Distinct Influence of Different Boreal Summer Intraseasonal Oscillation Modes on Western North Pacific Tropical Cyclone Activity

Abstract Recent studies have identified three distinct modes of propagation associated with the boreal summer intraseasonal oscillation (BSISO): canonical (northeastward), northward dipole, and eastward expansion. However, the impacts of these three BSISO modes on western North Pacific tropical cyclone (TC) activity remain unclear. To address this question, we identified 98 BSISO events during the boreal summer from 1979 to 2021, comprising 47 canonical, 26 northward dipole, and 25 eastward expansion events. In terms of TC genesis, the canonical and eastward expansion modes primarily influence TC genesis over the South China Sea, whereas the northward dipole mode modulates TC genesis across the western North Pacific. These differences arise from BSISO-driven shifts in the large-scale environment and associated monsoon trough displacement. In terms of TC tracks and regional impacts, TCs tied to the canonical mode disproportionately affect southern China, whereas those linked to the northward dipole mode more strongly impact eastern China. The meridional migration of synoptic-scale waves and the zonal shift of the western Pacific subtropical high underpin the observed TC impacts across China driven by various BSISO modes. These findings highlight the distinct modulation of TC activity by different BSISO modes and offer useful insights for intraseasonal TC forecasting. Significance Statement Although the boreal summer intraseasonal oscillation (BSISO) has three main propagation types: northeastward (canonical), northward dipole, and eastward expansion, how these modes affect tropical cyclone (TC) activity in the western North Pacific is unclear. Analysis of 98 BSISO events (1979–2021) reveals distinct regional impacts. Canonical and eastward expansion modes primarily modulate TC genesis in the South China Sea, while the northward dipole mode influences TCs across the western North Pacific. Consequently, canonical mode TCs primarily impact southern China, while northward dipole mode TCs more strongly affect eastern China. These differences stem from shifts in the large-scale environment, monsoon trough, synoptic-scale waves, and the western Pacific subtropical high. These findings provide valuable insights for improving intraseasonal TC forecasts.