熱帯低気圧の温帯低気圧化の特徴と中緯度偏西風の変動との関係

Abstract

This study investigated the characteristics of the extratropical transition (ET) of tropical cyclones (TCs) to understand the difference in the typhoon-caused disasters. ET cases in the western North Pacific from 2016 to 2020 were classified into large (LA) and small (SM) types based on the radii of storm and gale-force winds during life times of TCs. We compared the two types in terms of structural changes represented by cyclone phase space (CPS) and their relationship with synoptic environments. The CPS analysis revealed that TCs of the LA type tended to experience larger structural change, especially to asymmetric features, whereas TCs of the SM type tended to experience smaller structural change. The mid-latitude westerlies associated with the LA type were characterized by relatively large-amplitude north-south meanders. When a TC of the LA type approached a westerly jet stream, their interaction caused a further increase in the amplitudes of the north-south meanders of the jet, and the jet then enhanced the asymmetry of the TC. In addition, northward acceleration of the TC of the LA type by the jet induced the ET occurrence at higher latitude. In contrast, a TC of the SM type did not reach a westerly jet stream, the mid-latitude westerlies remained to be zonal with relatively small-amplitude of the north-south meanders. Because the TC hardly interacted with the jet, the asymmetry of the TC was only enhanced slightly. Therefore, it was suggested that the difference between the LA and SM types was related to the interaction of the TC and the westerly jet stream, which induced to the increased amplitudes of the north-south meanders of the jet and the large structural change of the TC.