Influence Of Indo-Pacific SST Anomalies On The Early And Late Establishment Of Tropical Cyclone Seasons And The Rapid Intensification Of Interannual Variability

The prediction of tropical cyclone season onset(TCSO)date and intensity has always been the difficult problem of TC prediction.Less previous studies focus on TCSO,and it's difficult to understand physical mechanism related to TC rapid intensification.As one of the world's three largest oceans,the tropical Indian Ocean sea surface temperature anomalies(TIO SSTAs)changes play an important role on circulations and TC activities over the the Western North Pacific(WNP).This study: 1)examines the inter-annual variability of WNP TCSO related to neutral-tomoderate ENSO events on seasonal TC activity over the WNP basin and corresponding seasonal tropical cyclones activity;2)examines the possible impact of TIO SSTAs on the proportion of rapidly intensifying tropical cyclones(PRITC)over the WNP during the tropical cyclone peak season(July–November).This study explore the effects of large-scale environmental factors and the possible physical mechanism about the impact of TIO SSTAs on TCSO and PRITC over the WNP.Results shown that:The averaged TCSO is ?March 31 during early TCSO years,significantly earlier than the averaged onset date during late TCSO years(? June 8).There is a significant difference in seasonal variation of TC counts between the early and late TCSO years although no significant difference is found for the total TC counts.Compared with that during years with late TCSO,it is found that 1)a significant increase of ?3.5 TC during spring(March-May)mainly from the increase over the southeastern WNP,2)an obvious increased TCs in the southeastern WNP and a significant decreased TCs in the northwestern WNP making an insignificant difference of TC counts during summer(June-August),and 3)a significant decrease(?-4.2)of TC mainly from significant decreased TCs counts over the South China Sea(SCS)and Philippine sea during autumn(September-November).Accompanied by TC formation location tending to move eastward and equatorward during early TCSO years,the TC intensity becomes significantly stronger.These characteristics of seasonal TC activity are closely related to the development of sea surface temperature anomalies(SSTAs)over the tropical Indo-Pacific sector and eastcentral Pacific along with seasons.During spring in the early TCSO years,equatorial westerlies induced by the El Ni(?)o-like SSTAs pattern cause the increase of the low-level relative vorticity and thus enhancing TC formation in the southeastern quadrant.During summer and autumn in the early TCSO years,the persistent anomalously warming SSTAs over the Indo-Pacific sector associated with the El Ni(?)o development excite a Kelvin response that lowers the surface pressure in the equatorial region and leads to anomalous anticyclonic over the western WNP,thus favoring an decreased of TC over the northwestern quadrant.There is a robust interannual association(r=0.46)between TIO SSTAs and WNP PRITC during 1979–2018.Composite analyses between years with warm and cold TIO SSTAs confirm a significant impact of TIO SSTAs on WNP PRITC,with PRITC over the WNP basin being 50%during years with warm TIO SSTAs and 37% during years with cold TIO SSTAs.Tropical cyclone heat potential appears to be one of the most important factors in modulating the interannual change of PRITC over the WNP with a secondary role from midlevel moisture changes.Interannual changes in these large-scale factors respond to SSTA differences characterized by a tropics-wide warming,implying a possible global warming amplification on WNP PRITC.The possible footprint of global warming amplification of the TIO is deduced from 1)a significant correlation between TIO SSTAs and global mean SST(GMSST)and a significant linear increasing trend of GMSST and TIO SSTAs,and 2)an accompanying small difference of PRITC(?8%)between years with detrended warm and cold TIO SSTAs compared to the difference of PRITC(?13%)between years with non-detrended warm and cold TIO SSTAs.Global warming may contribute to increased TCHP,which is favorable for rapid intensification,but increased vertical wind shear is unfavorable for TC genesis,thus amplifying WNP PRITC.