The interdecadal variations of cross-month correlation feature of the NWPSH

The Northwest Pacific Subtropical High (NWPSH) presents a notable and even counterintuitive phenomenon: it shows a strong positive correlation between June and August (correlation coefficient: 0.55, significant at the 99.5% confidence level) during 1979–2005, while this correlation weakens sharply to -0.05 in 2006–2024. However, the relationships between June and July, and between July and August remain consistently weak throughout the entire period. Positive sea surface temperature (SST) anomalies in the tropical North Indian Ocean (TNIO) contribute to a persistent intensification of the NWPSH from June to August before 2005. Meanwhile, the Boreal Summer Intraseasonal Oscillation (BSISO) exhibits a stronger 60-day periodicity, which can cause opposite (similar) variations of NWPSH in adjacent month (cross-month). Under the combined effects of TNIO SST positive anomalies and BSISO, the NWPSH mainly exhibits the cross-month correlation feature, while the relationships between adjacent months are very weak. After 2005, the BSISO exhibits a marked shortening of its periodicity, and the key SST regions associated with the NWPSH in June, July, and August are also inconsistent, which induce the weakening of the cross-month correlation feature of the NWPSH. The shortening of the BSISO’s periodicity is attributed to warming over the Maritime Continent and the tropical western Indian Ocean, which intensify both the zonal Walker and meridional Hadley circulations. These changes enhance downward motions over the tropical eastern Indian Ocean and the northwestern Pacific, thereby suppressing the initiation of BSISO’ convection and accelerating its decay.