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<p>The southwest vortex (SWV) is a meso-α-scale low pressure system with cyclonic circulation generated in the lower troposphere east of the Tibetan Plateau. It is a key precipitation weather system in China. The SWV can move out of the source area under certain suitable large-scale circulation patterns, often resulting in widespread heavy precipitation in downstream areas. However, there is a lack of understanding of the differences in the circulation background of moving-out and non-moving-out SWVs. Based on ERA5 reanalysis data, in this study, the large-scale circulation characteristics of 39 moving-out SWVs and 76 non-moving-out SWVs generated in the Sichuan Basin during the summer from 2010 to 2019 are investigated through composite analysis. The main results show that the moving-out SWV is generally stronger and deeper than the non-moving-out SWV. When the moving-out SWV is active in the source area, the upper tropospheric subtropical westerly jet is strong, with its position at 15 degrees of longitude to the east and 3–4 degrees of latitude to the south, compared with the situation during the non-moving-out SWV activity. Thus, the moving-out SWV is located under the south side of the upper jet entrance area, which promotes further development of the SWV and then facilitates its eastward outflow. In the middle and lower troposphere, the western Pacific subtropical high pressure over the South China Sea is stronger and extends significantly westward. The strong southerly and southwesterly winds on its west side are conducive to water vapor transport in the source area, probably resulting in the moving-out SWV being preferentially strong and deep, with high precipitation intensity. The westerly winds from 25°N to 30°N is significantly strengthened due to an increase in the south-north pressure gradient, conducive to guiding the SWV to move eastward along 30°N. The low-level significantly colder and stronger northerly winds on the north side of the source area indicate that the cold air activity is more active, which may cause the moving-out SWV to have a relatively stronger baroclinicity.</p>

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Large-scale circulation characteristics of summer moving-out and non-moving-out Southwest vortices in the Sichuan Basin

  • 期刊论文

  • 发表日期：

    2025年9月6日

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  • Na Song,
  • Xinyuan Feng,
  • Caiyun Feng

摘要

The southwest vortex (SWV) is a meso-α-scale low pressure system with cyclonic circulation generated in the lower troposphere east of the Tibetan Plateau. It is a key precipitation weather system in China. The SWV can move out of the source area under certain suitable large-scale circulation patterns, often resulting in widespread heavy precipitation in downstream areas. However, there is a lack of understanding of the differences in the circulation background of moving-out and non-moving-out SWVs. Based on ERA5 reanalysis data, in this study, the large-scale circulation characteristics of 39 moving-out SWVs and 76 non-moving-out SWVs generated in the Sichuan Basin during the summer from 2010 to 2019 are investigated through composite analysis. The main results show that the moving-out SWV is generally stronger and deeper than the non-moving-out SWV. When the moving-out SWV is active in the source area, the upper tropospheric subtropical westerly jet is strong, with its position at 15 degrees of longitude to the east and 3–4 degrees of latitude to the south, compared with the situation during the non-moving-out SWV activity. Thus, the moving-out SWV is located under the south side of the upper jet entrance area, which promotes further development of the SWV and then facilitates its eastward outflow. In the middle and lower troposphere, the western Pacific subtropical high pressure over the South China Sea is stronger and extends significantly westward. The strong southerly and southwesterly winds on its west side are conducive to water vapor transport in the source area, probably resulting in the moving-out SWV being preferentially strong and deep, with high precipitation intensity. The westerly winds from 25°N to 30°N is significantly strengthened due to an increase in the south-north pressure gradient, conducive to guiding the SWV to move eastward along 30°N. The low-level significantly colder and stronger northerly winds on the north side of the source area indicate that the cold air activity is more active, which may cause the moving-out SWV to have a relatively stronger baroclinicity.