What do these extreme ocean events mean, how they are linked to climate change? For at least one researcher, they\u2019re an early sign of things to come.<\/p>\n
Published Dec 18, 2025 12:45 PM EST\t\t\t\t\t<\/p>\n
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Updated Dec 18, 2025 12:45 PM EST\n\t\t<\/p>\n
\n\tThe Boso Peninsula forms the eastern edge of Tokyo Bay. Normally, the Kuroshio Extension banks away from Japan and heads into the Pacific near this spot. But in recent years, it’s continued northward, bringing unprecedented warm water with it. (John S Lander\/LightRocket\/Getty Images via CNN Newsource)\n<\/p>\n
\n\t(CNN) \u2014 Bathtubs and pools mislead us about the ocean: Its surface is anything but flat.\n<\/p>\n
\n\tSeas pile up in some spots, pushed by trade winds or pulled by gravity toward big things like ice sheets. Amid it all, at the western end of large ocean basins, the fastest surface currents \u2014 veins of warm water \u2014 race toward the poles, causing additional slopes at the surface.\n<\/p>\n
\n\tThe ocean is uneven to begin with, and its unevenness is also changing. Maps of recent changes show intricate patterns of watery hills and valleys, but also call attention to one extraordinary location. Off the coast of Japan, one region of the ocean has been rising by nearly an inch every year, right next to another where it has been falling even faster.\n<\/p>\n
\n\tIt\u2019s the fingerprint of one of those surface currents changing its location, an event that has had dramatic repercussions. The Kuroshio, or \u201cBlack Current,\u201d is one of the largest streams of water anywhere in the world, and its recent movement has triggered record-warm ocean temperatures<\/a> and upended fisheries, an indelible staple of Japanese culture. Scientists say the warm waters have even amplified heatwaves on land and driven extreme rainfall.\n<\/p>\n \n\tAnd while there are signs some of the changes are now waning, fishing communities say they aren\u2019t yet back to normal. Meanwhile, scientists worry it could be a sign of more volatility to come.\n<\/p>\n \n\tThe position of the current could keep fluctuating, said Bo Qiu, a leading Kuroshio expert at the University of Hawai’i at Manoa. \u201cIt\u2019s hard to predict the future, but given the data we have so far, I can only see the intensity becoming larger and larger,\u201d he said.\n<\/p>\n \n\tThe deep, warm Kuroshio transports more than 200 times as much water as the Amazon River, traveling north from the equator and normally banking east around Japan\u2019s Boso peninsula, near Tokyo. Here, it becomes known as the Kuroshio Extension as it heads into the open Pacific.\n<\/p>\n \n\tBut in recent years, the current has been behaving in anything but the usual way, and the Extension, in particular, made a major divergence along Japan\u2019s coast. Its northern edge shifted as much as 300 miles farther poleward, leading to unprecedented warm waters in the surrounding region.\n<\/p>\n \n\t\u201cI was so surprised I don\u2019t even know if \u2018surprised\u2019 is the right word,\u201d said Shusaku Sugimoto, an associate professor at Tohoku University in Sendai, Japan, a northern coastal city.\n<\/p>\n \n\tSugimoto led a study<\/a> analyzing ocean temperatures off the coast in locations the Extension didn\u2019t historically reach, but has in recent years. \u201cThe fact that the temperature rose 6 degrees (Celsius) off the Sanriku coast, and\u00e2\u0080\u00afthe elevated temperature persisted for two years, represents a level of water temperature rise we\u2019ve never seen before,\u201d he said.\n<\/p>\n \n\tIt\u2019s not the only change.\n<\/p>\n \n\tIn August of 2017, the Kuroshio current south of Japan settled into a \u201clarge meander\u201d pattern, leaving the coastline and looping southward, taking its warm waters with it. This big shift in water temperatures south of Japan changes the distributions of fish species offshore.\n<\/p>\n \n\tLarge meanders themselves are a well-known recurring feature of the current, explained Shinichiro Kida, an oceanographer at Kyushu University. Records of these events<\/a> date back to the 1960s.During a lengthy meander event from 1975 to 1980, scientists saw a severe decline in anchovy in the Enshunada Sea, a major fishing region to the south of Japan\u2019s main island of Honshu. The anchovy were replaced by sardines, which favor the warmer water the current brought to the region.