Comparison of Intensity Changes of Western North Pacific Tropical Cyclones during Extratropical Transition

Two tropical cyclones over the western North Pacific that experienced an extratropical transition (ET), Yagi and Francisco, are examined in this study. The former underwent a 24-h reintensification process after the ET, whereas the latter continued to weaken until dissipation. On the basis of tropical cyclone data from the Japan Meteorological Agency and Final Operational Global Analysis data from the National Centers for Environmental Prediction, a comparison of the upper- and lower-level environmental fields during the ET revealed clear differences. A strong upper-level trough existed during the ET for Yagi; moreover, Yagi merged with a preexisting extratropical cyclone at the lower level during the ET. In contrast, the upper-level trough related to Francisco was weak during the ET, and Francisco dissipated after the ET. In addition, the reintensification of Yagi could be attributed to the following causes: (1) Yagi interacted with the upper-level jet stream ahead of the trough during the ET stage and was influenced by the upper-level divergence and lower-level convergence tendency caused by the jet stream, which was favorable for reintensification after the ET. Furthermore, the vertical wind shear increased when Yagi was crossing the jet stream, which can lead to baroclinic instability and the development of a lower-level frontal zone. The baroclinic energy in the frontal zone may have been converted into kinetic energy through this process and contributed to the reintensification of Yagi. (2) The downward extension of the moist potential vorticity related to the upper-level trough could have increased the positive vorticity in the lower level and induced cyclonic circulation, which strengthened Yagi after the ET. (3) The merging of Yagi with a preexisting extratropical cyclone brought cold air from high latitudes, which can enhance the horizontal temperature gradient and cause intense development of the lower-level frontal zone. Thus, Yagi evolved into a frontal cyclone and reintensified after the ET. In contrast, these patterns mentioned above did not exist for Francisco.