Weakened influence of ENSO on the East Asian summer monsoon since the early 2000s

Abstract

El Niño–Southern Oscillation (ENSO) was identified as the dominant factor influencing the East Asian summer monsoon (EASM), especially after the mid-1970s when the tropical Indian Ocean (TIO) response remarkably strengthened. Here, we find that the influence of ENSO on the EASM has been diminishing since the early 2000s. The EASM in wind anomalies associated with the positive phase of ENSO quickly disintegrates in August, changing from an anticyclone over the western North Pacific (WNPAC) to a cyclone over the western North Pacific (WNP), which exerts significant influence on the East Asia rainfall. These weakened EASM responses are closely linked to the changes in ENSO’s rate of decay around the early 2000s. During 1977–1999, ENSO events peaking in the boreal winter frequently display a gradual decay, triggering robust positive ocean–atmosphere feedback, which extends beyond the TIO and involves the WNP. The resultant North Indian Ocean (NIO) warming develops and persists through the decaying summer, maintaining the WNPAC in August. In contrast, ENSO events exhibit a faster decay during 2000–2022, leading to a weakened ENSO-induced TIO feedback. Additionally, the WNP warms up, accompanied by the collapse of the easterly wind response, contributing to the weak summer peak in the NIO. In turn, the weak NIO warming rapidly decays, which cannot sustain the WNPAC in August. This study emphasizes the crucial role of WNP air–sea coupling in the changing influences of ENSO on the EASM.