Bai et al. (2025) Influence of ENSO on the seasonal and extreme characteristics of ridges over Pacific-Western North American region

Identification

• Journal: Climate Dynamics
• Year: 2025
• Date: 2025-12-09
• Authors: Xiaoyu Bai, Paul Loikith, Dmitri A. Kalashnikov, Siiri Bigalke, Ping Liu, Deepti Singh
• DOI: 10.1007/s00382-025-07961-9

Research Groups

• School of the Environment, Washington State University Vancouver, USA
• Geography, Portland State University, USA
• Sierra Nevada Research Institute, University of California-Merced, USA
• School of Marine and Atmospheric Sciences, Stony Brook University, USA

Short Summary

This study investigates the seasonal and extreme characteristics of atmospheric ridges over Pacific-Western North America and their modulation by ENSO, finding that La Niña generally favors more frequent, larger, and more intense extreme ridges, especially in winter, with ENSO-forced shifts in mean flow largely explaining these patterns.

Objective

• To examine the characteristics (extent, intensity, location) of atmospheric ridges over Pacific-Western North America.
• To understand the characteristics and drivers of extreme ridges based on their extent and intensity.
• To evaluate the influence of ENSO on the characteristics of extreme ridges.

Study Configuration

• Spatial Scale: Pacific-Western North American region (30°–80° N, 165°–100° W for ridge centroids). Data initially at 0.25° x 0.25° resolution, interpolated to 2.5° x 2.5° for ridge identification.
• Temporal Scale: 1940–2024 (84-year period), analyzed seasonally (December-February, March-May, June-August, September-November).

Methodology and Data

• Models used: A feature-tracking algorithm was employed to identify ridge objects based on zonal daily eddy anomalies (Z*).
• Data sources:
  • ECMWF Reanalysis v5 (ERA5) for 500 hPa hourly geopotential data.
  • NOAA Extended Reconstructed SST (ERSST) V5 to calculate the Oceanic Niño Index (ONI) for defining El Niño and La Niña phases.

Main Results

• Ridge characteristics (location, frequency, extent, intensity) exhibit substantial seasonal variations. Ridge frequency and intensity are highest in winter and lowest in summer. Ridges tend to center along the Pacific coast and Gulf of Alaska in winter, spring, and fall, but shift further inland over the Intermountain West in summer.
• No significant long-term trends (1940–2024) were found in the aggregate frequency, extent, or intensity of ridges, but substantial interannual and multidecadal variability exists. Some emerging trends, such as an increasing spring ridge intensity of 3.69 GPM/decade since 2000, were noted.
• ENSO phases significantly influence extreme ridge characteristics, sometimes with opposite effects on seasonal versus extreme ridge characteristics. La Niña conditions are associated with a higher frequency, larger extent, and greater intensity of "Intense" ridges during winter, and more widespread "Large" ridges in spring and fall. El Niño conditions favor more intense ridges in spring and summer when considering all ridges.
• Extreme ridges are associated with hemisphere-scale circulation anomalies. Differences in winter extreme ridge circulation patterns during El Niño and La Niña largely reflect changes in the mean flow. For summer, these differences are less similar to mean flow differences, suggesting interactions with planetary-scale waves.

Contributions

• Provides a systematic, comprehensive climatology of atmospheric ridge characteristics (location, frequency, spatial extent, intensity) over Pacific-Western North America, extending beyond traditional blocking studies.
• Quantifies the characteristics and drivers of extreme ridges, distinguishing between "large" and "intense" events.
• Reveals the nuanced and seasonally varying influence of ENSO on both seasonal and extreme ridge characteristics, highlighting instances where ENSO's impact on extreme ridges differs from its impact on the overall ridge population.
• Links extreme ridge events to specific large-scale atmospheric circulation patterns and their associated surface impacts, enhancing understanding of predictability sources.
• Creates a valuable database of ridge objects for future research into surface impacts and risk management.

Funding

• NSF AGS-2206996
• NSF AGS-2206997

Citation

@article{Bai2025Influence,
  author = {Bai, Xiaoyu and Loikith, Paul and Kalashnikov, Dmitri A. and Bigalke, Siiri and Liu, Ping and Singh, Deepti},
  title = {Influence of ENSO on the seasonal and extreme characteristics of ridges over Pacific-Western North American region},
  journal = {Climate Dynamics},
  year = {2025},
  doi = {10.1007/s00382-025-07961-9},
  url = {https://doi.org/10.1007/s00382-025-07961-9}
}