Inconsistent Interannual Relationships of 10–25- and 30–60-Day Oscillations across the South China Sea–Western North Pacific Domain

Abstract The South China Sea (SCS)–western North Pacific (WNP) is a domain where the 10–25- and 30–60-day intraseasonal oscillations (ISOs) exhibit vigorous activities during summer with significant year-to-year variations. This study examines the relationship between the summer mean intensity of these two ISOs during 1979–2020. Notably, this relationship is found inconsistent across the domain, with a significant negative correlation over the SCS but transitioning into a positive one over the eastern WNP. To physically interpret this inconsistency, regional signatures of ISO behaviors and seasonal mean backgrounds regulating their intensities are explored and compared. Results indicate that the SCS 10–25-day (30–60 day) oscillation originates from the equatorial western Pacific (eastern Indian Ocean). The opposing anomalies in atmospheric backgrounds (including convection, winds, and vertical shear) between the above two equatorial regions facilitate one ISO emanation while suppressing the other, responsible for the anticorrelated intensity change of the two SCS ISOs. In contrast, eastern WNP ISOs exhibit distinct evolution features, particularly for the 30–60-day oscillation, manifested as a weakened connection with the Indian Ocean and an enhanced local process over the WNP. Therefore, the atmospheric backgrounds over the western Pacific, dominated by an extensive cyclonic anomaly, along with enhanced convection, increased moisture, and intensified easterly vertical shear, favor the simultaneous enhancement (or suppression with reversed conditions) of the two ISOs. These atmospheric backgrounds affecting SCS (WNP) ISOs are found to be associated with western Pacific (ENSO related) SST anomalies in concurrent summer (previous and subsequent winter). Significance Statement The 10–25- and 30–60-day oscillations are two dominant intraseasonal modes in the tropical atmosphere. In this study, we highlight that the relationship between the summer mean intensity of these two intraseasonal oscillations (ISOs) is region dependent, exhibiting a significant negative correlation over the South China Sea (SCS) but a positive correlation over the eastern part of the western North Pacific (WNP). Our objective is to reveal the underlying mechanism behind this inconsistent relationship by investigating ISO behaviors and influential factors governing ISO intensities. The derived results may contribute to advancing our comprehension and prediction of the intricate subseasonal variability within the SCS–WNP domain.