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–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), respectively. 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.