Contributions of the climate regime shift and historical global warming to explosive cyclone activity around Japan according to large-ensemble simulations

Using the Database for Policy Decision-Making for Future Climate Change (d4PDF), this study examined the impact of a tropical climate regime shift around 1998/1999 on explosive cyclone activity around Japan in boreal winter, highlighting cyclones moving along the Kuroshio Current (KC cyclones), especially northward-migrating (N-type, as defined in Tsukijihara et al., 2019) KC cyclones. Wave-train patterns along the Asian jet affect the track and rapid development of N-type cyclones, and the frequency of the top 10% of pronounced wave-train patterns using a teleconnection index (NI10%) has increased since 1998/1999. Tropical precipitation in the region close to the Bay of Bengal increased with the climate regime shift, leading to the increased frequency of NI10%. The downstream development of wave packets tends to form a ridge with a barotropic structure east of Japan that is able to force the northward shift of the KC cyclone. In reality, there are high correlations between the inter-annual variations in the ensemble mean frequencies of NI10%, N-type cyclones, and tropical precipitation around the Bay of Bengal, and all three indices have increased since 1998/1999. Furthermore, the above results are nearly the same in both historical and non-warming experiments, indicating that recent changes in the frequency of N-type cyclones are not directly related to historical global warming.