Strong landfall typhoons in Korea and Japan in a recent decade

Long-term changes in tropical cyclones (TCs) that made landfall in Korea and Japan during the TC seasons (June-October) are examined for the period 1977-2008. The TC activity is characterized by four parameters: power dissipation index (PDI), TC-induced rainfall, number of landfall TCs, and TC duration. The analysis period is divided into 2 decades (1977-1988 and 1997-2008). The PDI and TC-induced rainfall increase significantly in the later decade. This enhancement in the TC activity is because of the increase in the number of landfall TCs and the longer duration of the TCs over the two countries. The increase in the number of landfall TCs is associated with the enhanced northward steering flows over the East China Sea. The longer TC duration is mainly due to the high intensity of the approaching TCs prior to landfall. The other factors (i.e., tracks, translational speeds, mean drift lengths, and weakening rates of TCs) could also affect the TC duration, but they are found to be not significant. The results of our study reveal that the recent intensification of TCs is attributable to the changes observed in the later decade in the large-scale environments in the vicinity of the two countries. These changes include warmer sea surface temperature, highly humid midtroposphere, and weaker vertical wind shear over the region. In addition, another responsible factor is the anomalous upward motion driven by the relocation of secondary circulation near the jet entrance, which is highly related with weaker upper tropospheric jet stream in the recent decade.