Abstract
El Niño-Southern Oscillation (ENSO), characterized by anomalous sea surface temperature in the central-eastern equatorial Pacific, is a dominant interannual variability, impacting worldwide weather and socioeconomics. The ENSO cycle contains irregularity, in which La Niña often persists for more than two years, called “multi-year La Niña”. Observational records show that multi-year La Niña tends to accompany strong El Niño in the preceding year, but their physical linkage remains unclear. Here we show using reanalysis data that a strong El Niño excites atmospheric conditions that favor the generation of multi-year La Niña in subsequent years. Easterly wind anomalies along the northern off-equatorial Pacific during the decay phase of the strong El Niño are found crucial as they act to discharge ocean heat content (OHC) via an anomalous northward Ekman transport. The negative OHC anomaly is large enough to be restored by a single La Niña and, therefore, causes another La Niña to occur in the second year. Furthermore, analyses of the Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models support the abovementioned mechanisms and indicate that the occurrence frequencies of multi-year La Niña and strong El Niño are highly correlated.
Highlights
The El Niño-Southern Oscillation (ENSO) is a quasi-periodic variability with irregularity in several aspects such as amplitude and transition, and the life cycle is known to be locked with seasonal march: growing in boreal spring and summer, maturing in winter, and decaying in the following spring[1,2,3]
In addition to the observational analyses, we investigate the connection between strong El Niño and multi-year La Niña in 23 Coupled Model Intercomparison Project Phase 6 (CMIP6) models and demonstrate that the observed physical linkage is consistently seen in the climate model ensemble
By taking composites for these events of N3.4 and the equatorial Pacific ocean heat content (OHC) ( OHCeq: 120° E–60° W, 5.5° S–5.5° N, surface-500 m), the latter measuring the ENSO-related anomalies in the warm water volume, trajectories associated with the multi-year and single-year La Niñas are obtained in the phase space (Fig. 1a)
Summary
The El Niño-Southern Oscillation (ENSO) is a quasi-periodic variability with irregularity in several aspects such as amplitude and transition, and the life cycle is known to be locked with seasonal march: growing in boreal spring and summer, maturing in winter, and decaying in the following spring[1,2,3]. When strong El Niño decays from winter to spring, equatorial westerly wind anomalies are known to shift s outhward[16], causing equatorial thermocline shoaling and contributing to the rapid termination of strong El Niño[17,18,19] This atmospheric pattern with southward-shifted westerly anomalies is induced by a nonlinear coupling between ENSO’s interannual frequency and the annual cycle in the western Pacific, known as a combination mode (C-mode)[20,21,22]. Because the recharge after the peak of La Niña is ineffective with a wide meridional pattern of the wind stress curl, the spatial difference in the SST anomaly may contribute to the multi-year persistency of La Niña. A question of how extremely strong El Niño that decays r apidly[21] can influence the subsequent occurrence of multi-year La Niña is unclear
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