Circulation Features Associated with the Record-breaking Typhoon Silence in August 2014

摘要

Climatologically,August is the month with the most tropical cyclone (TC) formation over the western North Pacific (WNP) during the typhoon season.In this study,the reason for abnormal TC activity during August is discussed-—especially August 2014,when no TCs formed.The large-scale background of August 2014 is presented,with low-level large-scale easterly anomalies and anticyclonic anomalies dominating over the main TC genesis region,a weak monsoon trough system,and a strong WNP subtropical high (WPSH),leading to significantly reduced low-level convergence,upper-level divergence,and mid-level upward motion.These unfavorable large-scale conditions suppressed convection and cyclogenesis.In August 2014,equatorial waves were inactive within the negative phase of the Madden-Julian Oscillation (MJO),with fewer tropical disturbances.Although the low-level vorticity and convection of those disturbances were partly promoted by the convective envelopes of equatorial waves,the integral evolution of disturbances,as well as the equatorial waves,were suppressed when propagating into the negative MJO phase.Moreover,the upper-level potential vorticity (PV) streamers associated with anticyclonic Rossby wave breaking events imported extratropical cold and dry air into the tropics.The peripheral tropospheric dryness and enhanced vertical wind shear by PV streamer intrusion combined with the negative MJO phase were responsible for the absence of TC formation over the WNP in August 2014.