Cutoff low approaching routes to the Japan islands

Cutoff lows (COLs) are mid-latitude cyclones appear at the upper troposphere. They contain cold air mass derived from the polar region. COLs decrease stability of the lower troposphere (e.g., Hoskins et al. 1985) and often accompany severe weathers (e.g., Mohr et al. 2020), which sometimes causes great damages to mid-latitude regions, such as Japan. Besides, the approaching routes of COLs to the Japan Islands and their seasonal cycles have not been investigated with objective method. In this study, we attempt to elucidate them by utilizing depression detection scheme of Kasuga et al. (2021; K21), which can detect COLs from their early stage (pre-existing trough stage). We have implemented a tracking scheme of COLs by simply searching overlapping of area of COLs. Merge and split of COLs are also recorded to investigate variations of the generation/dissipation of COLs.Generally, COLs approach the Japan Islands from northwest advected by the subpolar jet throughout the seasons. They also approach from southeast in JJA and SON at 200 hPa passing around the North Pacific Subtropical High. These two routes were reported in different studies; we successfully track them with a consistent criterion and confirmed that they have longer durations compared to that of a geopotential minimum detection. Tracks shows different characters at 200 and 500 hPa levels as preceding studies have noted. We have found that strong class of COLs are most frequent in MAM. We are investigating climatological backgrounds of the seasonal variations of COLs and hoping to show the results in the conference.
The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)