Understanding the physical mechanism of record-breaking 2025 Korea–Japan heatwave

Abstract

The Korea–Japan (KJ) region has experienced warming faster than the global average, culminating in a record-breaking heatwave during the summer of 2025. Although the three summers from 2023 to 2025 all developed under a persistently negative Pacific decadal oscillation, only 2025 produced such an event. We show that the meridional structure of the underlying thermal forcing was decisive: in 2023, broad tropical–subtropical warming associated with El Niño distributed the thermal forcing across a wide latitudinal band; in 2024, a similarly broad tropical warming signal persisted; and in 2025, with the tropics having relatively cooled, warming was confined to the midlatitude North Pacific. This narrow midlatitude-confined forcing triggered the dynamical chain that drove the surface heatwave: a weakened upper-tropospheric temperature gradient on the equatorward flank of the jet, a weakened and poleward-displaced jet, persistent blocking, suppressed cloud cover, and enhanced surface shortwave radiation. The enhanced radiation then warmed the underlying ocean, and the air–sea coupling (ASC) further amplified and sustained the event. We further demonstrate that ASC over this region has strengthened since 2016, with 2025 representing its most extreme expression on record. These findings indicate that anticipating future KJ heatwave risk requires understanding the spatial structure of thermal forcing and the evolving strength of regional ASC, beyond projections of global-mean warming alone.