Climatology of Cutoff Lows Approaching Japan

Cutoff lows that approach Japan are often associated with hazardous weather. In this study, a basic climatology of cutoff lows approaching Japan was compiled by identifying tracks in reanalysis data for the period of 1979-2024. Cutoff low tracks approaching from the northwest (NWs) were observed most frequently in spring and least frequently in autumn and winter. NWs tended to occur where subpolar and subtropical jets merge. NWs were generally strong during autumn to spring, and relatively weak in summer. Cutoff low tracks from the southeast (SEs) were only observed in summer, and originated southeast of Japan in a region of climatologically weak winds and Rossby wave breaking. SEs were generally less frequent and weaker than NWs despite their association with hazardous weather. The frequency of SEs has significantly increased south of Japan in summer during the 46-year period of study, which may be associated with weakening of the midlatitude westerlies and strengthening of the subtropical easterlies in the upper troposphere.