9:45 AM - 10:00 AM
[AAS03-04] Large-scale factors for extreme heatwave over Japan in 2024 summer and late-July heavy rainfall in northern Japan
Keywords:heatwave, heavy rainfall, North Pacific subtropical high
A preliminary diagnosis of primary factors for the unprecedented hot summer over Japan in 2024 and the heavy rainfall over northern Japan in late July is presented based on 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 was the highest on record tied with 2023. Under a persistent above-normal SAT from mid-July through mid-August, record-high temperatures were successively observed locally, whereas heavy rainfall occurred in northern Japan in late July.
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 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 (Xie et al. 2009, 2016). In late July the NPSH also extended northward toward western Japan in association with the active phase of convection around the Philippines, including a typhoon. Both the associated persistent anomalous descent and increased downward solar radiation under the NPSH contributed to the record-high SATs over Japan. Meanwhile, the late-July heavy rainfall in northern Japan was attributable primarily to developed convective systems organized under the intensified moist westerly airflow continuous from intensified northward moist airflow toward the stagnated Baiu front between typhoon “GAEMI” and the intensified NPSH over western Japan.
The subsequent unprecedented heatwave in August was also attributable to the poleward-deflected STJ in common with that in July. Unlike in July, 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.
As in the preceding summer, 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 as indicated by Sato et al. (2024). In addition, significantly high SST around Okinawa/Amami could contribute to the record-high SATs in 2024 summer over this area through direct sensible heating of low-level air and also remotely to the heavy rainfall in late July over northern Japan through the enhanced supply of moisture to the northward airflow. 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 Niño event could also contribute to the extreme summer-mean SATs over Japan.
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 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 (Xie et al. 2009, 2016). In late July the NPSH also extended northward toward western Japan in association with the active phase of convection around the Philippines, including a typhoon. Both the associated persistent anomalous descent and increased downward solar radiation under the NPSH contributed to the record-high SATs over Japan. Meanwhile, the late-July heavy rainfall in northern Japan was attributable primarily to developed convective systems organized under the intensified moist westerly airflow continuous from intensified northward moist airflow toward the stagnated Baiu front between typhoon “GAEMI” and the intensified NPSH over western Japan.
The subsequent unprecedented heatwave in August was also attributable to the poleward-deflected STJ in common with that in July. Unlike in July, 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.
As in the preceding summer, 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 as indicated by Sato et al. (2024). In addition, significantly high SST around Okinawa/Amami could contribute to the record-high SATs in 2024 summer over this area through direct sensible heating of low-level air and also remotely to the heavy rainfall in late July over northern Japan through the enhanced supply of moisture to the northward airflow. 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 Niño event could also contribute to the extreme summer-mean SATs over Japan.