Mechanism Analysis Of Abnormal Tropical Cyclone Genesis Over The Northwestern Pacific In August

August is normally the month with most tropical cyclones(TCs)formed over the western North Pacific(WNP)during the typhoon season.In this paper,the reason for abnormal TC activity during August is discussed,especially for the August of 2014,which had no TC formed.Climatologic background analysis shows,when less TCs are generated in August,the monsoon trough and easterly vertical wind shear are weak,the tropospheric moisture is reduced,while the Western North Pacific subtropical high(WPSH),upper-level midlatitude jet and South Asia High(SAH)are strong,causing the reduction of low-level convergence and upper-level divergence.Those unfavorable large-scale conditions suppress moist convection and cyclogenesis.In the TC-active August,TCs genesis is favored when those large-scale conditions have reverse patterns over the WNP.Statistical results suggest no apparent relationship between the TC frequency in August and the developing El Nino(or La Nina)episode.While TCs genesis over the WNP is significantly modulated within the active(inactive)Madden-Julian Oscillation(MJO)and equatorial waves phases.With standardized squares of area-averaged filtered OLR variations for equatorial waves in Augusts,which can express the wave activeness.When the standlized value is larger than 0,which means the equatorial waves are active,we find more TCs are geneted in the active wave phases.In addition to the tropical processes,the mid-latitude systems,such as Rossby wave breaking(RWB),upper-level trough and mid-latitude jet can also modulate TCs genesis by affecting vertical wind shear,thermal structure,tropospheric moisture,and vertical motion.When the frequency of RWB is enhanced and high-PV intruded into low latitudes,the vertical motion is reduced,and tropospheric moisture is dry,which can inhibit the convection and cyclogenesis.Besides,when more RWB happens in August,the favaorable large-scale environmental conditions of the WNP are shifted to west,the TC frequency in this month is less and mean genesis location is shifted to west too.More TCs are geneted when less RWB frequency in August,and favorable large-scale conditions are extened to east as well as mean TC genesis longtitude.In August 2014,the monsoon trough and vertical wind shear were weaker than that in the TC-inactive August,while the WPSH and SAH were stronger.There was a negative MJO phase dominated over the WNP,causing a low-level anticyclonic circulation and easterly anomalies to suppress convection and cyclogenesis.The equatorial waves were inactive in this month,with less amount of tropical disturbances.Moreover,with the extratropical Rossby wave breaking and upper-level high-PV intrusions,the moisture in upper-to-mid troposphere was reduced by extratropical dry air.Consequently,the evolution of tropical disturbances was suppressed by the negative MJO phase and upper-level high-PV intrusions,making no TCs formed in the WNP eventually.