Zhang et al. (2025) A strong stratospheric harbinger for cold extremes: Weak polar vortex transition from displacement to split pattern

Identification

• Journal: Weather and Climate Extremes
• Year: 2025
• Date: 2025-11-17
• Authors: Murong Zhang, Xiao‐Yi Yang, Yipeng Huang
• DOI: 10.1016/j.wace.2025.100832

Research Groups

• State Key Laboratory of Marine Environmental Science, Center for Marine Meteorology and Climate Change, College of Ocean and Earth Sciences, Xiamen University, Xiamen, China
• Xiamen Key Laboratory of Straits Meteorology, Xiamen Meteorological Bureau, Xiamen, China

Short Summary

This study introduces a novel clustering method to identify "mixed-type" weak stratospheric polar vortex (WSPV) events, characterized by a transition from displaced to split patterns. These mixed-type events are found to induce more persistent and stronger negative Arctic Oscillation-like surface signatures, leading to robust cold anomalies over northern Eurasia and the central U.S. 10–39 days after onset, driven by synergistic tropospheric planetary wavenumbers 1 and 2.

Objective

• To quantitatively identify mixed-type Weak Stratospheric Polar Vortex (WSPV) events using clustering analysis on daily spatial patterns of the weak vortex in reanalysis data.
• To elucidate the climatological features, surface weather impacts, and tropospheric precursors of mixed-type WSPV events, distinguishing them from pure split- and displaced-type events.

Study Configuration

• Spatial Scale: Northern Hemisphere, focusing on the stratosphere (10 hPa geopotential height, polar cap poleward of 60°N/65°N) and troposphere (surface air temperature, sea level pressure, 500 hPa geopotential height, 100 hPa vertical wave-activity flux). Regions of interest include northern Eurasia, central U.S., North America, Alaska, North Pacific, and northern Scandinavia-eastern Siberia.
• Temporal Scale: 42-year period from 1979/1980 to 2020/2021 (December to March). WSPV events are defined as at least 5 consecutive WSPV days. Surface impacts are examined 10–39 days after event onset, and tropospheric precursors are analyzed up to 20 days prior to onset.

Methodology and Data

• Models used:
  • cost733class (Fortran Software package) for clustering analysis.
  • k-means and hierarchical clustering methods for identifying vortex patterns.
  • MATLAB R2020b for diagnostic analyses and figure generation.
• Data sources:
  • ERA5 daily mean atmospheric fields (Hersbach, 2020) for 1979/1980–2020/2021, including 10 hPa geopotential height, surface air temperature, sea level pressure (0.25° × 0.25° resolution), and 500 hPa wave analyses and NAM index (2.5° × 2.5° resolution).
  • NOAA CPC daily Arctic Oscillation (AO) index for 1979–2021.

Main Results

• A novel clustering method identified "mixed-type" WSPV events, characterized by a routine vortex evolution from displacement to split.
• Mixed-type WSPV events exhibit a longer average duration (21.71 days) compared to displaced-type (9.14 days) and split-type (16.83 days) events.
• These events are followed by a more persistent (10–39 days after onset) and stronger negative phase of the Arctic Oscillation (AO) at the surface.
• The sustained negative AO leads to significant cold anomalies over northern Eurasia and the central U.S. 10–39 days after event onset.
• Tropospheric precursors for mixed-type events involve a synergistic enhancement of planetary wavenumbers 1 and 2 in the mid-troposphere.
• Specifically, these precursors include a deepening of the Aleutian Low (enhancing wavenumber 1) and a strengthening of a dipole over northern Scandinavia-eastern Siberia (enhancing wavenumber 2).
• Displaced-type events are associated with wave reflection over North America and shorter durations, while split-type events are preceded by positive surface pressure anomalies over the North Pacific and enhanced wavenumber 2.

Contributions

• Introduces a novel quantitative method using clustering analysis to identify "mixed-type" WSPV events, which undergo a transition from displacement to split vortex patterns.
• Provides a comprehensive characterization of the climatological features, surface impacts, and tropospheric precursors unique to mixed-type WSPV events.
• Demonstrates that mixed-type WSPV events serve as strong stratospheric harbingers for more robust and persistent cold extremes over northern Eurasia and the central U.S., enhancing subseasonal-to-seasonal predictability.
• Identifies specific synergistic tropospheric planetary wave patterns (wavenumbers 1 and 2) and associated circulation anomalies (deepening Aleutian Low, northern Scandinavia-eastern Siberia dipole) as vital precursors for these high-impact events.

Funding

• National Natural Science Foundation of China (Grant 42176222, Grant 42305001)
• Open project of Xiamen Key Laboratory of Straits Meteorology (Grant HXQX202304, 2024KF02)
• Natural Science Foundation of Fujian Province (Grant 2023J011338)
• Fundamental Research Funds for the Central Universities (Grant 20720240029)

Citation

@article{Zhang2025strong,
  author = {Zhang, Murong and Yang, Xiao‐Yi and Huang, Yipeng},
  title = {A strong stratospheric harbinger for cold extremes: Weak polar vortex transition from displacement to split pattern},
  journal = {Weather and Climate Extremes},
  year = {2025},
  doi = {10.1016/j.wace.2025.100832},
  url = {https://doi.org/10.1016/j.wace.2025.100832}
}