Qian et al. (2025) Extratropical Cyclones Act as a “Bridge” to the Concurrent Impact of ENSO on the Arctic Oscillation During Boreal Winter

Identification

• Journal: Geophysical Research Letters
• Year: 2025
• Date: 2025-10-16
• Authors: Shengyi Qian, Haibo Hu, Kevin I. Hodges, Yimin Zhu, Xiu‐Qun Yang, Yuanheng Wang
• DOI: 10.1029/2025gl116719

Research Groups

• CMA Key Laboratory for Climate Prediction Studies, School of Atmospheric Sciences, Nanjing University, Nanjing, China
• Department of Meteorology and National Centre for Atmospheric Science, University of Reading, Reading, UK
• College of Meteorology and Oceanography, National University of Defense Technology, Changsha, China
• China Yangtze Power Co., Ltd., Yichang, China

Short Summary

This study reveals that extratropical cyclones (ECs) over the North Atlantic act as a crucial "bridge" for the concurrent impact of the El Niño-Southern Oscillation (ENSO) on the Arctic Oscillation (AO) during boreal winter, demonstrating this concurrent influence is more significant than the one-year-lagged effect. The mechanism involves ENSO-induced shifts in the westerly jet stream, modulating atmospheric baroclinicity and EC activity, which ultimately drives the AO pattern.

Objective

• To investigate whether the interannual variability of tropical Pacific sea surface temperature (SST) affects Arctic sea level pressure (SLP) and atmospheric circulation by modulating the activity of extratropical cyclones (ECs) over the North Atlantic.
• To determine the differences in the influences on the Arctic Oscillation (AO) among three distinct ENSO types (individual, double, and opposite events).
• To assess the relative importance of the one-year-lagged effect of ENSO on the AO (through poleward propagating atmospheric angular momentum) versus the concurrent effect of ENSO in the same winter.

Study Configuration

• Spatial Scale: Northern Hemisphere extratropics (20° N–90° N), North Atlantic, and tropical Pacific (5° N–5° S; 170° W–120° W for Nino3.4 index).
• Temporal Scale: Boreal winter (December-January-February, DJF) from 1870 to 2014, analyzing both concurrent and one-year-lagged impacts.

Methodology and Data

• Models used:
  • Automatic tracking algorithm TRACK (Hodges, 1994, 1995, 1996, 1999) for extratropical cyclone (EC) identification and tracking, using 850 hPa relative vorticity.
  • Diagnostic equations for barotropic energy conversion (BTEC) from mean kinetic energy (MKE) to eddy kinetic energy (EKE), and baroclinic energy conversion (BCEC) from eddy available potential energy (EAPE) to EKE.
  • Tendency diagnostic equation of meridional temperature gradient.
  • Wave activity flux (Takaya & Nakamura, 2001) and Eady growth rate (Lindzen & Farrell, 1980) for diagnostic analysis.
• Data sources:
  • NOAA‐CIRES‐DOE Twentieth Century Reanalysis project version 3 (20CRv3) (ensemble averaged and 80 members), with 3-hourly, daily, and monthly temporal resolution and 1° × 1° spatial resolution.
  • HadISST (Hadley Centre Sea Ice and Sea Surface Temperature) and HadSLP2 (Hadley Centre Sea Level Pressure) data sets for verification.

Main Results

• The concurrent influence of ENSO on the AO is more significant than the one-year-lagged influence, which is mediated by poleward propagating atmospheric angular momentum.
• During El Niño (La Niña) winters, a negative (positive) AO pattern is induced in the Arctic region. This concurrent impact is consistent across individual, double, and opposite ENSO event types.
• The mechanism involves:
  1. Anomalous tropical Pacific SST in El Niño (La Niña) winters leads to southward (northward) displacement of the westerly jet stream and anomalous atmospheric horizontal heat advection from the Pacific to the Atlantic.
  2. This shifted horizontal heat advection enhances (weakens) atmospheric baroclinicity over the subtropical North Atlantic.
  3. The altered baroclinicity drives intensified (reduced) baroclinic energy conversion from eddy available potential energy to eddy kinetic energy, while barotropic energy conversion is negligible.
  4. This shifts the genesis locations of ECs southward (northward) and suppresses (enhances) their poleward translation into the Arctic.
  5. Ultimately, through combined thermodynamic and dynamical forcing associated with EC activity, a negative (positive) AO pattern is generated in the concurrent winter.
• The anomalous atmospheric temperature distribution over the North Atlantic is primarily forced by atmospheric processes (horizontal heat advection) rather than local North Atlantic SST anomalies.

Contributions

• This study identifies and elucidates the physical mechanism by which extratropical cyclones over the North Atlantic serve as a "bridge" for the concurrent impact of ENSO on the Arctic Oscillation during boreal winter.
• It quantitatively demonstrates that this concurrent influence is more significant than the previously recognized one-year-lagged impact of ENSO on the AO.
• The research details the chain of atmospheric processes: from ENSO-induced westerly jet stream displacement and horizontal heat advection, to changes in North Atlantic baroclinicity, and subsequent modulation of EC genesis locations and poleward translation.
• It provides insights into the distinct behaviors of the AO in the following winter for different ENSO types (individual, double, and opposite events), highlighting the interplay between concurrent and lagged ENSO effects.

Funding

• National Key Program for developing Basic Science (Grants 2022YFF0801702 and 2022YFE0106600)
• National Natural Science Foundation of China (Grants 42175060)
• Jiangsu Provincial Innovation Center for Climate Change
• U.S. Department of Energy, Office of Science Biological and Environmental Research (BER) (for 20CRv3 data set)
• National Oceanic and Atmospheric Administration Climate Program Office (for 20CRv3 data set)
• NOAA Earth System Research Laboratory Physical Sciences Laboratory (for 20CRv3 data set)

Citation

@article{Qian2025Extratropical,
  author = {Qian, Shengyi and Hu, Haibo and Hodges, Kevin I. and Zhu, Yimin and Yang, Xiu‐Qun and Wang, Yuanheng},
  title = {Extratropical Cyclones Act as a “Bridge” to the Concurrent Impact of ENSO on the Arctic Oscillation During Boreal Winter},
  journal = {Geophysical Research Letters},
  year = {2025},
  doi = {10.1029/2025gl116719},
  url = {https://doi.org/10.1029/2025gl116719}
}