Title: ENSO Diversity and the Recent Appearance of Central Pacific ENSO.

Journal: Climate Dynamics, https://doi.org/10.1007/s00382-019-05005-7.

Authors: FENG Y., X. -Y. Chen, and K.-K. Tung

Abstract: ENSO diversity refers to the appearance in recent decades of different El Niño types in the tropical Pacific: In addition to the canonical El Niño, the newly discovered type, called Central Pacific (CP) El Niño, has its center of warming shifted more to the central equatorial Pacific. Whether these ENSO types are really distinct or are different manifestations of a continuum is currently under debate. The mechanisms for the recent appearance of the additional type of ENSO have not been clarified. While previously it was claimed that the increased CP El Niño occurrence in recent decades were due to tropical trade winds weakened by global warming, the observed trade winds actually intensified. We systematically study the variability of the Pacific sea-surface temperature (SST) using the new method of Rotated Principal Component Analysis (RPCA). The essential statistical characteristics of these two El Niño types are found to be describable by only three fundamental modes of pan-Pacific SST with different proportions: the ENSO-cycle, the pan-Pacific counterpart of the regionally defined Pacific Decadal Oscillation (PDO), and a new climate mode we call the Central Pacific Variability (CPV). It is shown that CP El Niño can be described by the PDO and the CPV. The latter has a horseshoe-shaped warm SST pattern in the Central Pacific flanked by cold SST to the east, roughly similar to the original definition of ENSO Modoki by Ashok et al. (J Geophys Res Oceans 112:C11007, 2007) as the second Empirical Orthogonal Function in a narrow tropical domain decomposition. The Principal Component (PC) of this mode, called Modoki PC (MPC) was thought to characterize the occurrence of this new phenomenon, but it has been shown to be unable to separate it from the canonical ENSO. In contrast, we demonstrate that Central Pacific El Niño occurs when the CPV index is larger than 0.5. The intensity of the CPV mode has increased dramatically since 1970s, likely as a result of the intensification of the easterly trade winds in the tropical Pacific, which tilts the thermocline along the tropical Pacific and raises the SST in the central equatorial Pacific. Moderate El Niños are more prone to stalling in the Central Pacific, becoming CP El Niños.