Influence Of MJO And Easterly Wave On Tropical Cyclogenesis And Their Mechanisms

Tropical cyclone(TC)forms over tropical oceans.Due to the lack of high-precision observation data at sea and the complexity of multi-scale interactions involved in TC genesis,it is a key issue still remaining unsolved in typhoon dynamics.Based on the diagnostic analysis of reanalysis data and the numerical simulation of mesoscale model,this work investigates the influences of Madden-Julian Oscillation(MJO)and easterly wave on TC genesis and their mechanisms.Three main conclutions are summarized as follows:(1)The modulation of MJO on TC genesis is investigated,and the difference between critical factors associated with MJO which impacts TC genesis in various ocean areas is revealed.Based on diagnostic analysis of the potential genesis index(GPI),we find that the key factor affecting TCs genesis in the western North Pacific(WNP)and Gulf of Mexico-western Caribbean(GOM)is moisture associated with MJO,while vertical wind shear associated with MJO is crucial in the eastern Atlantic(EAT).This regional difference is closely related to the background fields on MJO scale.The observations show that the characteristics of large-scale circulation during MJO active phase is favorable for the increase of cyclonic vortex,the enhancement of low pressure and the increase of moisture in the WNP and GOM.In contrast,the large-scale circulation during MJO active phase in the EAT exhibits a distinct westerly(easterly)anomaly in the lower(upper)level and relatively small change of moisture.With the numerical simulation of mesoscale WRF-ARW model,it is found that the experimental results are consistent with the observed results of GPI diagnosis.The MJO-scale moisture plays the most important role in the modulation of TC genesis in the WNP and GOM,while the MJO-scale vertical wind shear is a critical factor that impacts TC genesis in the EAT.(2)The TCs induced by easterly wave disturbances in the WNP are selected by statistical analysis,revealing the structure characteristics and the mechanisms of maintenance and development of such wave that can develop into a TC.In the summer of 2000-2009,there are24 TCs induced by easterly waves,accounting for 18% of the total TC frequencies.The horizontal structure of the easterly wave disturbance exhibits a slight east-west distribution,and the cyclonic-anticyclonic circulation is coupled with the enhanced-suppressed convection.In the vertical the easterly wave has an east tilt with height.Prior to three days of TC genesis,the convection center lags behind the center of circulation by a phase difference of approximately one-sixth.As TC gensis time approaches,the phase difference between circulation and convection gradually reduces.In addition,the upward motion is well matched with warm anomalies,and the downward motion is well matched with cold perturbations.The energy diagnosis of easterly wave disturbance shows that its maintenance and development are mainly related to the barotropic energy conversion in the lower-middle troposphere and baroclinic energy conversion in the middle-upper troposphere.Furthermore,the generation of EAPE in baroclinic conversion process mostly derives from the diabatic heating effects.(3)With a typical case of TC genesis induced by the easterly wave disturbance,the influence of easterly wave critical layer on the disturbance development and the trigger mechanisms that the easterly waves develop into TCs are revealed.The observation results show that the easterly wave critical layer provides a favorable background environment for TC development,and the interaction between monsoon trough and easterly wave induced energy accumulation may be a trigger mechanism for TC genesis.Furthermore,the simulation results reveal that vortex aggregation is a dynamic trigger mechanism for TC genesis.A brust of intense deep convection causes the formation of upper-level warm core,which further leads to the reduction of surface pressure through the static equilibrium equation and thus promotes the enhancement of disturbance.It belongs to a themodynamic trigger mechanism for TC genesis.