Li et al. (2026) The combined impacts of NAO and ENSO on the surface air temperature anomalies in boreal winter

Identification

• Journal: Climate Dynamics
• Year: 2026
• Date: 2026-04-01
• Authors: Xiaojie Li, Zhiwei Zhu, Xuefeng Liu, Lei Wang, Ying Yang
• DOI: 10.1007/s00382-026-08162-8

Research Groups

• State Key Laboratory of Climate System Prediction and Risk Management/Key Laboratory of Meteorological Disaster, Ministry of Education/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science and Technology, Nanjing, China
• School of Atmospheric Sciences, Nanjing University of Information Science and Technology, Nanjing, China
• Jiangsu Meteorological Observatory, Nanjing, China

Short Summary

This study investigates the combined impacts of the North Atlantic Oscillation (NAO) and the El Niño–Southern Oscillation (ENSO) on boreal winter surface air temperature (SAT) anomalies using reanalysis and numerical simulations. It reveals that the combined effects exhibit nonlinear characteristics, with ENSO modulating NAO-induced circulation anomalies through distinct Rossby wave trains, thereby amplifying or weakening regional SAT anomalies.

Objective

• To investigate the combined impacts and physical mechanisms of the North Atlantic Oscillation (NAO) and the El Niño–Southern Oscillation (ENSO) on surface air temperature (SAT) anomalies in boreal winter.

Study Configuration

• Spatial Scale: Northern Hemisphere, focusing on North America, Eurasia (mid-and-high-latitudes), tropical Pacific, North Atlantic, Indo-Pacific, central and eastern Siberia, Japan Sea Rim region, North Africa, and West Asia.
• Temporal Scale: Boreal winter (December, January, February mean) over the period 1961–2022, examining interannual variability.

Methodology and Data

• Models used:
  • Linear Baroclinic Model (LBM) (Watanabe and Kimoto 2000) with T42 horizontal resolution and 20 vertical sigma levels, integrated for 30 days.
• Data sources:
  • Monthly mean sea surface temperature (SST) from NOAA Extended Reconstructed SST, version 5 (ERSSTv5) (2° × 2° horizontal resolution).
  • Monthly mean precipitation from NOAA Precipitation Reconstruction (2.5° × 2.5° horizontal resolution).
  • Monthly mean 2-meter air temperature from Climatic Research Unit gridded Time Series (CRU TS version 4.08) (0.5° × 0.5° horizontal resolution).
  • Monthly mean 2-meter air temperature, geopotential height, zonal and meridional wind from ERA5 (European Centre for Medium-Range Weather Forecasts) (0.25° × 0.25° horizontal resolution).
  • NCEP/NCAR reanalysis for LBM climatological DJF mean (1961–2022).
  • Niño3.4 index (DJF mean SST averaged over 5°S to 5°N and 120°W to 170°W).
  • NAO index (difference in normalized sea level pressure zonally averaged over 80°W to 30°E between 35°N and 65°N).
  • Methods: Linear regression, composite analysis, Student’s t-test for significance, temperature equation analysis, and Rossby wave activity flux (WAF) calculation (Takaya and Nakamura 2001).

Main Results

• ENSO and NAO are statistically independent on interannual timescales, each exerting distinct impacts on boreal winter SAT. ENSO primarily induces a meridional dipole SAT pattern over North America and minimal influence on Eurasia, while NAO generates significant SAT anomalies across southeastern North America and mid-and-high-latitude Eurasia.
• The combined effects of ENSO and NAO on SAT exhibit nonlinear characteristics compared to single-factor impacts.
  • Positive NAO & Positive ENSO (pNAO&pENSO): Enhances positive SAT anomalies over Eurasia (extending to Northeast Asia) and shifts positive SAT anomalies northwestward over North America. This is attributed to enhanced subtropical westerlies over the North Pacific, creating a waveguide for Rossby waves from the tropical Pacific to North America, and Rossby waves from Indo-Pacific convection amplifying anticyclonic anomalies over eastern Siberia.
  • Positive NAO & Negative ENSO (pNAO&nENSO): Reduces the coverage and intensity of positive SAT anomalies over North America and northern Eurasia compared to pNAO alone. Suppressed convection in the tropical Pacific inhibits Rossby wave propagation to North America, and positive convection over the Maritime Continent induces a cyclonic anomaly over eastern Siberia, counteracting NAO-induced positive SAT.
  • Negative NAO & Positive ENSO (nNAO&pENSO): Intensifies negative SAT anomalies over southeastern North America and the Eurasian Continent. Enhanced westerlies facilitate Rossby wave propagation, reinforcing the NAO-induced cyclonic anomaly over southeastern North America; suppressed convection over the Maritime Continent leads to a cyclonic anomaly over Northeast Eurasia, strengthening NAO-induced negative SAT.
  • Negative NAO & Negative ENSO (nNAO&nENSO): Diminishes negative SAT anomalies over North America and central and eastern Siberia. Weakened westerlies inhibit Rossby wave propagation to North America, and enhanced convection over the Maritime Continent induces an anticyclonic anomaly over eastern Siberia, counteracting NAO-induced negative SAT.
• The modulation occurs through ENSO-induced tropical convection anomalies triggering two distinct poleward-propagating Rossby wave trains (one from the tropical Pacific to North America, another from the Indo-Pacific to Eurasia), which enhance or offset NAO-induced atmospheric circulation patterns.
• Numerical simulations using a Linear Baroclinic Model (LBM) confirm the observed atmospheric circulation responses to these combined forcings.
• Empirical Orthogonal Function (EOF) analysis shows the first two leading modes of Northern Hemisphere winter SAT anomalies explain nearly half of the total variance, with reconstructed principal components based on Niño3.4 and NAO indices showing high correlation (0.74 and 0.38, respectively) with observed PCs.

Contributions

• This study provides a comprehensive analysis of the nonlinear combined impacts of ENSO and NAO on boreal winter SAT anomalies, addressing a gap in existing literature that primarily focused on their individual effects.
• It elucidates the distinct physical mechanisms by which ENSO modulates NAO's influence: NAO primarily acts through regional anomalous circulations, while ENSO triggers specific poleward-propagating Rossby wave trains (from the tropical Pacific to North America and from the Indo-Pacific to Eurasia) that either amplify or weaken NAO-induced circulation patterns.
• The research highlights the critical role of ENSO in modulating the strength of the westerly jet over the North Pacific, which acts as a waveguide for Rossby waves, thereby influencing their propagation and impact on North America.
• The findings emphasize the dependence of SAT anomalies on specific ENSO–NAO phase combinations, offering valuable insights for real-time climate monitoring and subseasonal-to-seasonal prediction.

Funding

• National Key R&D Program of China (2022YFF0801702)
• Natural Science Foundation of Jiangsu Province (BK20250045)
• Jilin Provincial Natural Science Foundation (YDZJ202601ZYTS750)

Citation

@article{Li2026combined,
  author = {Li, Xiaojie and Zhu, Zhiwei and Liu, Xuefeng and Wang, Lei and Yang, Ying},
  title = {The combined impacts of NAO and ENSO on the surface air temperature anomalies in boreal winter},
  journal = {Climate Dynamics},
  year = {2026},
  doi = {10.1007/s00382-026-08162-8},
  url = {https://doi.org/10.1007/s00382-026-08162-8}
}