The role of mean state on changes in El Niño’s flavor

Abstract

Recently, many studies have argued for the existence of two types of El Nino phenomena based on different spatial distributions: the conventional El Nino [or Eastern Pacific (EP) El Nino], and the Central Pacific (CP) El Nino. Here, we investigate the decadal modulation of CP El Nino occurrences using a long-term coupled general circulation model simulation, focusing, in particular, on the role of climate state in the regime change between more and fewer CP El Nino events. The higher occurrence regime of the CP El Nino coincides with the lower occurrence regime of EP El Nino, and vice versa. The climate states associated with these two opposite regimes resemble the leading principal component analysis (PCA) modes of tropical Pacific decadal variability, indicating that decadal change in climate state may lead to regime change in terms of two different types of El Nino. In particular, the higher occurrence regime of CP El Nino is associated with a strong zonal gradient of mean surface temperature in the equatorial Pacific, along with a strong equatorial Trade wind over the area east of the dateline. In addition, the oceanic variables—the mixed layer depth and the thermocline depth—show values indicating increased depth over the western-to-central Pacific. The aforementioned climate states obviously intensify zonal advective feedback, which promotes increased generation of the CP El Nino. Frequent CP El Nino occurrences are not fully described by oceanic subsurface dynamics, and dynamical or thermodynamical processes in the ocean mixed layer and air–sea interaction are important contributors to the generation of the CP El Nino. Furthermore, the atmospheric response with respect to the SSTA tends to move toward the west, which leads to a weak air–sea coupling over the eastern Pacific. These features could be regarded as evidence that the climate state can provide a selection mechanism of the El Nino type.