| Summary: | In this study, the interdecadal change in the size of the Indo-Pacific warm pool (IPWP) and its connection with the intensity of the South China Sea summer monsoon (SCSSM) in May are investigated. The results show that the size of the IPWP experienced a marked interdecadal extension in the late 1980s, and its impacts on the SCSSM have also strengthened. Further analyses indicate that interdecadal changes in the connection between the IPWP and SCSSM variations are related to the changes in the atmospheric circulation anomalies over the tropical western Pacific due to the size of the IPWP. During 1951–1986, the IPWP was smaller than the climatologic size, which led to suppressed convection over the tropical western Pacific and easterly anomalies north of the equator. The Philippines and South China Sea (SCS) region was controlled by anomalous anticyclonic circulation, which was not conducive to the formation and development of the SCSSM. In contrast, during 1987–2018, the IPWP expanded significantly eastward. As a response to the IPWP size extension, convective activities were enhanced over the tropical western Pacific. Anomalous cyclonic circulation was located over the Philippines and SCS regions, which favored the enhancement of the SCSSM through wide westerly flow in the Bay of Bengal and tropical western Pacific. Moreover, it is shown that the interdecadally enhanced intraseasonal oscillations (ISOs) over the tropical western Pacific in the late 1980s also favored reinforcement of the SCSSM intensity.
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