DataSheet1_Dominant Role of Meridional Circulation in Regulating the Anomalous Subsidence of the Western Pacific Subtropical High in Early Summer 2020.docx

Anomalous subsidence over the western part of the western Pacific subtropical high (WPSH) caused record-breaking precipitation anomalies over the Yangtze-Huaihe River catchment in early summer 2020 (June–July 2020). The meridional circulation (MC) made a positive contribution to this anomalous subsidence, while the zonal circulation (ZC) made a negative contribution. The quantitative contributions of the MC and ZC to this anomalous subsidence were approximately 110% and –10% in June, 130% and –30% in July, and 120% and –20% for the mean of June and July, respectively, suggesting that the MC played a dominant role in the anomalous subsidence of the western part of the WPSH. The anomalous MC, with a rising branch located at the Maritime Continent and a descending branch located over South China, was forced by the warming of the northern tropical Indian Ocean and the rapidly developed La Niña event, which further resulted in the intensification and southwestward expansion of the WPSH and thus in heavy rainfall over the Yangtze River region.

2020年初夏（2020年6—7月），西太平洋副热带高压（western Pacific subtropical high, WPSH）西侧区域出现异常下沉运动，引发江淮流域（Yangtze-Huaihe River catchment）出现破纪录的降水异常。经向环流（meridional circulation, MC）对此次异常下沉运动起到正贡献，而纬向环流（zonal circulation, ZC）则产生负贡献。6月时，MC与ZC对该异常下沉运动的定量贡献分别约为110%和–10%；7月分别为130%和–30%；6—7月的平均值则分别为120%和–20%，这表明MC在WPSH西侧区域的异常下沉运动中占据主导地位。此次异常经向环流以海洋大陆（Maritime Continent）为上升支、以华南地区为下沉支，其形成受北热带印度洋增暖与快速发展的拉尼娜事件（La Niña event）驱动，进而推动WPSH增强并向西南方向扩张，最终引发长江流域强降雨。