Liang et al. (2025) The Asymmetry of the El Niño–Southern Oscillation: Characteristics, Mechanisms, and Implications for a Changing Climate

Identification

• Journal: Atmosphere
• Year: 2025
• Date: 2025-09-11
• Authors: Jin Liang, De‐Zheng Sun, Biao Jin, Y. Tony Yang, Cuijiao Chu, Minjia Tan
• DOI: 10.3390/atmos16091071

Research Groups

• Ocean College, Jiangsu University of Science and Technology, Zhenjiang, China
• Nanjing–Helsinki Institute in Atmospheric and Earth System Sciences, Nanjing University—Suzhou Campus, Suzhou, China
• Jiangsu Environmental Monitoring Center, Nanjing, China

Short Summary

This review synthesizes over two decades of research on El Niño–Southern Oscillation (ENSO) asymmetry, detailing its observed characteristics, evaluating competing physical mechanisms, and analyzing challenges in climate modeling. It concludes that ENSO asymmetry is driven by complex nonlinear atmospheric and oceanic processes, which current climate models largely underestimate, leading to uncertainties in future climate change projections.

Objective

• To systematically catalog the observed characteristics of ENSO asymmetry, from its signature in the tropical Pacific to its global teleconnections.
• To critically evaluate the current understanding of the competing physical mechanisms that generate ENSO asymmetry.
• To assess the persistent challenges in simulating ENSO asymmetry in state-of-the-art climate models and the implications for future climate projections.

Study Configuration

• Spatial Scale: Global, with a primary focus on the tropical Pacific (Eastern, Central, and Western Pacific) and its teleconnections to extratropical regions (e.g., Northern Hemisphere, Australia, Indian Ocean, Atlantic Ocean).
• Temporal Scale: Interannual to multi-centennial, covering over two decades of research (2000-2025), future projections for the 21st century and post-2100, and paleo-climate records (e.g., Pliocene).

Methodology and Data

• Models used: Coupled Model Intercomparison Project (CMIP3, CMIP5, CMIP6) models, Community Climate System Model (CCSM), Community Earth System Model (CESM), General Circulation Models (GCMs), recharge oscillator models, conceptual models, hybrid coupled ocean-atmosphere models.
• Data sources: Observational records (e.g., sea surface temperature anomalies, precipitation, wind stress), reanalysis data, and a comprehensive narrative literature review based on structured searches in scientific databases (Web of Science, Scopus).

Main Results

• ENSO exhibits inherent asymmetry, with El Niño (warm phase) typically having larger amplitudes, different spatial patterns (e.g., eastward propagation for strong El Niño post-1970s vs. westward for La Niña), and distinct temporal evolutions (e.g., multi-year La Niña events are more common than multi-year El Niño events).
• This asymmetry is primarily driven by nonlinear atmospheric mechanisms, including state-dependent westerly wind bursts (WWBs) that preferentially amplify El Niño, and a threshold-like response of deep atmospheric convection to sea surface temperature (SST) (e.g., strong convection increase above ~27.5 °C).
• Oceanic processes, such as nonlinear temperature advection, subsurface nonlinear dynamical heating, and asymmetric thermocline feedbacks, amplify and modulate the initial atmospheric asymmetry, contributing to the observed differences between El Niño and La Niña.
• State-of-the-art climate models (CMIP3, CMIP5, CMIP6) consistently underestimate ENSO asymmetry, often linked to systematic biases in the tropical Pacific mean state (e.g., equatorial cold tongue bias) and inadequate representation of key nonlinear air-sea interactions and cloud-radiative feedbacks.
• Projections suggest that ENSO amplitude asymmetry in SST may weaken post-2100 under sustained greenhouse warming, as the eastern Pacific mean state warms and atmospheric responses become more linear. However, the asymmetry of ENSO's global rainfall impacts may paradoxically be amplified in a warmer, moister climate.

Contributions

• Provides the first comprehensive synthesis focused squarely on the characteristics, mechanisms, and implications of ENSO asymmetry, integrating over two decades of dedicated research.
• Systematically catalogs the diverse manifestations of ENSO asymmetry, from tropical Pacific SST to global teleconnections, and critically evaluates the competing physical mechanisms (atmospheric vs. oceanic) driving it.
• Assesses the persistent challenges in simulating ENSO asymmetry in state-of-the-art climate models, linking model biases to underlying physical processes and highlighting implications for future climate projections.
• Offers an essential roadmap for future research, emphasizing the need for balanced model improvements, new diagnostic tools, and a framework linking asymmetry to the full spectrum of ENSO diversity to enhance prediction and projection capabilities.

Funding

• Jiangsu Province Industry-University-Research Collaboration Project (BY20240712).

Citation

@article{Liang2025Asymmetry,
  author = {Liang, Jin and Sun, De‐Zheng and Jin, Biao and Yang, Y. Tony and Chu, Cuijiao and Tan, Minjia},
  title = {The Asymmetry of the El Niño–Southern Oscillation: Characteristics, Mechanisms, and Implications for a Changing Climate},
  journal = {Atmosphere},
  year = {2025},
  doi = {10.3390/atmos16091071},
  url = {https://doi.org/10.3390/atmos16091071}
}