Mascolo et al. (2026) Rare events algorithm study of extreme double jet summers and their connection with heatwaves over the Northern Hemisphere

Identification

• Journal: Journal of Climate
• Year: 2026
• Date: 2026-09-17
• Authors: Valeria Mascolo, Francesco Ragone, Nili Harnik, Freddy Bouchet
• DOI: 10.1175/jcli-d-25-0454.1

Research Groups

• Laboratoire de Physique `a l’ENS de Lyon, CNRS, F-69342 Lyon, France
• School of Computing and Mathematical Sciences, University of Leicester, LE17RH Leicester, United Kingdom
• Department of Geophysics, Tel Aviv University, Tel Aviv, Israel
• Laboratoire de M´et´eorologie Dynamique, IPSL, ENS-PSL, CNRS, Paris, France

Short Summary

This study investigates the connection between extreme double jet summers and heatwaves over the Northern Hemisphere. The authors use a rare event algorithm to sample double jet structures in the CESM1.2 climate model, finding that persistent double jets are associated with long-lasting heatwaves.

Objective

• Investigate the relationship between extreme double jet summers and heatwaves over the Northern Hemisphere.
• Examine the connection between double jets and large-scale circulation patterns, such as the summer NAM and quasi-wave-3 teleconnection pattern.

Study Configuration

• Spatial Scale: The study focuses on the Northern Hemisphere, with a spatial resolution of 0.25° x 0.25° for ERA5 data and 1.25° x 0.9° for CESM1.2 simulations.
• Temporal Scale: The study considers summer months (June-July-August) from 1940 to 2022 for ERA5 data and a control simulation of 1000 years for CESM1.2.

Methodology and Data

• Models used: ERA5 reanalysis dataset, CESM version 1.2.2 in an atmosphere-land only setup.
• Data sources: Daily data for 2 m air temperature (𝑇2m), 500 hPa geopotential height (𝑍500), zonal wind 𝑈(averaged between 200 hPa and 350 hPa) from ERA5 and CESM1.2.

Main Results

• The double jet index correctly represents the atmospheric state in both CESM1.2 and ERA5 datasets.
• Persistent double jets are associated with long-lasting heatwaves, with a clear increase in the percentage of days in common between double jet states and heatwaves as the accumulation period increases.
• Double jets are also associated with strong polar cyclonic anomalies and wave 3 patterns in the 𝑍500 at midlatitudes.

Contributions

• This study provides new insights into the connection between extreme double jet summers and heatwaves over the Northern Hemisphere, highlighting the importance of persistent double jets in shaping large-scale circulation patterns.
• The use of a rare event algorithm allows for efficient sampling of extreme events, providing a more accurate representation of their characteristics.

Funding

• Not specified.

Citation

@article{Mascolo2026Rare,
  author = {Mascolo, Valeria and Ragone, Francesco and Harnik, Nili and Bouchet, Freddy},
  title = {Rare events algorithm study of extreme double jet summers and their connection with heatwaves over the Northern Hemisphere},
  journal = {Journal of Climate},
  year = {2026},
  doi = {10.1175/jcli-d-25-0454.1},
  url = {https://doi.org/10.1175/jcli-d-25-0454.1}
}