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It is observed that the significant negative relationship between the southern Tibetan Plateau precipitation (STPP) and the Southeast Asian summer monsoon (SEASM) uncovered recently has weakened apparently during the recent decade. The current study is aimed at investigating the interdecadal instability of this TP-SEASM connection and the possible impacting factors. During 1979–2010, the TP heating significantly weakened the SEASM through the interaction between the eastward-extended South Asian high (SAH) and the westward-extended western Pacific subtropical high (WPSH), while no significant change occurred in the SEASM associated with the enhanced TP heating during 2011–2020. Compared with the climate state of 1979–2010, sea surface temperature (SST) warmed in almost entire globe and low-level cyclonic circulation and precipitation were enhanced in the SEASM region during 2011–2020. Meanwhile, the WPSH intensified and its northern boundary extended northward. These oceanic and atmospheric features could be important factors that broke the interaction between the SAH and the WPSH and weakened the negative relationship between STPP and SEASM. The model experiment forced by observed global SST warming validates the above observational features and hypothesis. The warmer SST enhanced the low-level cyclone in the SEASM region and extended the northern boundary of WPSH northward, weakening the influence of SAH on the downstream SEASM region. Therefore, the SEASM region was not under the control of an anomalous high-pressure system so that the negative TP-SEASM relationship became weakened during the recent decade. • The negative relationship between Tibetan Plateau heating and Southeast Asian summer monsoon weakened apparently. • Warmer sea surface temperature is an external impacting factor for this weakened relationship. • Warming-induced convection interrupted the interaction between the South Asian high and the subtropical high. [ABSTRACT FROM AUTHOR]