Decadal change in the connection between the Pacific–Japan pattern and the Indian Ocean SST basin mode

The Pacific–Japan (PJ) pattern, which is characterized by meridional alternating anomalous cyclone and anticyclone along the East Asian coast, is the key circulation system modulating the East Asian summer climate. It is essential to explore the oceanic origins of the PJ pattern and examine their fidelity, with the aim to identify reliable seasonal predictability sources and therefore improve the seasonal prediction of the East Asian climate. This study reveals a decadal change in the connection between the PJ pattern and the Indian Ocean SST basin mode (IOBM) around 2006. Before 2006, the PJ pattern is closely associated with the IOBM, with a correlation coefficient of 0.59 (p < 0.05), whereas this tight connection disappears after 2006. We further find that the weakened relationship between the PJ pattern and the IOBM is mainly contributed by the decoupling of the Northeast Asian anomalous cyclone of the PJ pattern (PJ_C) and the IOBM after 2006. From the first to the second epoch (before and after the decadal change), the oceanic forcing of PJ_C changes from the IOBM to the tropical central/eastern Pacific (CEP) SSTA. Numerical experiments reproduce nicely the influence of the alternate oceanic drivers (i.e., the IOBM and CEP SSTA) on the formation of the PJ pattern. The shortened period of ENSO in the second epoch weakens the IOBM’s capacitor effect, which possibly leads to the decoupling of PJ_C and the IOBM.