台风"烟花"于2021年7月22日00时(世界时，下同)在台湾岛东侧突然向北转向，但美国、日本和中国的业务预报均没有正确预报出该过程。基于NCEP/NCAR提供的全球再分析资料(Final Operational Global Analysis，FNL)，采用Lanczos滤波方法分析发现，台风"烟花"转向前的西行减速主要受到与季风涡旋有关的季节内尺度环境引导气流分量的影响，向北转向及转向后的向北加速则主要受到季风涡旋和台风相互作用导致的天气尺度环境引导气流分量的引导，主要和beta涡旋对旋转、与季风涡旋的合并、以及加强的Rossby波能量频散这3个重要过程有关。台风"烟花"在纬向尺度约3 000 km的季节内尺度季风涡旋北部的偏东气流西移过程中，与季风涡旋相互作用导致beta涡旋对气旋性旋转，通风流经历东南风—东北风—西南风的转变，使得"烟花"向西减速并逐渐向季风涡旋中心移动，并于7月22日00时与季风涡旋合并，加强了 Rossby波能量频散，使得"烟花"东南边缘的天气尺度西南风加强，为缓慢移动的"烟花"提供了向北的引导，"烟花"突然向北转向。
Effects of monsoon gyre on sudden track changes of Typhoon"In-fa"
Typhoon"In-fa"(2106)experienced a sudden northward turn toward the Taiwan Island at 0000 UTC on July 22,2021,which was not predicted in all operational forecasts.By using FNL wind field data and the Lanczos filtering method,this paper found that the westward deceleration of Typhoon"In-fa"before the deflection was mainly affected by the intraseasonal component of environmental steering flows associated with the monsoon gyre,while the sudden northward turn and the subsequent northward acceleration were mainly attributed to the synoptic-scale component of environmental steering flows associated with the interaction between the monsoon gyre and the typhoon.These changes in multi-timescale steering flows were primarily caused by three important processes:the rotation of beta gyres,the coalescence process and the enhanced Rossby wave energy dispersion.Typhoon"In-fa"initially moved westward in the easterlies in the northern periphery of a monsoon gyre on the intraseasonal timescale with a zonal scale of about 3 000 km.The interaction between the typhoon and the monsoon gyre led to the cyclonic rotation of beta gyres.Thus,the ventilation flows between beta gyres correspondingly converted from the southeasterlies to the northeasterlies to the southwesterlies,which made the westward"In-fa"slow down and gradually move into the interior of the monsoon gyre.Eventually,"In-fa"coalesced with the monsoon gyre at 0000 UTC on July 22.The synoptic-scale southwesterlies in the southeastern periphery of"In-fa"were strengthened due to the enhanced Rossby wave energy dispersion caused by this coalescence process,which provided northward guidance for the slow-moving"In-fa".Therefore,Typhoon"In-fa"turned abruptly northward.
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