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<p>The late-summer-to-winter transition is a critical phase in the adjustment of atmospheric circulation, characterized by a shift in circulation patterns from summer to winter. This study examines the climatological withdrawal process of the South Asian High (SAH) and the factors that influence its dynamics during this transition. Analysis reveals that the SAH remains stable over the Tibetan Plateau (TP) until the fourth pentad of August, after which a notable shift occurs. From the fifth pentad in August to the second pentad in September, the SAH undergoes a southward displacement. From the third to the sixth pentads of September, the ridge line retreats and stabilizes over the southern Iran-TP region. In the following phase, the SAH shifts eastwards, reaching the Indo-China Peninsula (ICP) between the first and fourth pentads of October, before ultimately retreating to the ocean. The weakening and eventual dissipation of the Somali jet stream contributes to a reduction in convective activity. This reduction is exacerbated further by the weakening of westerly winds in the northern tropical Indian Ocean, which diminishes the moisture fluxes that were previously directed northwestwards towards the western and northern Bay of Bengal (BOB). Consequently, an active convection declines. The subsequent reduction in lower-tropospheric apparent heating over the TP triggers the southward movement of the SAH. This process is sustained by the continued decrease in heating over the BOB. Furthermore, enhanced apparent heating over the ICP and the northwestern tropical Pacific, particularly after the second pentad of October, likely facilitates the eastward retreat of the SAH, ultimately leading to its withdrawal over the ocean.</p>

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The Climatological Withdrawal Process of the South Asia High During the Transition From Late Summer to Winter

  • 期刊论文

  • 发表日期：

    2026年5月4日

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  • Xu Xue,
  • Wen Chen

摘要

The late-summer-to-winter transition is a critical phase in the adjustment of atmospheric circulation, characterized by a shift in circulation patterns from summer to winter. This study examines the climatological withdrawal process of the South Asian High (SAH) and the factors that influence its dynamics during this transition. Analysis reveals that the SAH remains stable over the Tibetan Plateau (TP) until the fourth pentad of August, after which a notable shift occurs. From the fifth pentad in August to the second pentad in September, the SAH undergoes a southward displacement. From the third to the sixth pentads of September, the ridge line retreats and stabilizes over the southern Iran-TP region. In the following phase, the SAH shifts eastwards, reaching the Indo-China Peninsula (ICP) between the first and fourth pentads of October, before ultimately retreating to the ocean. The weakening and eventual dissipation of the Somali jet stream contributes to a reduction in convective activity. This reduction is exacerbated further by the weakening of westerly winds in the northern tropical Indian Ocean, which diminishes the moisture fluxes that were previously directed northwestwards towards the western and northern Bay of Bengal (BOB). Consequently, an active convection declines. The subsequent reduction in lower-tropospheric apparent heating over the TP triggers the southward movement of the SAH. This process is sustained by the continued decrease in heating over the BOB. Furthermore, enhanced apparent heating over the ICP and the northwestern tropical Pacific, particularly after the second pentad of October, likely facilitates the eastward retreat of the SAH, ultimately leading to its withdrawal over the ocean.