[A-AS04] Extreme Events and Mesoscale Weather: Observations and Modeling

[AAS04-01] An Unprecedented Heatwave over Japan in 2023

*Hisashi Nakamura¹, Kazuto Takemura², Hirotaka Sato², Akira Ito², Takafumi Umeda², Shuhei Maeda² (1.Research Center for Advanced Science and Technology, University of Tokyo, 2.Japan Meteorological Agency)

Keywords:heatwave, subtropical jet, teleconnection, sea-surface temperature, typhoons

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 the unprecedented heatwave in late July through August, record-high surface air temperatures (SATs) were observed at many locations, especially over northern and eastern Japan. The heatwave was caused by natural climate variability superimposed on a long-term warming trend in air temperature, as suggested by an event attribution assessment with large-ensemble atmospheric model experiments. The late-July heatwave was attributable primarily to the poleward-deflected subtropical jet (STJ) and the markedly intensified surface North Pacific subtropical high over Japan. This situation occurred under the influence of the SilkRoad teleconnection and the Pacific–Japan (PJ) teleconnection. The latter was associated with northwestward-moving enhanced tropical convection, including typhoons, over the western North Pacific, under the local influence of warm sea-surface temperature (SST) anomalies and remote influence from the relatively cool tropical Indian Ocean. Those SST anomalies can be regarded as remnant and delayed impacts of a La Niña event that lasted until the preceding winter. The August heatwave was attributable to warm, moist low-level southerlies and their downslope-wind effects on the Japan Sea coast, occurring under the poleward-deflected STJ under active convection, including typhoons, over the tropical and subtropical western North Pacific. In addition, extremely high SSTs around northern Japan were likely to contribute locally to the record-setting summer-mean SATs by reducing low-level clouds and enhancing moisture supply through ocean evaporation. They both acted to maintain the high SSTs and record-setting SATs in coastal regions. The prominent positive SST anomalies to the east of northern Japan were due mainly to an extreme poleward meander of the Kuroshio Extension and a poleward retreat of the Oyashio.