South China has an important position and influence in economic,cultural and social development,and the SST anomalies in the Indian Ocean have a non-negligible impact on the weather and climate of South China.This paper explores the mechanism of precipitation anomalies during the second rainy season in South China to improve the theoretical system of precipitation anomalies in South China,and to provide a scientific basis for a deep understanding of drought and flooding anomalies,and work on prevention and mitigation of disasters in South China.In this paper,the relationship between IOD Modoki and precipitation anomalies during the second rainy season (July to September) in South China and possible mechanisms were investigated using CN05.1 Chinese regional high-resolution precipitation grid data for 1979—2019,SST data from Hadley Centre observations made in the UK,and ERA5 month-by-month atmospheric reanalysis data.The results show that the precipitation anomalies during the second rainy season in South China are significantly positively (negatively) correlated with the SST anomalies in the central (eastern and western) tropical Indian Ocean,manifesting as the spatial distribution of Indian Ocean IOD Modoki or Indian Ocean triple polariton events.After filtering out the influence of the ENSO signal,the precipitation anomalies during the second rainy season in South China are still more closely related to the IOD Modoki.The influences of positive IOD Modoki anomalies on the precipitation anomalies during the second rainy season in South China are as follows:1.The anomalous water vapour is transported westward from the eastern tropical Indian Ocean to the tropical Central Indian Ocean,then transported eastward to South China by the Koch force in the northern hemisphere,thus providing sufficient water vapour conditions in South China.The main water vapour convergence contributions to the positive precipitation anomaly in South China are the horizontal disturbance divergence term of the mean water vapour and the mean water vapour vertical advection term caused by the disturbance.2.Negative SST anomalies in the tropical southeast Indian Ocean cause southeasterly wind anomalies in the lower troposphere from the tropical southeast Indian Ocean to the tropical central Indian Ocean through the Mastuno—Gill response,enhancing the trans-equatorial flow near 70°E,which is then transported eastward to the northwest Pacific in the northern hemisphere,where it causes lower tropospheric cyclonic circulation anomalies.3.The anomalous low (high) tropospheric divergence (convergence) in the tropical East Indian Ocean and anomalous low (high) tropospheric convergence (divergence) anomalous convergence in South China enhance the local Hadley circulation in East Asia and favour precipitation generation in South China.4.The IOD Modoki causes the South Asian monsoon region to be controlled by anomalous downwelling motion,and causes positive vorticity anomalies around the eastern subtropical North Atlantic,North African desert region and western Mediterranean through the monsoon—desert mechanism.This in turn excites quasi-stationary Rossby waves propagating downstream along the rapids,and enhances the Japan Sea high pressure anomaly and lower tropospheric cyclonic circulation anomaly in South China and adjacent areas.All of these factors favour the production of precipitation in South China,while the absence of these factors is detrimental to the production of precipitation in South China.The above results are also verified in the numerical model.