In summer 2023, record-high temperatures were observed in many parts of the Northern Hemisphere, including Japan, where summer-mean temperature was the highest over the last 126 years. Under an unprecedented heatwave in late July through September, record-high temperatures were successively observed particularly over northern and eastern Japan. The late-July heatwave is attributable primarily to the markedly-intensified North Pacific Subtropical High over Japan, accompanied by the poleward-deflected subtropical jet (STJ). This situation occurred under the influence of the Pacific–Japan pattern driven by northwestward-moving enhanced tropical convection over the western North Pacific and the Silk-Road pattern. The enhanced convection was influenced by upper-level cyclonic vortices detached from the intensified mid-Pacific trough. Seemingly, it was also under the remote influence from positive sea-surface temperature (SST) anomalies in the western equatorial Pacific as well as negative ones in the central–eastern equatorial Indian Ocean, considered as remnant and delayed impacts of long-lasted La Niña until the preceding winter. The August heatwave occurred under the persistent poleward-shift of STJ as well as warm, moist low-level southerlies and their downslope-wind effects. Both extremely high SST around northern Japan and a long-term warming trend in air temperature could also contribute to the record-setting air temperature.
The winter response of the Asian jet stream to global warming is investigated using the Coupled Model Intercomparison Project Phase 5 (CMIP5) multi-model dataset under the RCP4.5 scenario. We first evaluate model performances in reproducing the current climatology in the upper troposphere and select the best 27 models. A multi-model ensemble projection by the selected models indicates that the jet stream over the Indochina peninsula and the South China Sea is intensified on its equatorial side in the late 21st century, while the jet stream shifts poleward over the Eurasian continent and the North Pacific. The strengthening of the jet stream on the south side is associated with cyclonic (anticyclonic) circulation anomalies in the upper (lower) troposphere over the southern part of China and decreased upper tropospheric divergence over the Maritime Continent. The strength of the upper tropospheric divergence and the amplitude of the anticylonic eddy streamfunction are strongly correlated. These findings suggest that future changes in the jet stream are related to the weakening of a Matsuno-Gill response to tropical heating.
Future changes in winter stationary waves are investigated using Coupled Model Intercomparison Project Phase 5 (CMIP5) models and a linear baroclinic model (LBM). The CMIP5 models showed a wave-like pattern of stationary waves from East Asia to the North Pacific and a weakening of horizontal divergence over the Maritime Continent. To investigate dynamical relationships among these changes, we performed LBM experiments using the zonal mean basic state and zonally asymmetric thermal forcing. The differences between the future and present experiments were similar to the changes projected by the CMIP5 models, although positions and amplitudes differed slightly. In addition, two of the LBM experiments showed that the change in the basic state explained most of the changes in the stationary wave, whereas the change in thermal forcing accounted for the eastward shift of the stationary wave. The storm track experiments conducted with the LBM to investigate the role of transient eddy feedback on stationary wave changes suggested that the feedback shifts the thermally forced stationary waves northeastward. This shift may explain the difference between the LBM experiments and the CMIP5 future projection.
An extreme rainfall event occurred over western Japan and the adjacent Tokai region mainly in early July, named "the Heavy Rain Event of July 2018", which caused widespread havoc. It was followed by heat wave that persisted in many regions over Japan in setting the highest temperature on record since 1946 over eastern Japan as the July and summertime means. The rain event was attributable to two extremely moist airflows of tropical origins confluent persistently into western Japan and large-scale ascent along the stationary Baiu front. The heat wave was attributable to the enhanced surface North Pacific Subtropical High and upper-tropospheric Tibetan High, with a prominent barotropic anticyclonic anomaly around the Korean Peninsula. The consecutive occurrence of these extreme events was related to persistent meandering of the upper-level subtropical jet, indicating remote influence from the upstream. The heat wave can also be influenced by enhanced summertime convective activity around the Philippines and possibly by extremely anomalous warmth over the Northern Hemisphere midlatitude in July 2018. The global warming can also influence not only the heat wave but also the rain event, consistent with a long-term increasing trend in intensity of extreme precipitation observed over Japan.
This study investigates future changes in temperature and precipitation extremes over Japan by the end of the 21st century using ensemble simulations under the Representative Concentration Pathway 8.5 scenario projected by a convection-permitting regional climate model with a grid spacing of 2 km.
