Impacts of Multi-Scale Disturbances on Tropical Cyclone Genesis over the Northern Part of the Western North Pacific

Recent studies consistently highlight a northward shift in tropical cyclone (TC) activity in the western North Pacific (WNP); however, the impacts of the underlying drivers governing this shift remain insufficiently explored. This study focuses on TC genesis over the northern WNP (north of 20°N) and identifies a significant meridional seesaw relationship in TC genesis frequency between the northern and southern WNP (bounded by 20°N). Furthermore, we quantitatively diagnose the relative contributions of three key atmospheric disturbances—the Madden–Julian Oscillation (MJO), Quasi-Biweekly Oscillation (QBWO), and synoptic-scale disturbance (SSD)—to the interannual variability of northern WNP TC genesis. Results show that these multi-scale disturbances together explain 39.2% of the total variance in northern WNP TC genesis, with their individual contributions accounting for 40.9% (MJO), 35.1% (QBWO), and 24.0% (SSD). The joint effect of these disturbances induces a Matsuno–Gill-like circulation pattern across the tropical Indo-Pacific, which drives downward vertical motion over the off-equatorial WNP. Additionally, tropical interactions triggered by these disturbances activate the meridional Pacific–Japan (PJ) or East Asia–Pacific (EAP) atmospheric teleconnection pattern. This teleconnection generates anomalous cyclonic winds and enhances the water vapor supply north of 20°N, creating favorable conditions for TC genesis in the northern WNP. This study clarifies the relative importance of multi-scale disturbances in regulating northern WNP TC genesis and highlights the necessity of incorporating SSDs into future analyses of interannual TC variability.