Large-Scale Factors for the Persistent Extreme Heatwave over Japan in 2024 Summer and Late-July Heavy Rainfall in Northern Japan

Abstract

In summer 2024, record-high temperatures were observed over Japan, where the area-averaged summer-mean temperature was tied highest with 2023. The extreme heatwave in July was attributable primarily to the poleward-deflected upper-tropospheric subtropical jet (STJ). The low-level North Pacific Subtropical High (NPSH) intensified to the immediate south of Japan under the remote influence of enhanced cumulus convection over the northern Indian Ocean. Persistent anomalous descent and increased solar radiation associated with the NPSH contributed to the record-high temperatures in the southern portion of Japan. In August, the persistent heatwave, particularly over western Japan, was attributable to the poleward-deflected STJ and enhanced convection associated with a lower-tropospheric cyclonic gyre to the southeast of Japan where several typhoons sequentially formed. Meanwhile, heavy rainfall over northern Japan in late July was attributable primarily to developed convective systems organized just to the south the Baiu front under the intensified moist westerly airflow to the north of the markedly-extended NPSH over western Japan. Other factors that could contribute to the extreme heatwave and heavy rainfall in 2024 summer include extreme warmth of the surrounding ocean, global warming, and extremely high zonal-mean temperatures in the midlatitude Northern Hemisphere troposphere during a post El Niño summer.