Monsoon Trough Influences On Multiple Tropical Cyclones Events In The Western North Pacific

Under some circumstances,multiple tropical cyclones(MTCs)develop successively over the western NorthPacific(WNP).Generally,the interactions among multiple TCs will lead to a larger forecasting error and thus a great challenge to the operational center.Prediction of multiple TCs activity is essential to better prepare for and mitigate TC-induced disasters.Hence,the purpose of this study is to investigate the possible roles of large-scale environmental conditions on the MTCs events.The observations show that both thermodynamical and dynamical conditions modulate the frequency of the MTCs events.During the active years,the enhanced monsoon trough(MT)extends eastward,leading to a strong low-level relative vorticity,upward motion and moist environment that are favorable for the occurrence of the MTCs events.The percentage contributions to the genesis potential index(GPI)change are from moisture field(75.9%),absolute vorticity(18.5%),vertical shear(-24.2%),vertical motion at 500 hPa(37.3%),suggesting the moisture field is the most important factor in the MTCs event.The enhanced moisture and convection processes likely contribute to more MTCs events through a positive convection–circulation–moisture feedback.The barotropic kinetic energy conversion diagnosis suggests that,in the active years,the synoptic-scale disturbances will develop through gaining the energy from the mean flow with the eastward-extending MT.Possible linkages between monsoon trough and MTCs events are proposed.Sensitivity experiments are conducted by specifying different initial conditions.The results show that a MTCs event is produced in the ACTIVE,whereas no MTCs event occurs in the INACTIVE.In the ACTIVE,the first TC is generated faster through the greater barotropic kinetic energy conversion.Once the first TC is generated,the TC energy dispersion induced Rossby wave train acts as a precursor of the second TC,and thus a MTCs event is developed eventually.Dynamically,a strong easterly shear prevails over the MT region,which favors a strong low-level synoptic-scale disturbance.Furthermore,the upper level asymmetric outflow jet induces an imbalance,which acts as a dynamical forcing to induce vertical motion in the wake of the pre-existing TC.It is speculated that the ascending motion is enhanced when the upper-level divergence and the low-level convergence in phase.The possible pathway for the formation of MTCs event is TC energy dispersion.The results of RH?SWITCH revealed that both thermodynamical and dynamical conditions associated with the MT are important in triggering MTCs event over the western NorthPacific basin.