Recent Enhancement in Co-Variability of the Western North Pacific Summer Monsoon and the Equatorial Zonal Wind

The western North Pacific summer monsoon (WNPSM) is an important subcomponent of the Asian summer monsoon. The equatorial zonal wind (EZW) in the lower troposphere over the western Pacific may play a critical role in the evolution of the El Niño-Southern Oscillation (ENSO). The possible linkage between the EZW over the western Pacific and the off-equatorial monsoonal winds associated with the WNPSM and its decadal changes have not yet been fully understood. Here, we find a non-stationary relationship between the WNPSM and the western Pacific EZW, significantly strengthening their correlation around the late 1980s/early 1990s. This observed shift in the WNPSM–EZW relationship could be explained by the changes in the related sea surface temperature (SST) configurations across the tropical oceans. The enhanced influence from the springtime tropical North Atlantic, summertime tropical central Pacific, and maritime continent SST anomalies may be working together in contributing to the recent intensified WNPSM–EZW co-variability. The observed recent strengthening of the WNPSM–EZW relationship may profoundly impact the climate system, including prompting more effective feedback from the WNPSM on subsequent ENSO evolution and bolstering a stronger biennial tendency of the WNPSM–ENSO coupled system. The results obtained herein imply that the WNPSM, EZW, ENSO, and the tropical North Atlantic SST may be closely linked within a unified climate system with a quasi-biennial rhythm occurring during recent decades, accompanied by a reinforcement of the WNPSM–ENSO interplay quite possibly triggered by enhanced tropical Pacific–Atlantic cross-basin interactions. These results highlight the importance of the tropical Atlantic cross-basin influences in shaping the spatial structure of WNPSM-related wind anomalies and the WNPSM–ENSO interaction.