Exceptional Heat and Basin-Scale Connections in the Kuroshio-Oyashio Region in the Early 2020s

Abstract This study investigates recent extreme conditions in the North Pacific, which began in the late 2010s and have intensified significantly by 2024, focusing on the Kuroshio-Oyashio region. The anomalies appear to have originated in the western North Pacific, linked to changes in the Kuroshio and Oyashio currents, while developments in the eastern North Pacific remained somewhat distinct. Sea surface temperature anomalies in the Kuroshio-Oyashio region have reached record-breaking levels, with deviations up to 6°C above the 1993–2022 average. Subsurface temperatures have also increased notably, accounting for 15.7% and 12.3% in the western and eastern North Pacific (28% combined) of the global ocean heat content increase in the 0–300 m layer between 2020s and 2010s. In the 2020s, the Kuroshio current extended northward to approximately 42°N along Japan’s coast, forming a loop-like pattern with a southward return flow, while the Kuroshio Extension in wider zonal extent also shifted northward. These oceanic anomalies are likely influenced by wind changes, with Sverdrup theory suggesting weakened subtropical gyres and a northward shift of the subtropical-subpolar gyre boundary. Further research is needed to fully understand, monitor, and predict these changes and their impacts on regional ecosystems.