南海-西北太平洋地区大气准双周振荡对TC生成的调节作用

通过对南海-西北太平洋地区大气10～20 d准双周振荡(QBWO)不同位相的划分(A～D),研究了QBWO对南海-西北太平洋海域热带气旋(TC)生成的调节作用。将TC分为强热带风暴及以下级别(TS)和台风及以上级别(TY),并将QBWO分为干湿位相,发现南海海域生成的TS(TY)在干湿位相的比与西北太平洋海域生成的TS(TY)在干湿位相的比相等,这表明QBWO对TS(TY)生成的调节作用在南海和西太平洋地区可能相同。从A位相到C位相,南海和西北太平洋地区TC的生成频数均逐渐增多,D位相时期,TC生成最少,多数TC发生在QBWO的对流活动湿位相,少数TC发生在干位相。南海-西北太平洋海域TC的生成受到QBWO的明显调制。从位相A到位相C,低频对流和低频风场逐渐向西北方向移动,低频对流强度持续加强,低频风场逐渐由异常西风-东风-西风转为异常东风-西风-东风配置,西北太平洋地区季风槽加强,使得TC生成频数逐渐增多。此外,在QBWO活跃位相,非绝热加热增强和纬向风垂直切变减弱也有利于TC的生成。

THE MODULATION EFFECT OF QUASI-BIWEEKLY OSCILLATION ON TROPICAL CYCLONES OVER SOUTH CHINA SEA-WESTERN NORTH PACIFIC

The modulation effect of tropical storms/typhoons over the South China Sea-western North Pacific(SCS-WNP) by quasi-biweekly oscillation(QBWO) is studied,through the classification of four phases with daily NCEP/NCAR reanalysis data,OLR of daily NOAA data and tropical cyclones(TCs) dataset from CMA-STI best track dataset for tropical cyclones in the western North Pacific.In this article,the TCs are divided into strong tropical storm and the level below(generally called tropical storm,refered to as TS),and typhoon level and the above(generally called typhoon,refered to as TY).The results show that the ratio of the number of TS(TY) generated in the wet phase to that of TS(TY) generated in the dry phase in the SCS area is roughly equal to the ratio in the WNP area.This shows that the modulation of TCs’ generation by QBWO is the same both in the SCS and in the WNP.QBWO has an important influence on the frequency of TCs’ occurrence and the modulation is obvious during phase C.The number of the TC genesis is more in the wet phase of quasi-biweekly convection but less in the dry phase.The QBWO shows the feature of northwest propagation.The westerly phase of the quasi-biweekly zonal wind is conductive to TC formation.Form phase A to phase C,in the process of moving to the west,the monsoon trough shifts to the north slightly and the average position of TC generation also shifts to the northwest.The majority of TCs occurred on the north side of the monsoon trough.In addition,strong diabatic heating and weak zonal wind vertical shear are conductive to the occurrence of TCs.