Fagan et al. (2025) Upper Troposphere Lower Stratosphere Composition Change in Tropical Cyclones: Assessments From 17 Years of Satellite Observations

Identification

• Journal: Journal of Geophysical Research Atmospheres
• Year: 2025
• Date: 2025-11-15
• Authors: Hannah E. Fagan, Cameron R. Homeyer
• DOI: 10.1029/2025jd044717

Research Groups

• University of Oklahoma Graduate College
• School of Meteorology, University of Oklahoma
• CCC research group (Dr. Cameron Homeyer's group)

Short Summary

This study investigates the impact of tropical cyclones (TCs) on upper troposphere and lower stratosphere (UTLS) ozone and water vapor composition using 17 years of satellite observations, revealing that composition changes are highly sensitive to TC intensity, distance from the center, and environmental wind shear.

Objective

• To examine how tropical cyclone intensity, radial distance from the center, and environmental deep-layer wind shear relate to changes in upper troposphere and lower stratosphere (UTLS) ozone (O3) and water vapor (H2O) composition, quantifying these impacts in terms of vertical distance from the tropopause.

Study Configuration

• Spatial Scale: Global, covering North Atlantic (NA), South Indian (SI), East Pacific (EP), West Pacific (WP), and South Pacific (SP) ocean basins. Satellite profiles are retrieved every 160 km along orbit. Reanalysis data has a horizontal resolution of 0.625° × 0.5° longitude-latitude with 72 vertical levels (approximately 1100 m grid spacing in the UTLS). TC profiles are flagged within 1000 km of a TC location.
• Temporal Scale: 17 years (2005–2021) of satellite observations. Environmental conditions are from 3-hourly reanalysis assimilations.

Methodology and Data

• Models used: NASA Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) reanalysis for environmental conditions (tropopause pressure, deep-layer wind shear).
• Data sources:
  • Satellite: Aura Microwave Limb Sounder (MLS) (version 5 profiles for O3 and H2O).
  • Observation: International Best Track Archive for Climate Stewardship (IBTrACS) for tropical cyclone track information, intensity, and location.

Main Results

• Accurate assessment of UTLS composition changes within TCs requires accounting for varying tropopause heights (using a tropopause-relative logarithmic pressure framework) and basin-specific background composition.
• Tropical cyclones are associated with increased water vapor throughout most of the UTLS and dehydration at the tropopause level, while ozone is significantly reduced in the upper troposphere.
• All diagnosed UTLS composition changes demonstrate significant sensitivity to distance from TC center and TC intensity:
  • Intensity Dependency: More intense TCs lead to greater UTLS composition changes. Major hurricanes (sustained winds ≥49.4 m/s) show nearly an 80% average decrease in upper tropospheric O3 and up to 70-80% average increase in upper tropospheric H2O. Major hurricanes also exhibit a significantly drier tropopause (approximately 20% lower H2O concentration) compared to tropical storms (18.0–32.5 m/s). Lower stratospheric hydration is strongest for major hurricanes, with H2O increases exceeding 10% at approximately 2 km above the tropopause.
  • Distance Dependency: Effects on UTLS O3 and H2O generally decrease with distance from the TC center, particularly beyond 400 km. The strongest stratospheric hydration signal is found within 100 km from the TC center, at altitudes approximately 1.5–3.5 km above the tropopause. A dehydrated tropopause is observed within TCs, especially prominent 50–100 km from the TC center.
  • Shear Dependency: UTLS H2O changes show substantial sensitivity to deep-layer wind shear. Weaker shear environments (< 5 m/s) are associated with more pronounced tropospheric hydration and tropopause dehydration. Upper tropospheric O3 deficits are stronger on the downshear side of the storm (e.g., approximately 70% downshear vs. 50% upshear in strong shear environments). Tropopause dehydration is most prominent in low shear environments (approximately 20% H2O decrease) and is absent in strongly sheared environments (> 10 m/s).

Contributions

• First study to employ a tropopause-relative framework for evaluating UTLS composition change within tropical cyclones.
• First to calculate basin-relative background composition for global samples, demonstrating its necessity due to significant spatial variability in UTLS background O3 and H2O concentrations.
• Leverages the longest analyzed observational record (17 years) of TC effects on the UTLS, providing robust statistics and a comprehensive global assessment.
• Provides a detailed exploration of the relationship between UTLS composition impacts from TCs and storm characteristics (intensity, radial distance, environmental wind shear) in multiple coordinate systems.
• Quantifies these impacts in terms of vertical distance from the tropopause, which is critical for improving understanding of how TCs affect Earth’s radiation budget and climate.

Funding

No explicit funding information was provided in the paper text.

Citation

@article{Fagan2025Upper,
  author = {Fagan, Hannah E. and Homeyer, Cameron R.},
  title = {Upper Troposphere Lower Stratosphere Composition Change in Tropical Cyclones: Assessments From 17 Years of Satellite Observations},
  journal = {Journal of Geophysical Research Atmospheres},
  year = {2025},
  doi = {10.1029/2025jd044717},
  url = {https://doi.org/10.1029/2025jd044717}
}