[A-AS04] Extreme Events: Observations and Modeling

[AAS04-03] Unprecedented heatwave around northern Japan in summer 2021 and its global linkages

★Invited Papers

*Shoshiro Minobe¹, Fumiaki Ogawa¹, Takeshi Enomoto², Toru Miyama³, Masuo Nakano⁴, Shion Sekizawa⁵ (1.Faculty of Science, Hokkaido University, 2.Disaster Prevention Research Institute, Kyoto University, 3.Japan Agency for Marine-Earth Science and Technology, Application Laboratory, 4.Japan Agency for Marine-Earth Science and Technology, 5. Research Center for Advanced Science and Technology, University of Tokyo)

Keywords:heatwave, blocking, jetstream

An unprecedented heatwave that occurred around northern Japan occurred from mid-July to early August 2021 was investigated with respect to local features, regional atmospheric circulation anomalies, and global teleconnections. This heatwave brought record-breaking consecutive hot-summer-days (daily maximum temperature higher than 30 °C) in Sapporo, the largest city on Hokkaido Island, and temperature and sunshine durations averaged over the heatwave period were record high in many places in Hokkaido accompanied by moderate precipitation reduction. The Hokkaido heatwave is a part of a high-temperature anomaly in the entire troposphere centered over Sakhalin Island in Russia, north of Japan, related to anticyclonic circulation aloft. This anticyclonic circulation anomaly and accompanying weaker cyclonic circulation to the south formed a blocking north-south dipole in the upper troposphere. The blocking was initiated by a Rossby wave breaking and lasted three weeks. The main branch of the jetstream flowed north of the blocking, bypassing Japan to the north. This jetstream shift brought warm tropical air over Japan, and this is likely to be the main cause of the heatwave. Furthermore, since the jetstream usually advects moist air from Eurasia to Japan, the jetstream shift northward from Japan reduced the inflow of moist air into Japan causing the longer-than-usual sunshine duration. As for teleconnections, the Pacific Japan (PJ) pattern, which was third highest in the record since 1958, played an important role in shaping the anomalous atmospheric circulation, in association with the enhanced convection northeast of the Philippines. Moreover, a propagating signal crossing the Eurasia continent with about two week periodicity found by Fukutomi et al. (2012) may be important. The arrival of the propagating signature, referred to as Northern Eurasia quasi-BiWeekly Oscillation (NE-BWO) roughly coincided with the onset of the blocking. Therefore, it is suspected that NE-BWO triggered the anomalous atmospheric circulation and heatwave around northern Japan. Possible relationships of regional changes under global warming to future heatwaves were discussed.