Influential role of inter-decadal explosive cyclone activity on the increased frequency of winter storm events in Hokkaido, the northernmost island of Japan

This study examined explosively developing extratropical cyclone activity in the vicinity of Japan in boreal winter from 1979/1980 to 2016/2017 mainly using the Japanese 55-year Reanalysis data. The in situ observation and reanalysis data revealed increased frequencies of both strong winds and heavy precipitation in Hokkaido, the northernmost island of Japan, after the end of the last century, which is consistent with the fact that the cyclones migrating along the Kuroshio Current (KC cyclones) have tended to approach the Hokkaido region in recent years. To examine factors controlling the cyclone track and rapid development, we performed composite analyses of the northwards migrating type (N type) and eastwards migrating type (E type) KC cyclones. In the N-type cyclone, wave trains prevailed along the polar front and subtropical jets over the Eurasian continent, thereby forming an upper-level trough (ridge) to the west (east) of the cyclone. An upper-level divergence accompanied by rapid development of the N-type cyclone excited wave packets downstream as a Rossby wave source, leading to further reinforcement of the upper-level ridge east of the cyclone. The anomalous anticyclone with a barotropic nature is capable of forcing the cyclone track to shift northwards. Furthermore, a combination of the N-type cyclone and the anomalous anticyclone to its east facilitates moisture import into the cyclone system from lower latitudes, contributing to enhanced moisture flux convergence. In reality, diabatic heating in the vicinity of the cyclone centre is larger in the N-type cyclone than in the E-type one. It is also suggested that the pronounced Rossby wave propagation along the subtropical jet related to a climate regime shift in the Tropics is responsible for the recent increase in the number of N-type cyclones.