Ryan et al. (2025) A worldwide climatology of extreme air masses

Identification

• Journal: Theoretical and Applied Climatology
• Year: 2025
• Date: 2025-12-15
• Authors: J. M. Ryan, Ben Kravitz, Travis O’Brien, Scott M. Robeson, Paul W. Staten
• DOI: 10.1007/s00704-025-05917-x

Research Groups

• Department of Earth and Atmospheric Sciences, Indiana University, Bloomington, IN, USA
• Atmospheric, Climate, and Earth Sciences Division, Pacific Northwest National Laboratory, Richland, WA, USA
• Climate and Ecosystem Sciences Division, Lawrence Berkeley National Lab, Berkeley, CA, USA
• Department of Geography, Indiana University, Bloomington, IN, USA

Short Summary

This study quantifies regional exposure to extreme hot and cold air masses (EHAMs/ECAMs) by tracking their frequencies, movements, trends, and sources/sinks globally, revealing a net increase in extreme events due to EHAMs increasing faster than ECAMs decrease.

Objective

• To quantify and compare the frequencies, movements, trends, and sources/sinks of extremely hot and cold air masses (EHAMs/ECAMs) globally, using a consistent definition for both, to better understand their formation, propagation, and predictability.

Study Configuration

• Spatial Scale: Global, with a focus on land regions, at a resolution of 1° latitude by 1.25° longitude.
• Temporal Scale: 41 years, from January 1980 to December 2020, using daily data.

Methodology and Data

• Models used: A feature tracking algorithm was developed in MATLAB (R2024b) to identify and track extreme air masses based on temperature, area, and duration criteria.
• Data sources:
  • Daily 2-meter temperature (T2m) data from the Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) reanalysis.
  • Daily temperature data from nClimGrid (observational dataset for the contiguous United States) for comparison.

Main Results

• For most land regions, Extremely Cold Air Masses (ECAMs) occur more often than Extremely Hot Air Masses (EHAMs), with ECAMs being more common in each hemisphere's winter.
• EHAMs have become more common almost everywhere on land, while ECAMs have decreased, with the strongest trends observed in the Northern Hemisphere autumn, particularly in the Arctic.
• Globally, the number of EHAMs is increasing at a rate of +2.32 events per year, which is higher than the decrease in ECAMs at -1.27 events per year, leading to a net increase in extreme air masses.
• The average duration of EHAMs is increasing (+0.021 days per year), while ECAM durations are decreasing (-0.019 days per year).
• The Arctic exhibits the strongest trends, with fewer ECAM days in spring and autumn and more EHAM days, especially in autumn (e.g., Kara-Barents Sea).
• The Southern Ocean and parts of the North Atlantic show decreasing EHAMs and increasing ECAMs, consistent with observed regional cooling.
• On land, EHAMs generally move eastward, while ECAMs generally move eastward and equatorward.
• EHAMs are more likely to form on the western sides of continents and dissipate on the eastern sides, particularly in the Northern Hemisphere. ECAM sources are predominantly north of 50°N, with sinks typically located between 20°N and 40°N.

Contributions

• Provides the first consistent, parallel global climatology of large-scale extreme hot and cold air masses (EHAMs/ECAMs), addressing a gap in previous studies that often treated these events differently.
• Quantifies not only the frequency but also the movement, sources, and sinks of individual extreme air masses, enhancing the understanding of their formation and propagation, which is crucial for improving predictability.
• Documents the asymmetric and regionally varying trends in frequency and duration for EHAMs and ECAMs, revealing a net increase in extreme air masses globally.
• Highlights the distinct processes driving hot (primarily upper-level processes, subsidence, radiation, and mixing) versus cold (primarily temperature advection and synoptic-scale weather) extremes.

Funding

• Regional and Global Model Analysis (RGMA) component of the Earth and Environmental Systems Modeling (EESM) program of the U.S. Department of Energy’s Office of Science (HiLAT-RASM project in collaboration with the CASCADE project).
• Indiana University Environmental Resilience Institute (for Ben Kravitz and Travis A. O’Brien).
• NSF award number 2140235 (for Paul W. Staten).
• Lilly Endowment, Inc. (through the Indiana University Pervasive Technology Institute).

Citation

@article{Ryan2025worldwide,
  author = {Ryan, J. M. and Kravitz, Ben and O’Brien, Travis and Robeson, Scott M. and Staten, Paul W.},
  title = {A worldwide climatology of extreme air masses},
  journal = {Theoretical and Applied Climatology},
  year = {2025},
  doi = {10.1007/s00704-025-05917-x},
  url = {https://doi.org/10.1007/s00704-025-05917-x}
}