Impact Of ENSO On South China Sea Typhoon Under Monsoon Conditions

ENSO has a large impact on South China Sea typhoon genesis under southwest monsoon conditions.Based on multiple El Ni?o–Southern Oscillation(ENSO)indices,ERA-Interim reanalysis data,NECP reanalysis data,etc.we analyze the intensity and timefrequency variability of ENSO events,classify the intensity of South China Sea summer monsoon(SCSSM).The relationship between ENSO and the intensity of the SCSSM is discussed,and the South China Sea typhoon generate under southwest monsoon conditions in extreme ENSO events and no ENSO events are compared.The results are as follows:(1)During the period 1951~2016 over the last 65 years,there are 22 warm events(El Ni?o)and 13 cold events(La Ni?a).The intensity of ENSO events are divided into 5 levels,El Ni?o events occurs in spring are strong or extremely strong,the peak period of ENSO tend to be winter.Frequency analysis is applied to indexes of ENSO,indicate that the percentage of strong warm events months is much more than strong cold events months.Three methods of time–frequency——continuous wavelet analysis,cross wavelet analysis,wavelet coherency—are used,and the results are in good agreement with the principal period(2~7-yr)and the interdecadal variability(10~16-yr)of ENSO cycle.(2)The intensity of the South China Sea summer monsoon is calculated and divided into 5 levels.Based on the analysis of the relationship between ENSO and the intensity of the SCSSM,we found that there is no El Ni?o event in all the strong SCSSM years,and ENSO events occurs before the onset of SCSSM in all the weak or slightly weak years.When the SST over the central and eastern equatorial Pacific is below normal(La Ni?a),there must be SCSSM anomalies.When the SST over the central and eastern equatorial Pacific is above normal(El Ni?o),SCSSM is normal or weak.The probability of SCSSM being weak or slightly weak as the intensity of El Ni?o over moderate is 70%.Indicate that SCSSM is in good response to ENSO events.(3)Two South China Sea typhoon are selected: one is typhoon Dianmu which generate between extremely strong El Ni?o and extremely weak La Ni?a,the other is typhoon Bebinca which generate under no ENSO conditions.Under the influence of extreme ENSO events,the SST of the western Pacific and Beibu Gulf is above normal and warm,the western Pacific subtropical high is north-eastward and weak,and the intensity of SCSSM is strong,which provides abundant water supply for typhoon Dianmu.The northwestern Pacific monsoon trough is north-eastward,and the convective activity in the control region is very active.Typhoon Dianmu is generated in the monsoon depression and MCS plays an important role in the formation process.Kinetic energy of the typhoon system increases obviously,which provides favorable conditions for the enhancement of typhoon.Without the influence of ENSO,the warm SST region of the western Pacific is smaller,the western Pacific subtropical high is westward and weak,and the intensity of SCSSM is normal,result in weaker water vapor flux transport.The northwestern Pacific monsoon trough is southward,and the convective activity in South China Sea,Beibu Gulf and northwestern Pacific is weaker.The genesis of typhoon Bebinca is mainly affected by the traction of Typhoon Leepi.The convective activity of Bebinca is weaker than that of Dianmu,and the kinetic energy in each stage of typhoon system does not increase obviously,which is not conducive to the enhancement of typhoon.