Climatological characteristics of the extratropical transition of tropical cyclones along different tracks in the western North Pacific (1979–2022)

Based on datasets from the International Best-Track Archive for Climate Stewardship (IBTrACS) and the fifth major global reanalysis produced by ECMWF (ERA5), we found that 29% of tropical cyclones (TCs) in the western North Pacific underwent extratropical transition (ET) from 1979 to 2022, with the frequency of ET events showing a slow decreasing trend. The extratropical transition tropical cyclones (ETCs) are classified into four clusters using the k -means clustering method based on their track patterns: recurving ETCs, westward ETCs, northwestward ETCs, and abnormal track ETCs. The transition process of recurving ETCs mostly occurs after the recurvature is completed, while 63.7% of the westward ETCs complete their transition after landfall. Abnormal track ETCs undergo transition over high-latitude oceans. Northwestward ETCs have the longest duration and slowest transition speed during the ET period, resulting in a prolonged impact. The ET process occurs at the edges of the western Pacific subtropical high (WPSH), with higher frequency during westward extension and lower during eastward retreat. While westward ETCs transition through surface friction effects, others complete ET in the northwest baroclinic zone of the WPSH. 本研究基于国际气候管理最佳路径档案(IBTrACS)和欧洲中期天气预报中心第五代再分析数据集(ERA5), 发现1979-2022年间西北太平洋有29%的热带气旋经历了温带变性(ET)过程, 且ET事件的发生频率呈缓慢下降趋势.通过k-means聚类方法, 将变性的热带气旋(ETCs)按路径特征划分为四类:转向路径ETCs,西移路径ETCs,西北移路径ETCs和异常路径ETCs.转向路径ETCs主要在完成转向后开始变性.西移路径ETCs大多数在登陆后完成变性(63.7%).异常路径ETCs通常在高纬度洋面完成变性.西北移路径ETCs在变性期间所用时间最长, 移动速度较慢, 因此影响持续时间较长.ET过程均发生在西太平洋副热带高压(WPSH)边缘, WPSH西伸时ET频率较高, 东退时频率较低.除西移路径ETCs受地表摩擦主导外, 其他类型多在WPSH西北侧的强斜压区完成转变.