of the extreme EN of 1982-83 and1997-98, as they appear to correspond to a different dynamical regime from the rest of EN due to the nonlinear activation of deep convection in the cold eastern Pacific (Takahashi et al 2011;Takahashi and Dewitte 2016). These extreme events have been associated with intense warming in the FEP and disproportionally large rainfall anomalies in the arid western coast of South America (e.g.…”