Delayed Tropical Asian Summer Monsoon Onset in Recent Decades

Abstract This study reveals that the onset of tropical Asian summer monsoon (TASM) experienced a pronounced interdecadal delay (about one week) after the late‐2000s. Both the large‐scale seasonal transition reflected by EOF analysis and the monsoon onset dates defined by local rainfall exhibited a clear interdecadal change. Correspondingly, there appears reduced rainfall and low‐level easterly anomalies over tropical Asia in May, indicating that the monsoonal westerly winds and convective activities cannot be stably established. Further analysis suggests that the SST warming in the equatorial western Indian Ocean serves as a possible driver of the delayed TASM onset. The warm SST anomalies trigger a Kelvin wave‐like response to the east, inducing anticyclonic circulation and easterly anomalies over tropical Asia, which hinder moisture transport and reduce rainfall. The reduced rainfall further strengthens anomalous easterly winds, thereby favoring the delayed onset of the summer monsoon.