Lynne et al. (2025) Climatology, trends, and variability of cold-season northern hemisphere extratropical cyclone characteristics during 1950–2023 in ERA5

Identification

• Journal: Climate Dynamics
• Year: 2025
• Date: 2025-11-25
• Authors: Matthew S. Lynne, Aiguo Dai
• DOI: 10.1007/s00382-025-07956-6

Research Groups

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

Short Summary

This study analyzes the climatology, trends, and variability of cold-season Northern Hemisphere extratropical cyclones (ETCs) from 1950 to 2023 using an impact-based tracking algorithm on ERA5 reanalysis data, revealing significant regional shifts and intensity changes in storm tracks, particularly a westward shift in the North Pacific and a poleward shift and strengthening in the North Atlantic.

Objective

• To examine the climatology, long-term trends, and variability of cold-season Northern Hemisphere extratropical cyclone (ETC) frequency, intensity, area, lifespan, and travel velocity from 1950 to 2023 using an impact-based tracking algorithm.
• To determine how these impact-based ETC characteristics have spatially changed across the Northern Hemisphere since 1950.
• To compare long-term trends to interannual and decadal variability in regions of significant changes.
• To align these changes with trends in environmental characteristics such as baroclinicity.

Study Configuration

• Spatial Scale: Northern Hemisphere extratropical regions (22°–80° N) on a 1° × 1° grid.
• Temporal Scale: Cold season (October–April) from 1950/51 to 2023/24, using hourly data.

Methodology and Data

• Models used: ERA5 reanalysis (European Centre for Medium-Range Weather Forecasts Reanalysis-Version 5).
• Data sources:
  • Hourly mean sea-level pressure (MSLP) from ERA5 (1° × 1° grid) for ETC detection and tracking.
  • 10 m u and v wind components from ERA5 (0.25° resolution) for ETC intensity calculation.
  • Monthly mean 500 hPa and 850 hPa u wind and temperature from ERA5 for Eady Growth Rate (baroclinicity) calculation.
  • A modified ETC tracking algorithm (based on Crawford and Serreze (2016) and Hanley and Caballero (2012)) was applied, with a key modification to record ETC statistics at all grid cells encompassed by an ETC’s outermost isobar.
  • Long-term trends were estimated using the slope of the least squares linear regression line, and significance was assessed using the Mann–Kendall trend test (p-value ≤ 0.05).

Main Results

• Climatology:
  • All ETC characteristics (frequency, intensity, area, lifespan) exhibit high values over the North Pacific (NP) and North Atlantic (NA) and low values over Eurasia and North America, except for travel velocity which peaks over East Asia and eastern North America.
  • Highest mean cold-season ETC intensities are greater than 9.5 m/s, and areas are approximately 6 × 10^12 m^2 over the NP and NA oceans.
  • Highest mean cold-season ETC lifespans are 3.46 × 10^5 to 4.32 × 10^5 seconds (4–5 days) over oceanic regions.
  • Highest median cold-season ETC velocities are approximately 13.89 m/s (50 km/h) over eastern North America and Asia.
• Trends (1950–2023):
  • North Pacific (NP): A westward shift of the storm track, particularly in November, with significant decreases in ETC frequency of approximately -1.07% per decade in the northern NP and increases of approximately 0.65% per decade in East Asia. A poleward shift and strengthening were observed in December–February. Decreases in ETC area over most of the NP during November were approximately -2.0 × 10^10 m^2 per decade. Increases in lifespan over parts of the NP were approximately 2160 seconds per decade (0.6 hours per decade) to 2268 seconds per decade (0.63 hours per decade). Decreases in median ETC velocity were observed over the northern and eastern NP.
  • North Atlantic (NA): A poleward shift and strengthening of the storm track, particularly in January–March. Decreases in ETC frequency over the southeastern United States extending into the central NA, with increases over the high-latitude NA (e.g., -0.57% per decade in eastern North America/western NA). Increases in ETC intensity over large regions of the NA were approximately 0.19 m/s per decade (central NA). Increases in ETC velocity over parts of North America extending into the western NA were approximately 0.0167 m/s per decade.
  • Kara Sea: Significant increases in ETC frequency and intensity (e.g., 0.13 m/s per decade) occurred, most prominently in November and December.
  • Eastern Europe/Western Eurasia: A persistent region of decreasing ETC intensity (e.g., -0.005 m/s per decade) was observed during most months.
  • Baroclinicity: Increasing trends in Eady Growth Rate (on the order of 1.157 × 10^-8 s^-1 per decade) were found over the central NA (from the eastern North American coast to northwestern Europe), most prevalent in February and March, aligning with NA storm track shifts and strengthening. Baroclinicity trends were generally weaker over the NP.
• Variability: Historical trends for each region are roughly half of the decadal standard deviation, suggesting that a continuation of these trends for approximately one century could result in ETC frequencies exceeding historical interannual variability. Interannual standard deviation is typically 4 to 5 times greater than the decadal standard deviation.

Contributions

• Developed and applied a modified "impact-based" ETC tracking algorithm to hourly ERA5 data (1950–2023) for the Northern Hemisphere cold season, which records ETC statistics (frequency, intensity, area, lifespan, velocity) at all grid cells encompassed by an ETC’s outermost isobar. This method provides a unique visualization of cyclone impacts and increases the sample size for robust trend analysis, addressing limitations of previous center-based methods.
• Provided a detailed spatial and temporal (monthly) analysis of the climatology, long-term trends, and interannual-to-decadal variability of five key ETC characteristics over a 74-year period.
• Physically contextualized the observed ETC characteristic trends with changes in baroclinicity (Eady Growth Rate), particularly over the North Atlantic.
• Quantified the relationship between long-term trends and natural variability, indicating that a continuation of current trends could lead to ETC characteristics exceeding historical variability within one century.

Funding

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

Citation

@article{Lynne2025Climatology,
  author = {Lynne, Matthew S. and Dai, Aiguo},
  title = {Climatology, trends, and variability of cold-season northern hemisphere extratropical cyclone characteristics during 1950–2023 in ERA5},
  journal = {Climate Dynamics},
  year = {2025},
  doi = {10.1007/s00382-025-07956-6},
  url = {https://doi.org/10.1007/s00382-025-07956-6}
}