Sun et al. (2026) Mechanisms of the interannual variability in occurrence numbers of the Southwest China vortex in summer

Identification

• Journal: Climate Dynamics
• Year: 2026
• Date: 2026-01-06
• Authors: Xiaoting Sun, Minghao Wang, Yueqing Li, Yihui Ding, Qingquan Li
• DOI: 10.1007/s00382-025-07962-8

Research Groups

• China Meteorological Administration Key Laboratory for Climate Prediction Studies, National Climate Centre, Beijing, China
• Institute of Plateau Meteorology, China Meteorological Administration, Chengdu, Sichuan, China
• Heavy Rain and Drought-Flood Disasters in Plateau and Basin Key Laboratory of Sichuan Province, Chengdu, Sichuan, China

Short Summary

This study establishes an objective dataset of Southwest China Vortex (SWCV) events from 1979-2024 and investigates the interannual variability of summer SWCV occurrence, revealing its modulation by the mid-latitude Mongolian Cyclone (MONC) via circumglobal teleconnection and the subtropical Western North Pacific Anticyclone (WNPAC) influenced by Indian Ocean Basin Mode-like warming.

Objective

• To analyze the spatiotemporal evolution characteristics of Southwest China Vortex occurrence numbers (SWCV_NUM) in China from 1979 to 2024.
• To identify the dominant circulation patterns and underlying physical mechanisms driving the interannual variability of summer SWCV_NUM.

Study Configuration

• Spatial Scale: Eastern Tibetan Plateau, Sichuan Basin, Southwest China (26–33°N, 100–109°E), Yangtze River Basin, Huaihe River Basin, North China, Northeast China, South China, Mongolia, Western North Pacific, Indian Ocean.
• Temporal Scale: 1979–2024 (46 summers), with 6-hourly data for reanalysis and daily for observations. SWCV events typically persist for 1–3 days.

Methodology and Data

• Models used: An objective identification algorithm for the Southwest China vortex (SWCV) was developed. No specific climate models were used for simulations, but reanalysis data were extensively utilized.
• Data sources:
  • China’s first-generation global atmospheric reanalysis (CRA-40): 6-hourly, 0.25° × 0.25° resolution, 37 pressure levels (1000–100 hPa) of geopotential height, temperature, vertical velocity, zonal and meridional wind components, specific humidity, surface pressure, and land surface reanalysis (1979–2024).
  • Fifth-generation European Centre for Medium-Range Weather Forecasts Reanalysis (ERA5): 6-hourly geopotential height data, 0.25° × 0.25° resolution (1979–2024).
  • CN05.1 gridded observational dataset: Daily precipitation data from China's National Meteorological Information Center, 0.25° × 0.25° resolution, based on over 2400 surface stations.
  • National Oceanic and Atmospheric Administration (NOAA) Extended Reconstructed SST version 5 (ERSSTv5): Monthly sea surface temperature (SST) data, 2° × 2° horizontal resolution (1979–2024).
  • Southwest Vortex Yearbook (2012–2022) for comparative evaluation.

Main Results

• An objective SWCV dataset for 1979–2024 was established, identifying 1,184 SWCV events over 46 summers, averaging 25 events annually.
• Summer SWCV occurrence numbers (SWCV_NUM) exhibit pronounced interannual variability (nearly twofold differences between active and inactive years) and a weak declining trend over the 46-year period.
• SWCV processes are frequently accompanied by heavy to torrential rainfall: 42% of events coincide with torrential rain (30–69.9 mm / 12 h), 36% with heavy rain (15–29.9 mm / 12 h), 14% with moderate rain (5–14.9 mm / 12 h), and 7% with heavy rainstorms (70–139.9 mm / 12 h).
• Enhanced summer SWCV activity is associated with a pronounced precipitation band extending from northern Southwest China to the Huaihe River basin, alongside suppressed convection over southern China.
• The interannual variability of summer SWCV_NUM is significantly correlated with two dominant circulation systems: the mid-latitude Mongolian Cyclone (MONC) and the subtropical Western North Pacific Anticyclone (WNPAC).
• The MONC is primarily modulated by a quasi-barotropic circumglobal teleconnection (CGT) pattern, which enhances SWCV_NUM through eastward-propagating wave activity flux from western Europe across Siberia and Mongolia to Southwest China.
• Indian Ocean Basin Mode-like warming (0–25°N, 50–120°E) generates anomalous low-level easterlies in the subtropical western Pacific to the Bay of Bengal, inducing anticyclonic shear that strengthens the WNPAC. This circulation pattern enhances tropical moisture and heat transport into Southwest China, promoting SWCV formation.
• A combined MONC-WNPAC index demonstrates a significant correlation (r = 0.54) with summer SWCV_NUM, effectively capturing its interannual variability and suggesting these systems drive its observed declining trend.

Contributions

• Developed a comprehensive whole-process objective identification method for the Southwest China vortex, establishing a high-quality, objective, and extended (1979–2024) SWCV activity dataset.
• Provided an improved understanding of the climatological features and interannual variability of SWCV occurrence numbers in summer.
• Identified and elucidated the distinct physical mechanisms by which the mid-latitude Mongolian Cyclone (modulated by circumglobal teleconnection) and the subtropical Western North Pacific Anticyclone (influenced by Indian Ocean Basin Mode-like warming) drive the interannual variability of summer SWCV_NUM.
• Implemented an operational SWCV monitoring and prediction module within the National Climate Center's Climate Analysis and Prediction System (CIPAS3.1), providing real-time products for forecasting.

Funding

• National Key Research and Development Program of China (2023YFF0805104, 2022YFE0136000, 2016YFA0602200)
• Innovative Development Special Project of China Meteorological Administration (CXFZ2025Q013, CXFZ2023J003)
• National Natural Science Foundation of China (42305018, U2242207)
• Natural Science Foundation of Anhui Province of China (2208085UQ08)
• Youth Innovation Team of China Meteorological Administration (CMA2023QN15)
• Key Innovation Team of Sichuan Meteorological Bureau (SCQXTD202402)

Citation

@article{Sun2026Mechanisms,
  author = {Sun, Xiaoting and Wang, Minghao and Li, Yueqing and Ding, Yihui and Li, Qingquan},
  title = {Mechanisms of the interannual variability in occurrence numbers of the Southwest China vortex in summer},
  journal = {Climate Dynamics},
  year = {2026},
  doi = {10.1007/s00382-025-07962-8},
  url = {https://doi.org/10.1007/s00382-025-07962-8}
}