Influence Of Different Intensity ENSO Events On The Duration Of Tropical Cyclone Quiescent Period Over The Western North Pacific

The tropical cyclone(TC)over the western North Pacific(WNP)is one of the most severe weather systems affecting China.The relationship between ENSO and TC activity has always been a frontier research topic for scientists.This study starts with the duration of tropical cyclone quiescent period(DTCQP)to further explore the relationship between ENSO events and TC activities over the WNP.Through statistical analysis of the distribution of the DTCQPs during different intensity and different development phases of ENSO events from 1951 to 2016.It is found that the super El Ni(?)o and strong La Nina events have the most significant impact on the DTCQP.Therefore,by analyzing the anomalous distribution of the SST and the atmospheric circulation and combining the anomalous characteristics of the potential generation index(GPI)of TCs,the main physical factors affecting the DTCQP during the super El Ni(?)o and the strong La Ni(?)a events are further explored.The main conclusions are summarized as follows:The average DTCQP is shorter than the normal during the El Ni(?)o developing(EN)year,while the average DTCQP is long during the El Ni(?)o decaying(EN+1)year.The impact of the El Ni(?)o event on the DTCQP depends on its development intensity.In other words,the longer DTCQP tend to appear in stronger EN+1 year.In the four super EN+1 years,the DTCQP is about 200 days,which is about 3 months longer than the 1951-2016 mean DTCQP.In contrast,the average DTCQP is longer than normal during the La Ni(?)a developing(LN)year,especially during the strong LN year,the characteristic of the long DTCQP is more significant.The anomalous characteristic of the DTCQP is not obvious in La Ni(?)a decaying(LN+1)year.Through the analysis of the SST and atmospheric circulation conditions during the super El Ni(?)o and strong La Ni(?)a events,it is found that the atmospheric circulation caused by the SST anomalies plays a key role in the DTCQP when the SSTs meet the thermal condition for TC generation in the WNP.The SSTs in the TCs producing region of the WNP are above 27 oC during the super El Ni(?)o and strong La Ni(?)a,which meet the critical temperature condition for the occurrence and development of TCs,while the atmospheric circulation conditions over the WNP are significantly different.During the super EN+1 year,the abnormally long DTCQP is due to the persistence of the WNP anomalous anticyclone(WNPAC),which is enhanced during the mature phase of El Ni(?)o but maintained through the subsequent spring and summer.The anomalous development and persistence of the WNPAC significantly influence the monsoon trough activity,which leads to delayed and weak development of the monsoon trough in the summer of super EN+1 year.From April to June of the super EN+1 year,there is almost no monsoon trough activity over the WNP and the large-scale environmental conditions are unfavorable to TC genesis.Corresponding to the strong convergence at 200 hPa,strong divergence in the lower troposphere,strong vertical wind shear,and small relative humidity in the middle troposphere,which makes the GPI a significant negative anomaly in most parts of the WNP.Therefore,the number of TCs is significantly reduced and the DTCQP is abnormally long during the super EN+1 year.In contrast,from April to June in the super EN years,the WNP is dominated by anomalous cyclonic circulation,which leads to the early and strong development of the monsoon trough.The active monsoon trough provides favorable environmental conditions for the formation and development of TCs.The anomalous distribution of atmospheric circulation during the strong La Ni(?)a event is roughly opposite to that during the super El Ni(?)o event.From April to June of the strong LN year,the WNP is mainly dominated by anomalous anticyclonic circulation which leads to delayed and weak development of the monsoon trough.The corresponding large-scale environmental conditions are not conducive to TCs genesis,resulting in a small number of TCs,a westward shift,and a long DTCQP.However,From April to June of the strong LN+1 year,the WNP is affected by the anomalous cyclonic circulation.The monsoon trough activity is active,and the convective development is strong,which is conducive to the development of low-pressure disturbances and the development of TCs.