Contrasting transition complexity between El Niño and La Niña - Observations and CMIP5/6 models - June 2020 revised

Oct 29, 2026

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The observed El Niño and La Niña show distinct complexities in event-to-event transition patterns. El Niño transitions follow episodic, cyclic, and multi-year ordering, while La Niña shows the reversed order. A subtropical Pacific onset mechanism explains the difference by triggering events through subtropical processes and nonlinear responses to the tropical Pacific mean SST state. CMIP5/6 models reproduce El Niño transition complexity but fail for La Niña due to a weaker-than-observed subtropical mechanism and a cold bias in model tropical Pacific mean SSTs.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages25

Reading Time1 h 3 min

AccessFree to download

Key points

Abstract

Plain Language Summary

1. Introduction

ENSO event transition complexity

Two primary onset mechanisms of ENSO

Tropical Pacific onset (TP-onset)

Subtropical Pacific onset (SP-onset)

1 2 3 4 Contrasting transition complexity between El Niño and La Niña: 5 Observations and CMIP5/6 models 6 7 8 9 Shih-Wei Fang and Jin-Yi Yu* 10 11 Department of Earth System Science, University of California, Irvine, CA, USA 12 13 14 15 16 17 18 Revised, June 2020 19 Submit to Geophysical Research Letters 20 21 22 23 Key points: 24 􀁸 El Niño transitions are dominated by, in order, episodic, cyclic, and multi-year 25 patterns, but the reversed order is found for La Niña. 26 􀁸 This asymmetry is caused by a subtropical Pacific mechanism that produces more 27 episodic (multi-year) transitions for El Niño (La Niña) . 28 􀁸 CMIP5 models fail to simulate the asymmetry due to a cold bias in their tropical 29 mean states and an overly weak subtropical mechanism. 30 31 *Corresponding Author: Dr. Jin-Yi Yu, Department of Earth System Science, 32 University of California, Irvine, CA, USA. Email: [jyyu@uci.edu](jyyu@uci.edu) 33 Abstract 34 The observed El Niño and La Niña exhibit different complexities in their 35 event-to-event transition patterns. The El Niño is dominated in order by episodic, 36 cyclic, and multi-year transitions, but the reversed order is found in the La Niña. A 37 subtropical Pacific onset mechanism is used to explain this difference. This 38 mechanism triggers El Niño/La Niña events via subtropical processes and is 39 responsible for producing multi-year and episodic transitions. Its nonlinear responses 40 to the tropical Pacific mean state result in more multi-year transitions for La Niña 41 than El Niño and more episodic transitions for El Niño than La Niña. The CMIP5/6 42 models realistically simulate the observed transition complexity of El Niño but fail to 43 simulate the transition complexity of La Niña. This deficiency in CMIP5 models 44 arises from a weaker than observed subtropical onset mechanism and a cold bias in 45 the tropical Pacific mean sea surface temperatures in the models. 46 47 Plain Language Summary 48 A new asymmetry is found between the warm (i.e., El Niño) and cold (i.e., La Niña) 49 phases of El Niño-Southern Oscillation (ENSO) in their event-to-event transition 50 patterns. The observed El Niño transitions is dominated in order by the episodic, 51 cyclic, and multi-year patterns, but the reversed order is found in the La Niña 52 transitions. This difference in the transitions arises from a subtropical Pacific forcing 53 mechanism that triggers ENSO events. The subtropical onset mechanism is found to 54 generate more episodic transitions for El Niño than La Niña and more multi-year 55 transitions for La Niña than El Niño. This asymmetry is due to nonlinear responses of 56 the subtropical mechanism to the tropical mean sea surface temperatures (SSTs) . 57 State-of-art global climate models realistically simulate the observed transition 58 complexity of El Niño but fail to reproduce the transition complexity of La Niña. This 59 deficiency arises from a weak subtropical onset mechanism and a cold bias in the 60 tropical Pacific mean SSTs in the models. 61 62 1. Introduction 63 El Niño-Southern Oscillation (ENSO) is a complex phenomenon that involves 64 wide ranges of different patterns, amplitudes and temporal evolutions (Kao and Yu 65 2009; Capotondi et al. 2015; Wang et al. 2017; Yu et al. 2017; Timmermann et al. 66 2018; Yu & Fang 2018) . One important part of the complexity appears in the way that 67 one ENSO event transitions to another. An El Niño (La Niña) event can be preceded 68 by a La Niña (El Niño) event to result in a cyclic transition, by another El Niño (La 69 Niña) event to become a multi-year transition, or by a neutral (non-ENSO) condition 70 to become an episodic transition (Yu & Fang 2018) . ENSO onset mechanisms control 71 how anomalies in sea surface temperature (SST) are established in the equatorial 72 Pacific and play critical roles i

FAQs

How do El Niño and La Niña differ in event-to-event transition complexity?

El Niño is dominated by episodic, cyclic, and multi-year transitions in that order, while La Niña shows the reversed order. This creates a clear asymmetry between warm and cold phases in transition patterns.

What mechanism is proposed to explain the asymmetry between El Niño and La Niña transitions?

A subtropical Pacific onset mechanism is used to explain the difference. It triggers ENSO events via subtropical processes and produces more multi-year transitions for La Niña and more episodic transitions for El Niño due to nonlinear responses to the tropical Pacific mean SST state.

Why do CMIP5/6 models fail to reproduce La Niña transition complexity?

The models fail because the subtropical onset mechanism is weaker than observed and the tropical Pacific mean SSTs have a cold bias. As a result, CMIP5/6 can simulate El Niño transition complexity but not the La Niña pattern.

Oct 29, 2026

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