Thual et al. (2025) A fresh perspective on ENSO nonlinearity: the ENSO pattern continuum metric

Identification

• Journal: npj Climate and Atmospheric Science
• Year: 2025
• Date: 2025-11-05
• Authors: Sulian Thual, Boris Dewitte
• DOI: 10.1038/s41612-025-01233-3

Research Groups

• Mercator-Ocean International, Toulouse, France
• School of Computing, Clemson University, Clemson, USA
• CECI, Université de Toulouse, CERFACS, CNRS, Toulouse, France
• Centro de Estudios Avanzados en Zonas Aridas (CEAZA), La Serena, Chile
• Departamento de Biología Marina, Facultad de Ciencias del Mar, Universidad Católica del Norte, Coquimbo, Chile

Short Summary

This study introduces a "pattern continuum" metric, based on Warm Pool Edge Position (WPEP) quantiles, to synthesize and understand the spatial diversity, asymmetry, and nonlinearity of El Niño-Southern Oscillation (ENSO) sea surface temperature (SST) patterns. It demonstrates that ENSO's complex features can be effectively approximated by a simple "shifted-mean" framework, where a fixed SST structure shifts zonally, providing a novel interpretation for the quadratic relationship between SST principal components.

Objective

• To propose and implement new diagnostics to quantify ENSO spatial shifting in climate data, specifically focusing on how the pattern diversity of sea surface temperature anomalies (SSTA) relates to the zonal displacement of total SST.
• To interpret key dynamical features of ENSO, such as its spatial diversity, asymmetry, and nonlinearity, including the quadratic relationship between SST principal components, through the lens of a "pattern continuum" and a "shifted-mean" framework.

Study Configuration

• Spatial Scale: Equatorial Pacific (120° E–80° W longitude, 5° N–5° S latitude).
• Temporal Scale: Monthly data from 1980–2022 (GODAS), 1920–2005 (CESM-LE), and 1870–2024 (ERSSTv5, HadISST).

Methodology and Data

• Models used:
  • "Pattern continuum" diagnostic: SST maps are ordered and smoothed according to Warm Pool Edge Position (WPEP) quantiles.
  • "Shifted-mean flow" framework: A conceptual model where a constant SST structure shifts zonally with WPEP, derived from observed data.
  • Principal Component Analysis (PCA): Applied to SSTA to analyze leading modes and their relationships.
• Data sources:
  • NCEP Global Ocean Data Assimilation System (GODAS) reanalysis.
  • NCAR Community Earth System Model Large Ensemble Project (CESM-LE) historical simulations.
  • NOAA Extended Reconstructed SST V5 (ERSSTv5).
  • Hadley Centre Global Sea Ice and SST V1.1 (HadISST).
  • Warm Pool Edge Position (WPEP) is estimated as the 29 °C isotherm (or 28.5 °C for CESM-LE) of non-seasonal SST.

Main Results

• The "pattern continuum" reveals a fundamental organization of ENSO, capturing major patterns of diversity (gradual transition from broad Central Pacific La Niña cooling to Central then Eastern Pacific El Niño warming) and asymmetry (La Niña cooling extends more westward, extreme Eastern Pacific El Niño events are much more intense).
• This observed pattern continuum can be effectively approximated by a "shifted-mean" flow, where a fixed SST structure shifts zonally, demonstrating that ENSO SSTA patterns emerge from the geometrically nonlinear differences between a shifting SST structure and itself at rest.
• The shifted-mean flow successfully reproduces the non-Gaussian statistics of SST in Niño regions (e.g., positive skewness in Niño 3 and 3.4, negative in Niño 4), attributing this to the basic geometrical nonlinearity of the shifting structure.
• The univariate but nonlinear shifted-mean flow necessarily projects onto two principal components (PCs) that are quadratically related, providing a simplified interpretation for the widely observed quadratic relationship between the leading SST principal components.

Contributions

• Introduces a novel "pattern continuum" diagnostic that provides a compact, continuum-based, and spatially resolved characterization of ENSO diversity and asymmetry, offering a fresh perspective beyond traditional event classification.
• Provides strong observational evidence supporting the theoretical "shifted-mean" framework (Thual and Dewitte, 2023), demonstrating how the spatial shifting of an approximately conserved structure can explain complex ENSO features.
• Offers a simplified, physically intuitive explanation for the emergence of the quadratic relationship between leading SST principal components, a key metric for ENSO nonlinearity, by linking it to the geometrical nonlinearity of the shifting SST structure.
• Develops a practical diagnostic tool useful for evaluating and intercomparing ENSO variability in climate models and other observational datasets.

Funding

• ANR (grant ANR-18-CE01-0012)
• ANID (Concurso de Fortalecimiento al Desarrollo Científico de Centros Regionales 2020-R20F0008-CEAZA, COPAS COASTAL FB210021 and Fondecyt Regular 1231174)
• ANR Templex (grant ANR-23-CE56-0002)

Citation

@article{Thual2025fresh,
  author = {Thual, Sulian and Dewitte, Boris},
  title = {A fresh perspective on ENSO nonlinearity: the ENSO pattern continuum metric},
  journal = {npj Climate and Atmospheric Science},
  year = {2025},
  doi = {10.1038/s41612-025-01233-3},
  url = {https://doi.org/10.1038/s41612-025-01233-3}
}