Influence of extreme northward meandered Kuroshio Extension during 2023–2024 on ocean–atmosphere conditions in the Sanriku offshore region, Japan

Abstract The ocean off the Sanriku coast, Japan, where the Kuroshio Extension (KE) meets the Oyashio, has unique dynamics and hosts rich marine resources. Since April 2023, the KE, typically flowing eastward around 36°N, exhibited an extreme northward meander, reaching 40°N by the winter of 2024. An anticyclonic eddy pinched off from the KE’s northern edge in late May 2024. The Japan Meteorological Agency conducted hydrographic surveys that cross this eddy and the KE along about 145°E in late May 2024. The hydrographic data revealed that thick warm-salty subtropical water replaced the cold-fresh subarctic water off the Sanriku coast, with temperatures 10 °C warmer at depths of 50–400 dbar compared to past decades. The Subtropical Mode Water, characterized by vertical uniformity, existed north of 40°N, more than 500 km further north of its conventional distribution area. From April 2023 to August 2024, satellite data indicated record-high sea surface temperature off the Sanriku coast (+ 4.9 °C), with intense marine heatwave conditions almost every day. In the winter of 2024, over the very warm ocean off the Sanriku coast, 300 W m –2 more heat than usual in the form of turbulent heat flux was released from the ocean to the atmosphere, and air temperature and humidity increased up to around 800 hPa level, from atmospheric reanalysis data. In addition to monitoring these unprecedented ocean conditions off the Sanriku coast, it is important to assess their impact on the marine environment, fisheries, and, ultimately, the local economy.