LAUSR

In this study, we investigate impact of the Pacific Decadal Oscillation (PDO) on the interannual relationship between the El Niño and South China Sea summer monsoon onset (SCSSMO) using observations and community atmosphere model (CAM) simulations. We find that the interannual relationship between El Niño and SCSSMO during 1948–2019 has significant decadal variation, namely, this variation is consistent with PDO phase change. Specifically, a high El Niño‐SCSSMO correlation appears during the positive PDO phase from the mid‐1970s to the mid‐1990s; in contrast, no significant correlation exists during the negative PDO phase from the mid‐1950s to the mid‐1970s and also from the 2000s to the 2010s. Further analyses suggest that the PDO‐related sea surface temperature anomalies (SSTAs) in the western North Pacific (WNP), rather than in the North Pacific, modulate the El Niño‐SCSSMO relationship. The SSTAs in the WNP can prolong the existence of the anomalous WNP anticyclone (WNPAC) from the El Niño mature winter to the following May, and the corresponding western Pacific subtropical high (WPSH) is stronger and extends further westward, thus causing a late SCSSMO during the positive PDO phase. In contrast, the WNPAC disappears and the WPSH withdraws earlier in May during the negative PDO phase, inducing an early onset of the SCSSM. The observed PDO's impact on the interannual relationship between the El Niño and SCSSMO is confirmed by CAM simulations.