Statistical Analysis Of Multiple Tropical Cyclone Events In The Western North Pacific

The western North Pacific(WNP)is the sea area with the most frequent tropical cyclogenesis in the world,and about half of the cases are generated in multiple tropical cyclone(MTC)events.Compared with a single tropical cyclone(STC),MTC events involve a wide range of scales,have a great impact and are difficult to forecast.At present,people still know less about it.In order to further understand the MTC events,by using ERA-Interim global reanalysis dataset,the best-track data from the Japan Meteorological Agency,the outgoing longwave radiation data from the National Oceanic and Atmospheric Administration and the bimodal ISO index from the University of Hawaii,we analyzed the statistical characteristics of MTC events in WNP from 1979 to 2018.Firstly,we consider the temporal and spatial conditions of tropical cyclogenesis and maintenance comprehensively,and then set the criteria so as to establish a dataset of MTC events in WNP.According to the number of typhoons in the event,the MTC events are divided into two categories,namely,MTC events with only 2 typhoon members(MTC2)and MTC events with 3 or more typhoon members(MTC3).The results show that compared with MTC2 and STCs,MTC3 events generate more frequently with stronger monsoon trough convergence,a larger range of high-temperature sea area and wetter mid-bottom atmosphere.Both dynamic and thermal elements are conducive to the development of typhoons.In addition,the 25-90 day subseasonal convection related to BSISO activities has a slower decay rate during MTC3 event.Its frequency has the strongest positive correlation with the subseasonal convection intensity in the 145-165°E and 5-20°N regions.Judging from the composition of the bimodal ISO index,MTC3 events vary more with the BSISO phase transition,but there is small difference in the average amplitude of the STCs and two types of MTC events.MTC events in WNP have obvious interdecadal changes,which averaged 4.4 times per year during 1979-1996,but suddenly decreased to 3.4 times after 1996/1997.The abrupt decrease was caused by the reduce of MTC3.To analyze the causes,we discuss the effects of large-scale environmental factors,tropical disturbances,and subseasonal BSISO from the multi-scale perspective.It shows that there was no significant mean change in the number of tropical vortex disturbances,but the typhoon generation rate during the same period remained low.During the typhoon season of 1997-2014,the vertical wind shear increased abruptly,meanwhile the 200 h Pa divergence and 850 h Pa convergence weakend in most areas where MTC3 events were active.Most of the dynamic factors were not favor of the development and maintenance of TCs.They offset the beneficial effects of sea surface temperature and relative humidity,and the influence of dynamic parameters was dominant.Further analysis shows that after 1996/1997,the number of days in active phase was shortened,along with the convection which could not extended further eastward and dissipated quickly.This made tropical disturbances only develop in a relatively short time and in a small area.The tropical disturbances that caused the typhoon energy dispersion effect dissipated under the unfavorable background in the eastern part of the MTC populated area,and ultimately induced the interdecadal variation of MTC3 events.But the impact on STCs and MTC2 events was small.