Impact of the boreal summer intraseasonal oscillation on tropical cyclone rapid intensification in the northwestern Pacific

The present study investigates the impact of the boreal summer intraseasonal oscillation (BSISO) on tropical cyclones (TCs) rapid intensification (RI) in the northwestern Pacific (NWP) during May to October from 1981 to 2024. The individual and combined effects of the two components of BSISO, BSISO1 (with a period of 30–60 days) and BSISO2 (with a period of 10–30 days), are discussed. The results show that both BSISO1 and BSISO2 significantly influence RI frequency and RI location, while their effects on RI intensification rate, RI duration and RI enhanced-magnitude are relatively weak. More (less) RI events occur in the convective (non-convective) phases of BSISO1 and BSISO2 when the convective (non-convective) signals dominate the main RI region. The locations of RI occurrence align with TC genesis locations, which show a northeastward (northwestward) shift with the same propagation of the convective signals during the phases of BSISO1 (BSISO2). Furthermore, the coefficient of variation and suppression ratio of the RI frequency exhibit greater values in BSISO2 compared to BSISO1, indicating a more pronounced modulation by BSISO2 on the RI frequency. Finally, we explore the underlying mechanisms of BSISO's impact: compared with the vertical wind shear and sea surface temperature, the mid-level relative humidity and low-level relative vorticity play a major role in modulating the impact of BSISO on TCRI. These results exhibit the significant modulation of BSISO on the characteristics of TCRI in the NWP.