Chris Mooney is a Pulitzer Prize-winning journalist and CNN climate contributor. He is currently a professor of practice at the University of Virginia’s Environmental Institute.
Bathtubs and swimming pools mislead us about the ocean: its surface is anything but flat.
Pushed by trade winds or driven by gravity, the ocean builds up in places and flows toward large objects such as ice sheets. Among them, at the western end of the ocean basin, the fastest surface currents (warm water veins) surge toward the poles, causing additional slopes on the surface.
The ocean is inherently uneven, and its unevenness changes. Maps of recent changes show complex patterns of water, mountains and valleys, but also draw attention to a remarkable site. Off the coast of Japan, one area of the ocean is rising nearly an inch a year, and another area immediately adjacent is declining at a faster rate.
It was a fingerprint in which a surface current changed its position, an event that had huge consequences. The Kuroshio, also known as the “Kuroshio”, is one of the world’s largest currents, and its recent movements have triggered record increases in ocean temperatures and upended fishing, an indelible staple of Japanese culture. Warmer waters are even exacerbating heat waves on land and triggering extreme rainfall, scientists say.
While there are signs that some changes are waning, fishing communities say a return to normal has not yet occurred. At the same time, scientists worry it could be a sign of increased volatility ahead.
Bo Qiu, a leading Kuroshio expert at the University of Hawaii at Manoa, said the current’s position may continue to fluctuate. “It’s hard to predict the future, but given the data we have so far, I can only see the intensity getting stronger,” he said.
The Boso Peninsula forms the eastern edge of Tokyo Bay. Normally, the Kuroshio current extends away from Japan and flows into the Pacific Ocean near this location. But in recent years, it has continued to move north, bringing unprecedented warm water. – John S. Rand/LightRocket/Getty Images
a river in the ocean
The deep, warm Kuroshio Current carries more than 200 times the volume of water as the Amazon River and flows north from the equator, often eastward around Japan’s Boso Peninsula near Tokyo. Here, as it enters the open Pacific Ocean, it is known as the Kuroshio Extension.
However, in recent years, the ocean current has behaved differently than usual, especially in the extended section, causing serious divergence along the coast of Japan. Its northern edge moved 300 miles poleward, causing unprecedentedly warm water in the surrounding area.
“I was very surprised, I don’t even know if ‘surprised’ is the right word,” said Shusaku Sugi, an associate professor at Tohoku University in Sendai, a coastal city in northern Japan.
Sugimoto led a study that analyzed ocean temperatures in coastal areas that the extension line historically did not reach but has in recent years. “The temperature along the Sanriku coast has increased by 6 degrees Celsius, and this high temperature has persisted for two years. This is a level of water temperature rise that we have never seen before,” he said.
That’s not the only change.
In August 2017, the Kuroshio in southern Japan formed a “big meander” pattern, leaving the coastline and looping southward, taking warm water with it. Dramatic changes in water temperatures in southern Japan have altered the distribution of offshore fish.
Kyushu University oceanographer Shinichiro Kida explains that large meanders themselves are well-known recurring features of ocean currents. Records of these events date back to the 1960s. During a long meandering event from 1975 to 1980, scientists found severe declines in anchovy populations in Enshu Nada, a major fishing area south of Japan’s main island of Honshu. Anchovies were replaced by sardines, which prefer the warm water brought to the area by currents.
But as far as we know, no major Kuroshio meander has lasted as long as this one. In August, the Japan Meteorological Agency finally announced the end of the event after nearly eight years. But overall, both changes have had a significant impact on a country of more than 100 million people.
Qiu said meandering and extensional displacement are related. He co-authored a new paper in the Journal of Climate that makes the same point, calling this configuration a “new dynamic.”
“I have worked on the Kuroshio expansion project for more than thirty years,” Qiu said. “I didn’t expect it to be like this.”
As the Kuroshio advances, it brings not only warmer water but, depending on its location, higher sea levels. Due to the warmth and speed of the current, the height of the ocean on either side of the current can differ by several feet.
But because of these factors, any new trend movement can have a huge impact.
