| The movement of tropical cyclones(TCs)in the northwest Pacific region is mainly guided by large-scale environmental flows,which include various atmospheric variables at different spatiotemporal scales.Different components of atmospheric waves may have corresponding impacts on the anomalous track of TCs.In order to further understand the importance of multiscale wave-induced flows in TC movement and the impact mechanism of different atmospheric variables on TC motion,this study systematically analyzed the influence of atmospheric multiscale waves on the abrupt track changes of TCs in the northwest Pacific region.The study mainly used reanalysis data and the WRF numerical model,and combined statistical synthesis analysis,spatiotemporal filtering methods,and segmental vorticity inversion methods to explore the guiding role and physical mechanism of multi-scale wave components on the abrupt track changes of TCs.The main conclusions are summarized as follows:(1)By statistically analyzing 52 tropical cyclones with abnormal track turnings in the north western Pacific over the past 20 years and synthesizing the composite large-scale environmental fields in which tropical cyclones are located,it is found that tropical cyclones with abnormal right turns are usually accompanied by deepening of the middle and upper tropospheric trough,weakening and eastward withdrawal of the subtropical high,and asymmetric enhancement of the southwest monsoon.When tropical cyclones turn leftward abnormally,there is a stronger easterly steering flow to the north of the center,mainly due to the latitudinal extension of the low-level monsoon trough and subtropical high,and the increase in meridional pressure gradient between them.In addition,according to the track turning type and dominate wave components affecting the occurrence of track turning,52 tropical cyclones are divided into five categories,including three abnormal right-turning categories dominated by synoptic fluctuation with time scale less than 10 days,Intra-seasonal oscillation(ISO)fluctuation with time scale within 10-90 days,and background flow with time scale greater than 90 days,and two abnormal left-turning categories dominated by ISO fluctuation and background flow.These five categories of tropical cyclones are named R10,R1090,R90 and L1090,L90 respectively.Further synthesis averaging of the filtered fields in which five categories of tropical cyclones are located shows that: The northeast side of R10 category tropical cyclones is a northwestsoutheast synoptic wave train,with significant southerly anomalies near the center of the cyclone,which is conducive to abnormal rightward turning.The center of R1090(L1090)category tropical cyclones is located in the southeast(northwest)part of a large ISO-scale cyclonic circulation and is affected by significant southerly(easterly)anomalies to turn right(left).The background field in which R90(L90)category tropical cyclones are located is a weaker meridional(latitudinal)stretching anticyclonic circulation,with TC center of located on the east(south)side of this anticyclonic circulation.(2)On the basis of statistical analysis,an in-depth exploration of the typical Typhoon GONI with an abnormal right turning track was carried out.The results showed that during the movement of GONI,it was strongly influenced by the steering flow of the surrounding environmental field.The main influencing systems include the subtropical high,monsoon gyre,peripheral anticyclone of typhoon and neighboring Typhoon Atsani.Using the piecewise potential vorticity inversion method,it was quantitatively analyzed that before turning,the movement was jointly contributed by the monsoon gyre and subtropical high,with a ratio of about 4:6;after that,the southerly steering flow generated by the peripheral anticyclone on the southeast side of the typhoon played a key role in the abnormal northward turning;after turning,the peripheral anticyclone and subtropical high jointly contributed to the northward movement component of the typhoon,with an average contribution ratio being equivalent.In the vertical direction,the steering flow components at all height levels are basically consistent with the actual typhoon movement components,and the projection ratios along TC movement are all greater than 0.6;before turning occurs,southerly steering vectors begin to appear in lower troposphere first,indicating that systems developed in lower atmosphere contribute more importantly.In addition,analysis of wave component contributions shows that the abnormal northward turning process of GONI is dominated by synoptic scale fluctuations.The active convection and non-convection zones of synoptic wave trains are basically distributed alternately along latitudes.As wave trains and typhoon interact and develop,TC center experiences a southeastward shift from the north side of synoptic scale cyclonic anomaly.The anticyclonic anomaly vortex in wave train gradually strengthens in southeast direction of typhoon center,resulting in significant southwesterly anomaly steering flow around typhoon,which is consistent with quantitative analysis of circulation.(3)Compared with Typhoon GONI,the main influencing systems of another typhoon FITOW with an abnormal left turning track include the subtropical high,monsoon gyre and mid-latitude trough.According to the quantitative analysis results of piecewise potential vorticity inversion,it is pointed out that before turning,the movement of FITOW was jointly contributed by the subtropical high and mid-latitude trough,with a contribution ratio of about3:1;then the influence of mid-latitude trough weakened,the subtropical high extended westward,and the easterly steering flow generated by monsoon gyre strengthened,with an average projection ratio along TC movement of about 0.8,leading to an abnormal westward turning process.The baroclinicity of steering flow components at all height levels significantly increased before and after FITOW turned,and always relied on contributions from middle and lower troposphere.Wave analysis points out that ISO-scale fluctuations have a significant impact on the westward turning of Typhoon FITOW.Numerical experiment results emphasize that if all ISO fluctuations are removed(i.e.,NOISO experiment),the high-latitude westward turning characteristics of FITOW will no longer appear.The piecewise potential vorticity inversion results also confirm that in NOISO experiment,the northward steering flow provided by mid-latitude trough(westerly trough)system is significantly weakened,while subtropical high circulation on northeast side continues to extend westward,providing latitudinal guiding flow for stable westward track.In addition,according to statistical and spatiotemporal filtering analysis results,the disturbance distribution of eastward and westward components of ISO fluctuations during abnormal westward turning process is not the same.The westward ISO component is manifested as a cyclone-anticyclone alternating wave train placed in northwestsoutheast direction;while eastward ISO component is manifested as a pair of dipole circulation anomalies placed in southwest-northeast direction.Therefore,additional results from four sets of sensitivity experiments show that if ISO westward component and westward equatorial Rossby wave are removed,typhoon’s abnormal westward turning characteristics will no longer appear.Initially,it mainly weakens trough system on north side and has no obvious impact on subtropical high;subsequently,subtropical high has obvious enhancement and extension to west and dominates;while if ISO eastward component and eastward MJO wave are removed,typhoon’s westward turning characteristics still exist.The circulation evolution is close to control experiment,but the degree and occurrence time of track westward turning have changed.Compared with eastward component,westward ISO wave component contributes more prominently to abnormal westward turning track of Typhoon FITOW,especially in early stage of turning. |
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