Weakening relationship between East Asian summer monsoon and Asian-Pacific Oscillation after 1990s

The East Asian summer monsoon (EASM), which is an important influencing factor of the summer climate of East Asia, is associated with large-scale change of the land–sea thermal contrast. The Asian–Pacific Oscillation (APO) at middle-high latitude over the Asian-Pacific region can modulate the EASM because it not only represents the upper–tropospheric zonal land–sea thermal contrast over Asia and the Pacific region, but it also affects the sea surface temperature (SST) over the North Pacific, which can tune the land–sea thermal contrast for the EASM. This study revealed weakening of the APO–EASM relationship since the 1990s. It was found that the relationship between the APO and the EASM during 1948–1990 (1991–2016) was statistically significant (insignificant). Further study indicated the APO was concurrent with significant positive SST in the central North Pacific and subtropical central–western Pacific during 1948–1990, which contributed to the shift of the Pacific Decadal Oscillation (PDO) from its cold to warm phase and led to a weakened EASM. The APO-related SST and atmospheric circulation anomalies were found statistically insignificant during 1991–2016, which indicates weakening of the influence of the APO on the shift of the PDO, and even a weaker link to the EASM.