| Tropical cyclone(TC)itself is a weather-scale system,but its genesis and spatial distribution have significant differences on multiple time scales such as interannual and intra-seasonal scales.Therefore,improving the understanding of TC genesis at different time scales is beneficial to improve the accuracy of forecasting.Prior studies have emphasized the influence of the boreal summer intra-seasonal oscillation(BSISO)at its active and inactive phases relative to specific basin and global TC genesis,with less studies on the variability of BSISO and its effects on interannual time scales,which inevitably have different influences on weather and climate events.This study therefore explores the impact of BSISO interannual variability on western North Pacific(WNP)TCs,which is conducive to an improved understanding of different time scales of TC genesis and contributes to the climate prediction of WNP TC.Main conclusions are as follows:(1)Study indicates that interannual meridional variability in the BSISO modulates the meridional migration of WNP TC genesis and is related to changes in BSISO phase structure.These structural changes are characterized by occurrence frequency changes for individual BSISO phases between the equatorial region over the eastern Indian Ocean-Maritime Continent(EIO-MC)regions and the subtropical region over the South China Sea-western North Pacific(SCS-WNP)regions.This interannual north-south change of the BSISO appears to be associated with changes in sea surface temperature over the MC region and WNP mid-to-low tropospheric moisture advection,mainly via remote forcing of a Gill-type Rossby response to BSISO convection and a large meridional asymmetry of the low-level background moisture distribution.During years with increased BSISO residence time over the SCS-WNP regions,more TCs occur over the northern WNP due to TC-favorable BSISO-associated convection and circulation.In contrast,more TCs occur over the southern WNP in years with increased BSISO convection residence time over the EIO-MC regions.(2)The results show that the increase in frequency of BSISO phase occurrence over the subtropical SCS-western Pacific since the late-1990 s has potentially contributed to a recent poleward shift of WNP TC genesis.We then decomposed the annual-mean latitude of TC location metrics and found that the poleward shift of WNP TC genesis during recent decades is mainly associated with increased BSISO residence time over the subtropical SCS-WNP and increased co-variability of TC genesis frequency and TC genesis latitude..(3)The interannual meridional variability in the BSISO is significantly correlated with the El Ni(?)o-Southern Oscillation(ENSO)in the pre-winter.This interannual variation in the BSISO can be predicted in advance by the different phases of ENSO in winter.During the summer after the El Ni(?)o reaches its mature phase,BSISO tends to propagate northward and stay in the SCS-WNP regions,resulting in increased TC genesis in this regions.This process may be related to the Pacific-East Asian teleconnection mechanism.Given the relationship with ENSO and ENSO’s predictability,there is potential for inter-annual prediction of the BSISO and thus TC genesis distribution over the WNP basin. |
