Habitable Japan Sustainability of atmospheric and oceanic environment as a survival basis of island country Japan Grant-in-Aid for Transformative Research Areas (A) (2024–2028)

The Kuroshio is one of the world’s strongest warm currents. It flows along the southern coast of Japan and continues into the Pacific Ocean off the Boso Peninsula in Chiba Prefecture, where it is known as the “Kuroshio Extension” (top in Fig. 1). However, around the end of 2022, the Kuroshio Extension began to veer northward. It passed Ibaraki, Fukushima, Miyagi, and Iwate Prefectures, and by the winter of 2023, it had reached off Aomori Prefecture (Fig. 1). This is the first extreme northward meandering since satellite observations began in 1993, and it has drastically altered the marine environment off Sanriku—one of the world’s three major fishing grounds. This extreme meandering persisted until around February 2025. At present (September 2025), the Kuroshio Extension is flowing off Ibaraki Prefecture in a relatively calm state.

During the two years of this extreme northward meandering (February 2023–January 2025), sea surface temperature (SST) off Sanriku remained about 6 °C above normal (Fig. 2). This is the largest increase in the world’s oceans, and since 1850, no other area of ​​the ocean has experienced SST increase of 6°C for as long as two years. The ocean off Sanriku has become “the most intensely warmed in the world.”

How deep did this warming extend? Research cruise data conducted by the Japan Meteorological Agency revealed that the warming extended far below the surface, reaching depths of 700 m. Under these conditions, changes have begun to appear in the ecosystem. Fish species native to warm southern oceans, such as the cardinalfish and the southern yellowtail, have been confirmed off Miyagi Prefecture for the first time. Fish that had never been caught before are now being caught, and this is having a serious impact on fishing grounds and the local fishing industry.

This “unprecedented ocean” eventually caused “unprecedented atmosphere” conditions. Because the ocean is so hot, vigorous heat is released from the ocean into the atmosphere in winter, increasing the air temperature off Sanriku, with effects extending up to 2 km above the sea surface (Fig. 3). In other words, the extreme northward meandering of the Kuroshio Extension changed not only the ocean temperature but also the atmospheric structure.

To understand the changing state off Sanriku, we conducted research cruises aboard the R/V Shinsei-maru and T/V Seisui-maru from late June to early July, 2025. We are currently analyzing the sample data collected to understand the changes that have occurred in the ocean. Please stay tuned for further information.

Although the Kuroshio Extension is currently in a calmer state, its future path remains unpredictable. There is a possibility that the Kuroshio Extension will once again meander extremely northward, and depending on its course, the sea and sky off Sanriku, and even our lives, will be greatly affected. That is precisely why continuous on-site monitoring by research vessels capable of observing seamlessly from the depths of the ocean to the heights of the sky is indispensable.

For more details,
Sugimoto, S., A. Kojima, T. Sakamoto, Y. Kawakami, and H. Nakano (2025): Influence of extreme northward meandered Kuroshio Extension during 2023–2024 on ocean–atmosphere conditions in the Sanriku offshore region, Japan. Journal of Oceanography, 81, 203–215, doi:10.1007/s10872-025-00747-x.

(Shusaku Sugimoto@A01-1. September 2025)