Abstract In 2025 Japan underwent record-high summer-mean temperature exceeding the preceding records set in 2023 and 2024. The prolonged heat in 2025 was attributable to a markedly persistent poleward shift of the subtropical jet (STJ) over the Asian continent and around Japan, associated with the early onset and persistent enhancement of Asian monsoonal convection. The early termination of the Baiu period caused extremely dry conditions nationwide in July. The remarkably-enhanced convection around the Philippines, where several typhoons sequentially formed, contributed to the marked northwestward extension of the North Pacific Subtropical High (NPSH) toward Japan through the Pacific-Japan (PJ) teleconnection in July. Sub-monthly events of Rossby-wave propagation along the STJ occasionally enhanced the extension of the upper-level Tibetan High toward Japan, causing the accentuated extreme heat. Persistent anomalous descent and increased insolation under the NPSH contributed to the record-high surface air temperatures. Global warming as well as above-normal sea-surface temperatures around Japan and over the midlatitude North Pacific may also have contributed to the record heatwave.In early August, intra-seasonally suppressed convection around the Philippines and Rossby-wave propagation over the Asian continent caused the southward-meandered STJ, with ascent along enhanced low-level moist airflow to cause locally heavy rainfall over Japan.
Takemura et al. (Fri,) studied this question.