Climatological Characteristics Of Western North Pacific Multiple Typhoon Event And Its Possible Mechanism

Typhoon is a kind of harmful synoptic system accompanied by torrential wind and rain,which often causes huge economic losses to landing areas and even threatens people's lives.China,adjacent to western North Pacific–the world's largest genesis location of typhoons,is one of the countries seriously affected by typhoons.Sometimes typhoons occur continuously in a group and do more harm to the society.By now,such kind of typhoon cluster has no clear and unified definition,and its mechanism has not been clearly studied yet.Therefore,it is meaningful to study the issue of typhoon cluster in the western North Pacific.With the best track dataset issued by the Joint Typhoon Warning Center(JTWC)of the U.S.Navy from 1945 to 2018,the western North Pacific multiple typhoon event(MTYE)is defined,and characteristics of its temporal-spatial distributions and possible mechanisms are analyzed.It is found that MTYE mainly occurs during the season from July to October,and its major genesis location is around 10°N between 135°E-180°over the western North Pacific.Compared to those typhoons occurring individually,the typhoons in MTYE have much larger intensity and longer lifetime,thus bigger power dissipation index(PDI).The ratio of typhoon frequency in MTYE to the total mainly shows interannual variability with certain increasing trend.In the MTYE active year,sea surface temperature is greatly warmed in the mid-eastern tropical Pacific and subtropical Pacific in northern hemisphere,which could generate Gill-type Rossby wave response and anomalous Walker circulation,and produce low-level positive relative vorticity anomaly,mid-level positive relative humidity anomaly and easterly vertical shear anomaly in turn,providing essential background state for MTYE genesis.Besides,enhancements of eddy kinetic energy at multiple sub-seasonal time scales also have important contributions to the MTYE.The super MTYE in 1996 is composed of 16 consecutive typhoons,which lasts for more than 90 days from early July to early October with at most five typhoons coexisting at the same time.The major genesis location of the super MTYE is 135 ° E-175 ° E,5 ° N-20 ° N.The negative low-level vorticity anomaly and the negative mid-level relative humidity anomaly averaged from July to October in 1996 are not conducive to the formation of MTYE,whereas the warm SST anomaly and the easterly vertical shear are the favorable factors.Five days before the formation of MTYs,the convective activity on the 10-30-day time scale is weakened,so the weather is clear,which allows more solar radiation reaching the ocean surface,causing much warmer SST.At the same time,the low-level relative vorticity anomaly is positive on the 30-60-day time scale,which creates favorable intraseaonal atmospheric and oceanic conditions for the formation of MTYs together with the warmer SST on the 10-30-day time scale.Five days after the formation of MTYs,the convective activity increases on10-30-day time scale,so does the mid-level relative humidity,moreover,positive vorticity anomaly occurs in the lower troposphere with anomalous divergence in the upper level.Meanwhile,the easterly vertical shear anomaly occurrs on both 30-60-day and 60-90-day time scales,providing the atmospheric condition favorable for typhoon formation at longer time scales.Therefore,the joint effect of air-sea anomalies at multiple sub-seasonal time scales leads to the continuous genesis of typhoons,which is the essential reason for the occurrence of the super MTYE in 1996.