\n<\/p>\n \n\tBut no Kuroshio large meander that we know of has lasted as long as this one. In August, the Japan Meteorological Agency finally declared<\/a> that it had ended after nearly eight years. But overall both changes have had a big effect on this country that is home to more than 100 million.\n<\/p>\n \n\tThe meander and the Extension shift are connected, Qiu said. He co-authored a new paper<\/a> in the Journal of Climate arguing as much<\/a>, and calling the configuration a \u201cnew dynamic regime.\u201d\n<\/p>\n \n\tFishing boats sail toward a kelp harvest area in the Northern Territories, 3.7 kilometers off Nemuro, Hokkaido, Japan on June 2, 2018. Fish and kombu seaweed harvested in this region are crucial to Japan’s food culture, but recent changes in a warm ocean current called the Kuroshio have damaged these fisheries. (The Asahi Shimbun\/Getty Images via CNN Newsource)\n<\/p>\n \n\t\u201cI\u2019ve been working on the Kuroshio Extension for more than threedecades,\u201d Qiu said. \u201cI never expected this.\u201d\n<\/p>\n \n\tAs the Kuroshio powers along, it not only brings warmer water but also, depending on its location, higher sea level. There can be a several-foot difference between ocean heights on different sides of the current, thanks to its warmth and speed .\n<\/p>\n \n\tBut because of these factors, any new movement of the current can have dramatic effects.\n<\/p>\n \n\tThe large meander, for example, triggered a sea level fall in one region and pronounced sea level rise \u2014 by as much as half a foot \u2014 along the coast of the island of Honshu south of Tokyo. In October of 2017, when Typhoon Lan struck the Shizuoka Prefecture along this coastline, the higher sea level amplified the damage, according to the Japan Meteorological Agency.\n<\/p>\n \n\tIt is not only the height of the sea: As the Kuroshio is a warm current, and one traveling northward into cooler waters, its arrival in a new location has enormous effects on ocean temperatures.\n<\/p>\n \n\tThe big question is how much the recent phenomena are part of a natural cycle, and how much it\u2019s influenced by something else, like climate change. (And how much it\u2019s both, combined.)\n<\/p>\n \n\tThe impact of climate change on the large meander is unclear, because of its long history. But as far as the Extension shift goes, there are mounting reasons to implicate greenhouse gases and a changing climate.\n<\/p>\n \n\tKombu kelp is laid in the sun to dry Wakkanai, Hokkaido, on July 15, 2012. This seaweed is a vital part of Japan’s food culture and economy. But it’s becoming harder to harvest. (Hiroaki Murata\/The Yomiuri Shimbun\/AP via CNN Newsource)\n<\/p>\n \n\tTo understand the recent changes in the Kuroshio is to understand how water moves through the world\u2019s oceans, especially the large rotating \u201cgyres\u201d found in the world\u2019s major ocean basins.\n<\/p>\n \n\tFive of these basins \u2014 North and South Pacific, North and South Atlantic, and Indian \u2014 feature a similar pattern. Warm water travels west along the equator, then turns toward the poles. The currents carrying the warm water north or south on poleward transits are called \u201cwestern boundary currents<\/a>,\u201d and they include the Kuroshio and its four famed cousins: the Gulf Stream, the Brazil, the East Australian, and the Agulhas.\n<\/p>\n \n\tScientists are now seeing most of these currents changing in a similar way \u2014 getting warmer and pushing even farther poleward.\n<\/p>\n \n\tThis change is rooted in a phenomenon known as the Hadley Cell<\/a>, a global zone of warm, rising air across the tropics. It\u2019s now expanding due to climate change.\n<\/p>\n \n\t\u201cThis expansion shifts not just rainfall patterns but also the zones of sinking air that anchor high-pressure systems, such as the Pacific High,\u201d explains Brown University climate scientist Emanuele Di Lorenzo.\n<\/p>\n \n\tIn the middle latitudes, these massive high-pressure systems are the driving force behind currents like the Kuroshio. So, when the winds move, the currents do too.\n<\/p>\n \n\tBoth models and data suggest the Kuroshio Extension, in particular, has been moving north partly because of the atmospheric shift, Di Lorenzo explained.