Effects of Storm Size on the Interactions between Mid-Latitude Westerlies and Tropical Cyclones during Extratropical Transition in the Western North Pacific

In 2019, serious disasters were caused by local strong winds associated with Typhoon Faxai and wide torrential rains with Typhoon Hagibis. Although both tropical cyclones (TCs) followed similar tracks and underwent extra-tropical transition after the recurvature (ETR), their storm sizes and structures were distinct: Faxai was a small axisymmetric TC, whereas Hagibis was a large asymmetric TC. The purpose of this study is to clarify the effect of storm size on a TC that undergoes ET and its associated synoptic environment. Hagibis causes a larger amount of precipitation more widely than Faxai. A large amount of diabatic heating closely associated with the precipitation leads to low potential vorticity (PV) production downstream of Hagibis in the upper troposphere and the enhancement of the ridge. By contrast, the diabatic heating is relatively small, and the production of low PV area is indistinct downstream of Faxai. Besides the case studies, large (LA) and small (SM) TCs that undergo ETR (LA-ETR and SM-ETR TCs, respectively) in the western North Pacific from 2016 to 2020 are statistically compared using cyclone phase space and composite analyses with the best track and Japanese 55-year Reanalysis datasets. As observed in the case studies, the LA-ETR TCs are characterized by a larger amount of diabatic heating and a more enhancement of the downstream ridge than the SM-ETR TCs. The LA-ETR TCs change into asymmetric structures more drastically than the SM-ETR TCs while moving northward along the westerly jet with increasing the amplitude of the north-south meander. By contrast, the amplitude of the north-south meander of the westerly jet does not increase around the SM-ETR TCs. Therefore, the larger the storm size is, the larger the amplitude of the north-south meander of the westerly jet is, resulting in a more drastic asymmetric structural change of the TC.