10:15 AM - 10:30 AM
[ACG32-06] Role of the Midlatitude Ocean in the Unprecedented Heatwave over Japan in 2023
Keywords:heatwave, sea-surface temperature, low-level clouds, Kuroshio Extension
In summer 2023 Japan, as several other regions in the Northern Hemisphere, suffered from an unprecedented heatwave. Record-high surface air temperatures (SATs) were observed at many locations particularly over northern and eastern Japan, due to natural climate variability superimposed on a long-term warming trend. The heatwave was attributable primarily to the markedly intensified surface North Pacific subtropical high around Japan under the poleward-deflected subtropical jet (STJ), which occurred under the influence of the Silk-Road and Pacific–Japan (PJ) teleconnections. The latter was associated with enhanced tropical convection over the western North Pacific under the local influence of positive sea-surface temperature (SST) anomalies and remote influence from the relatively cool tropical Indian Ocean. 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 evaporative moisture supply from the ocean. The former acted to increase insolation, while the latter acted to enhance the greenhouse effect locally. They both reinforced the high SSTs and record-setting SATs in coastal regions. The prominent 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. Furthermore, positive SST anomalies in major oceanic frontal zones both in the midlatitude North Pacific and Atlantic may contribute to persistent poleward displacement of the STJ and thus tropospheric warm anomalies over the midlatitude Northern Hemisphere.