Oral

IAMAS

JMP03 - High-impact Weather and Climate Extremes

Large-Scale Factors for the Extreme Heat over Japan in 2024 Summer

Primary large-scale factors for an unprecedented hot summer over Japan in 2024 are discussed on the basis of analysis by the JMA Advisory Panel on Extreme Climate Events. In summer 2024, record-high surface air temperatures (SATs) were observed in Japan, where the summer-mean area-averaged SAT tied for the highest record set in 2023. Under persistent above-normal SATs from mid-July through mid-August, record-high temperatures were successively observed locally, especially over western and eastern Japan as well as over Okinawa/Amami island region. The unprecedented heatwave from July was attributable primarily to the persistent poleward-deflected subtropical jet (STJ). In July, the low-level North Pacific subtropical high (NPSH) extended zonally to the immediate south of Japan in association with enhanced convection over the northern Indian Ocean through the Kelvin-Wave Induced Ekman Divergence (WIED) process. In late July the NPSH also extended northward toward western Japan in association with the sub-seasonally enhanced convective activity around the Philippines, including a typhoon. Both the associated persistent anomalous descent and increased insolation under the NPSH contributed to the record-high SATs over the southern portion of Japan. The subsequent unprecedented heatwave in August was also attributable to the poleward-deflected STJ as in July. Unlike in July, however, a large-scale low-level cyclonic gyre with enhanced convective activity formed to the southeast of Japan, maintained by southwestward-intruding upper-level cold cyclonic vortices and the lower-level anomalous anticyclonic circulation in the South China Sea forced through the Kelvin-WIED process. Markedly high SST around northern Japan could contribute to the extremely high SATs around northern Japan mainly through direct sensible heating and enhanced greenhouse effect by increased evaporation, although the contribution of the marine heatwave appeared to be somewhat less dominant than in the preceding summer. Rather, another marine heatwave with significantly high SST around the Okinawa/Amami region could contribute locally to the record-high SATs in 2024 through direct sensible heating of low-level air. Moreover, both a long-term warming trend in the troposphere under the global warming and pronounced above-normal tropospheric temperatures over the midlatitude Northern Hemisphere partly due to delayed influence of the latest El Ni?o event could also contribute to the extreme summer-mean SATs over Japan in 2024.