Effect of the Boreal Summer Intraseasonal Oscillation on Interannual Variation of Rainy Season Onset in Southwest China

Abstract Southwest China (SWC) is a significant disaster-prone area, where approximately 80% of the annual precipitation is concentrated during the rainy season. The rainy season onset date (RSOD) in SWC directly influences the precipitation amount during the rainy season, which has crucial implications for drought and flood conditions, as well as geological disasters such as landslides and debris flows. Based on meteorological station data and reanalysis data from 1981 to 2023, this study reveals that the rainy season in SWC advances from east to west, with an average RSOD of 27 May superimposed with interannual variations. It is found that the first component of the boreal summer intraseasonal oscillation (BSISO1) plays a phase-locked role in the interannual variation of RSOD in SWC. Phases 1–3 of BSISO1 are conducive to the late RSOD, while phases 4–6 are conducive to the early RSOD. BSISO1 affects the moisture and dynamic conditions over SWC through whether convection propagates northward or not, resulting in an early or late onset of the rainy season in SWC. Moreover, in negative phase years of the Indian Ocean dipole (IOD), the longitudinal asymmetry of water vapor and convergence anomalies at the convective center is conducive to the northward propagation of BSISO1 convection and thereby the rainy season onsets earlier, which is the opposite in positive IOD years. These results shed some light on the mechanisms for interannual variations and the prediction of RSOD in SWC. Significance Statement Southwest China (SWC) is a region seriously affected by droughts, floods, and landslides, with most of its annual rain falling during the rainy season. The rainy season onset date (RSOD) in SWC shows strong interannual variations with an average start date around 27 May. This study finds that the first component of boreal summer intraseasonal oscillation (BSISO1) plays a key role in these variations. When BSISO1 convection is over the equatorial Indian Ocean (phases 1–3), the rainy season starts later. When it moves to the Bay of Bengal (phases 4–6), the RSOD begins earlier. BSISO1 can modulate moisture and dynamic conditions over SWC by regulating the northward propagation of convection, thereby determining an early or late rainy season onset. Furthermore, the negative Indian Ocean dipole (IOD) years promote BSISO1 convection spread north, leading to an earlier RSOD, while positive IOD years block this movement, causing delays. These findings advance the understanding of RSOD interannual variability and provide insights for improving its predictability in SWC.