\n<\/p>\n \n\tBetween 1993 and 2021,driven by wind changes, the northern edge of the Kuroshio Extension shifted north by about 130 miles, a recent study found<\/a>. This was before an even larger shift in 2023 and 2024.\n<\/p>\n \n\tThe Kuroshio Extension\u2019s leap in 2023 and 2024 was an extreme event. The northern edge reached nearly to the northern tip of Honshu, Japan\u2019s largest island. In a different study<\/a>, Sugimoto and colleagues with the Japan Meteorological Agency and the Meteorological Research Institute took oceanographic measurements in the Extension\u2019s new location. They found water as much as 18 degrees Fahrenheit warmer than usual, extending to a depth ofaround 400 meters, or 1,300 feet.\n<\/p>\n \n\tThe authors note that for an entire year and a half between April 2023 and August 2024, the region saw \u201cintense marine heatwave conditions almost every day.\u201d\n<\/p>\n \n\tAnd it was not just in the water: the Japan Meteorological Agency also found<\/a> that the extreme ocean conditions contributed to record summer heat over land in northern Japan in 2023. Another group of scientists, meanwhile, linked<\/a> the warm offshore current to extreme rainfall over Japan\u2019s Chiba Prefecture, near Tokyo, in September of 2023.\n<\/p>\n \n\tThese ocean changes have altered the distributions of fish populations along Japan\u2019s Pacific Coast, creating extreme impacts on Japan\u2019s iconic fisheries.\n<\/p>\n \n\tIn central Japan, for instance, a major mackerel fishery was upended, and fishers say that even though the large meander has now ended, that doesn\u2019t mean things can instantly go back to the way they were.\n<\/p>\n \n\t\u201cImmediate recovery is not a reality, and while conditions may gradually improve, at present, there has been no recovery in catches,\u201d said Osamu Nagai, executive director of the Mie Gaiwan (Outer Bay) Fisheries Cooperative Association.\n<\/p>\n \n\t\u201cThe catch has fallen to less than half of what it was 10 years ago \u2014 we\u2019re now only catching about 20 to 30 percent of the mackerel. This is a major blow,\u201d Nagai said.\n<\/p>\n \n\tNortheastern Japan\u2019s Sanriku coast, known for its rich fisheries, is a different story, but still a bad one. Here, the southward flowing Oyashio current traditionally brings down cool waters and supports rich fisheries. But when the Kuroshio Extension moved northward into this region, it displaced the Oyashio, bringing on a stark change in ocean temperature.\n<\/p>\n \n\tAnd it is not just fish species, like Pacific salmon and saury, that became harder to catch.\n<\/p>\n \n\t\u201cWhat matters most\u00e2\u0080\u00afis the foundation of Japan\u2019s most important food culture, which is the flavor made out of Kombu seaweed \u2014\u00e2\u0080\u00afit can only be harvested in Hokkaido, near Japan,\u201d said Yoshihiro Tachibana, a professor who specializes in climate dynamics at Mie University.\n<\/p>\n \n\t\u201cKombu stocks are declining dramatically. Dashi (the fundamental Japanese soup stock) culture might collapse. It\u2019s declining. We can\u2019t get any at all. So I believe this has a tremendous impact on our food culture as well,\u201d he said.\n<\/p>\n \n\tRecently, the northern edge of the Kuroshio extension has retreated a bit. It has moved back to around 37 degrees North Latitude, according to Qiu. That\u2019s still a high location historically, but hardly as extreme as before.\n<\/p>\n \n\tBut the questions remain: what do these extreme ocean events mean, how they are linked to climate change? For at least one researcher, they\u2019re an early sign of things to come.\n<\/p>\n \n\t\u201cIt\u2019s a great opportunity to learn what the oceans will be like 100 years from now,\u201d said Sugimoto, of Tohoku University in the northeastern region of Honshu, Japan\u2019s largest island.\n<\/p>\n \n\t\u201cAn unprecedented ocean phenomenon is now occurring by chance in Tohoku,\u201d he continued. \u201cUnderstanding how this has altered the seas of Tohoku offers a chance to understand how the world\u2019s oceans will change in the future.\u201d\n<\/p>\n \n\tElisabeth Doty contributed to this report.\n<\/p>\n \n\tThe-CNN-Wire\u2122 & \u00a9 2025 Cable News Network, Inc., a Warner Bros. Discovery Company. All rights reserved.\n<\/p>\n