Unprecedented oceanographic conditions in October 2024 around the Oyashio and Coastal Oyashio off the southeastern coast of Hokkaido, Japan

During October 2024, unprecedented oceanographic conditions were observed around the Oyashio and Coastal Oyashio in the North Pacific Ocean off the southeast coast of Hokkaido, Japan. A very strong anticyclonic mesoscale eddy, the “Kuroshio Warm Core Ring (KWCR)”, detached from the Kuroshio Extension, which had abnormally overshot, and approached the southeast coast of Hokkaido. Extremely large, positive anomalies of temperature and salinity with a maximum of 14.1 °C and 1.61, respectively, were detected on the slope where the Oyashio flows in normal years. Isopycnal surfaces with extremely large downward displacements from their mean positions reached a maximum of 413 dbar. The approach of the KWCR to the coast caused unusual oceanographic conditions on the shelf. Kuroshio Water with salinities ≥33.7 and temperatures ≥5 °C spread from the KWCR and was identified at a regular monitoring station on the shelf for the first time since 1987. The Coastal Oyashio, a nearly stable westward coastal boundary current, disappeared from the western part of the shelf and was replaced by eastward currents parallel to the anticyclonic flows of the KWCR. The intrusion of the Oyashio into the study region weakened in October 2024. However, because a similar tendency of weakening has been apparent since autumn 2019, the weakening of the Oyashio was not attributable solely to the stagnation of the KWCR. • The Kuroshio Extension (KE) has exhibited abnormal overshoot since the end of 2022. • A Kuroshio Warm Core Ring (KWCR) detached from the KE generated corollary extremes. • The KWCR stagnated on the slope where the Oyashio flows in normal years. • Extra-large anomalies of temperature > 14 °C and salinity >1.6 were measured. • The Coastal Oyashio disappeared from the western part of the shelf.