Lynne et al. (2025) Connections of cold-season northern hemisphere extratropical cyclone characteristics to common climate modes during 1950–2023

Identification

• Journal: Climate Dynamics
• Year: 2025
• Date: 2025-11-20
• Authors: Matthew S. Lynne, Aiguo Dai
• DOI: 10.1007/s00382-025-07958-4

Research Groups

• Department of Atmospheric and Environmental Sciences, University at Albany, SUNY, Albany, NY, USA

Short Summary

This study investigates the connections between six cold-season Northern Hemisphere extratropical cyclone (ETC) characteristics and five common climate modes (PNA, ENSO, IPO, NAO, and decadal NAO) from 1950 to 2023, revealing similarities and key differences in their influences across interannual and decadal timescales through correlative and composite analyses.

Objective

• To examine the regions where ETC impacts are most strongly influenced by the five climate modes.
• To investigate how the influences on ETC characteristics by the decadal climate modes (decadal-smoothed NAO and IPO) differ from those from unsmoothed NAO, ENSO, and PNA.
• To reveal the physical mechanisms behind these influences by the climate modes on interannual and decadal time scales.

Study Configuration

• Spatial Scale: Northern Hemisphere extratropical regions (22° N to 80° N), specifically focusing on the North Pacific, North Atlantic, North America, East Asia, and Europe. Data grids are 1° × 1° for MSLP and 0.25° for 10 m wind speed.
• Temporal Scale: Cold-season months (October–April) from 1950/1951 to 2023/2024. Hourly data for MSLP and 10 m wind speed; 6-hourly data for temperature, geopotential height, and u and v winds.

Methodology and Data

• Models used:
  • Impacts-based ETC tracking algorithm (Lynne and Dai, 2025) to identify and track cyclones and record statistics over all cyclone-impacted grid cells.
  • Pearson’s correlation coefficient for statistical analysis.
  • Random phase test (Ebisuzaki, 1997) for significance assessment.
  • Lanczos filter (11-year cutoff, 15-year moving window) for IPO index and associated ETC characteristics.
  • 11-year running average for decadal NAO index and associated ETC characteristics.
  • Calculation of maximum Eady growth rate (baroclinicity), Barotropic Energy Conversion (BTEC), and Baroclinic Energy Conversion (BCEC) to analyze physical mechanisms.
• Data sources:
  • European Centre for Medium Range Weather Forecasts Reanalysis version 5 (ERA5) hourly mean sea-level pressure (MSLP) and 10 m wind speed.
  • ERA5 6-hourly temperature, geopotential height, and u and v winds.
  • Extended Reconstructed Sea Surface Temperature, Version 5 (ERSSTv5) for ENSO and IPO indices.
  • NCEP-NCAR reanalysis for NAO and PNA indices (derived from 500 hPa geopotential height anomalies).

Main Results

• Pacific Climate Modes (PNA, ENSO, IPO):
  • Positive phases of PNA, IPO, and El Niño generally favor larger and more frequent ETCs across the north-central North Pacific and southeastern North America (including the Gulf of Mexico), associated with upper-level troughing and a stronger subtropical jet stream.
  • Conversely, these positive phases lead to less frequent ETCs over western North America (Rocky Mountain lee storm track) and northeastern Asia.
  • Positive PNA and ENSO phases are linked to slower ETCs across North America but faster ETCs off the coast of East Asia.
  • Key Differences: El Niño exhibits higher ETC frequencies over East Asia, a relationship not clearly observed for PNA or IPO. ETC lifespan over eastern Canada is negatively correlated with ENSO (shorter during El Niño) but not with PNA or IPO. Troughing over western Canada during a negative PNA is shifted slightly eastward compared to La Niña.
  • Physical Mechanisms: Positive phases favor a southward enhancement and eastward extension of baroclinicity, but lower barotropic energy conversion. A key difference in baroclinic energy conversion is that a positive PNA favors greater conversion near the coast of Japan, while for ENSO, this pattern is weaker and shifted eastward, leading to higher ETC frequencies farther east over the North Pacific during El Niño.
  • IPO impacts on ETCs are generally weaker and less robust than PNA and ENSO, partly due to fewer observed cycles.
• Atlantic Climate Modes (NAO, Decadal NAO):
  • A positive NAO phase favors more frequent, stronger, and larger ETCs across the North Atlantic and Europe, associated with a strengthened North Atlantic jet stream and enhanced baroclinicity.
  • Higher ETC frequencies, intensities, and areas over central and eastern North America occur during the negative phase of the unsmoothed NAO.
  • ETC velocity over the east coast of the northeastern United States and southeastern Canada is positively correlated with the NAO.
  • Key Differences (Decadal vs. Interannual NAO): The decadal NAO's influence is weaker and less coherent. The canonical meridional shift of the North Atlantic storm track is apparent only over the central and eastern North Atlantic and parts of western Europe on a decadal scale, as opposed to covering more of Europe and North America on an interannual scale. Negative correlations of ETC area over central and eastern North America are significantly weaker or non-existent in the decadal case.
  • Physical Mechanisms: On interannual scales, a positive NAO favors greater energy extraction from enhanced temperature gradients over eastern North America and the North Atlantic. On the decadal scale, this greater energy extraction disappears over North America and is much weaker over the North Atlantic.

Contributions

• Applied a novel "impacts-based" extratropical cyclone (ETC) tracking algorithm that records statistics over all cyclone-impacted regions, providing a more comprehensive view of ETC influence compared to methods focusing solely on the cyclone center.
• Provided a direct and detailed comparison of the influences of common climate modes on ETC characteristics across both interannual and decadal timescales, highlighting significant differences in spatial extent and magnitude.
• Elucidated the physical mechanisms (e.g., changes in geopotential height, baroclinicity, barotropic and baroclinic energy conversion) that explain the observed connections between climate modes and ETC behavior.
• Identified subtle yet important differences in ETC responses to closely related climate modes like PNA and ENSO, and between interannual and decadal phases of the NAO, contributing to a more nuanced understanding of climate-ETC interactions.

Funding

• University at Albany of SUNY
• NSF (Grants OISE-1743738 and AGS-2015780)

Citation

@article{Lynne2025Connections,
  author = {Lynne, Matthew S. and Dai, Aiguo},
  title = {Connections of cold-season northern hemisphere extratropical cyclone characteristics to common climate modes during 1950–2023},
  journal = {Climate Dynamics},
  year = {2025},
  doi = {10.1007/s00382-025-07958-4},
  url = {https://doi.org/10.1007/s00382-025-07958-4}
}