Influences Of Boreal Summer Intraseasonal Oscillation On Synoptic-scale Disturbances And Tropical Cyclones Over The Western North Pacific

Duringboreal summer,the intraseasonal oscillation(ISO)and synoptic-scale disturbances,including tropical cyclones(TCs),are all active and tight connected.The boreal summer ISO(BSISO)has two prominent modes:the 10-30-day quasi-biweekly oscillation(QB WO)and the 30-90-day Madden-Julian Oscillation(MJO).In this study,climate statistics analysis indicated that the connection of QBWO with synoptic-scale disturbances(including TC)is more significant than that of MJO.With enhanced eddy kinetic energy(EKE)and more TC genesis,the TC growth rate and the probability of TC intensification increase significantly during the active phases of QBWO.The effects of QBWO on the synoptic-scale disturbances and TC intensification are mainly through altering the large-scale dynamic conditions.During the active convection phase of QBWO,the 10-30-day cyclonic vorticity and convergence at the lower troposphere,divergence at the higher troposphere and the moisture convergence provide a favorable environment for TC growth.The thermodynamic effects of atmospheric instability,SST and ocean heat potential show weaker contributions to TC intensification.Based on diagnosis of the EKE budget,it is found that the enhanced synoptic-scale eddies gain more kinetic energy from mean kinetic energy(MKE)through the enhanced barotropic energy conversion during grow phases of QBWO while the interaction between eddy momentum flux and rotational part of ambient field plays a key role in causing the enhancement of EKE.Also,the Princeton dynamic-core Atmospheric General Circulation Model(AGCM)sensitivity experiments indicate that QBWO affects the synoptic-scale wave train by altering the wind field and moisture field.Besides,BSISO and synoptic-scale disturbances,including TC show pronounced interannual variability over the WNP.During active years of QBWO/MJO,when the QBWO/MJO variance over the WNP is larger than one standard deviation,synoptic-scale disturbances are enhanced and more TCs form over the WNP.Based on diagnosis of the EKE budget,it is found that the enhanced synoptic-scale eddies,including TCs,gain more kinetic energy from the cyclonic anomalies associated with seasonal mean state and the two modes of ISO through the enhanced barotropic energy conversion during active years of QBWO/MJO.The current study implies that the amplitude of ISO plays a role in the forecasting of TC,due to the active ISO-TC interaction at the interannual timescale.