Impact of Victoria Mode on Late-Season Tropical Cyclone Genesis Over the Western North Pacific

Abstract

The Victoria Mode (VM) plays a vital role in shaping Pacific Sea Surface Temperature (SST) variability. However, its impact on late-season tropical cyclone genesis (TCG) over the western North Pacific (WNP) remains underexplored. This study investigated the influence of VM on WNP TCG from 1965 to 2020. Significant relationships emerge in interannual (6 year) and decadal (11 year) timescales. On the interannual timescale, VM correlates positively with TCG west of 145°E and negatively to the east. On the decadal timescale, the negative relationship spans the entire WNP. Victoria Mode directly modulates vertical motion and induces low-latitude anticyclonic circulation across the low-latitude North Pacific by altering surface wind convergence and diabatic heating. This, in turn, shapes environmental conditions, affecting TCG. Dynamic factors are more important in WNP TCG changes than thermodynamic factors. Critical environmental conditions in VM's impact on TCG include four dynamic factors: relative vorticity, vertical velocity, meridional gradient of zonal wind, and vertical shear of zonal winds. Vertical shear of zonal winds is particularly influential. Distinct correlation patterns and mechanisms are diagnosed between different timescales, attributed to anomalous SST observed over the WNP on the interannual timescale but not on the decadal timescale. Furthermore, this study illustrates the complexity of region- and time-dependent relationships between environmental conditions and TCG. Such dependencies pose challenges for conventional genesis potential index models in accurately capturing the spatial characteristics of the VM-TCG relationship. Overall, our findings uncover VM's substantial influence on WNP TCG, enhancing understanding of key environmental conditions and mechanisms driving interannual and decadal TCG variations.