For example, the Great Meander triggered a drop in sea levels in one area and caused significant sea level rise, as much as half a foot, off the coast of Honshu, south of Tokyo. When Typhoon Ran struck coastal areas of Shizuoka Prefecture in October 2017, rising sea levels exacerbated the damage, according to the Japan Meteorological Agency.
It’s not just the height of the sea level: Because the Kuroshio is a warm current and moves north into cooler waters, its arrival in a new location can have a huge impact on ocean temperatures.
The big question is to what extent the recent phenomenon is part of a natural cycle, and to what extent it is influenced by other factors such as climate change. (And what the sum of the two is.)
Because of its long history, the impact of climate change on the Grand Meander is unknown. But in terms of delaying the transition, there is growing evidence that greenhouse gases and climate change are linked.
When the tropics come to you
To understand recent changes in the Kuroshio is to understand how water moves through the world’s oceans, specifically the large rotating “gyres” found in the world’s major ocean basins.
Five of the basins—the North and South Pacific, the North and South Atlantic, and the Indian basins—had similar patterns. Warm water flows westward along the equator, then turns toward the poles. Ocean currents that carry warm water north or south on their poleward transit are known as the “Western Boundary Currents,” which include the Kuroshio and its four famous cousins: the Gulf Stream, the Brazilian Ocean, the East Australian Ocean, and the Agulhas Ocean.
Scientists are now finding that most ocean currents are changing in a similar way—warming and pushing farther toward the poles.
The change stems from a phenomenon known as the Hadley cell, a band of warm, rising air across the tropics. It is now expanding due to climate change.
“This expansion not only changes rainfall patterns, but also the regions of sinking air that support high-pressure systems, such as the Pacific High,” explained Brown University climate scientist Emanuele Di Lorenzo.
In mid-latitudes, these giant high-pressure systems are the driving force behind ocean currents such as the Kuroshio. So when the wind moves, so does the water flow.
DiLorenzo explained that both models and data suggest that the Kuroshio extension has been moving northward, in part due to atmospheric changes.
A recent study found that between 1993 and 2021, the northern edge of the Kuroshio Extension moved about 130 miles north due to changes in wind direction. This is before a bigger shift occurs in 2023 and 2024.
The crossing of the Kuroshio extension line in 2023 and 2024 is an extreme event. The northern edge reaches almost to the northern tip of Japan’s largest island, Honshu. In a separate study, Sugimoto and colleagues from the Japan Meteorological Agency and the Institute of Meteorology conducted oceanographic measurements at the expanded new site. They found water temperatures 18 degrees Fahrenheit warmer than normal and depths extending to about 400 meters (1,300 feet).
The authors note that for a full year and a half from April 2023 to August 2024, “intense marine heatwave conditions will occur almost every day” in the region.
And not just in the water: The Japan Meteorological Agency also found that extreme ocean conditions contributed to record high temperatures in 2023 over land in northern Japan. Meanwhile, another group of scientists linked offshore warm currents to extreme rainfall in September 2023 in Chiba Prefecture, Japan, near Tokyo.
On August 15, 2025, the first batch of saury of the season was landed at a port in Nemuro, Japan’s northernmost main island of Hokkaido. This species is particularly affected by changes in the Kuroshio Current. – Kyodo News/Getty Images
The future of fisheries is uncertain
These ocean changes have altered the distribution of fish stocks along Japan’s Pacific coast, greatly impacting Japan’s iconic fisheries.
In central Japan, for example, a major mackerel fishery has been damaged, and fishermen say that although the Great Meander is now over, that doesn’t mean things can immediately return to the way they were before.
“Immediate recovery is not a reality, and although the situation may gradually improve, catches have not yet recovered,” said Osamu Nagai, executive director of the Mie Waiwan Fisheries Cooperative.
“The catch has dropped to less than half of what it was 10 years ago, and now we only catch about 20 to 30 percent of the mackerel. This is a major blow,” Nagai said.
The Sanriku coast in northeastern Japan is known for its rich fisheries, and while the situation is different, it’s still bad. Traditionally, the southward-flowing Oyashio Current brought cooler waters and supported abundant fisheries. But as the Kuroshio extension moves north into the area, it replaces the Oyashio tide, causing significant changes in ocean temperatures.
It’s not just fish like Pacific salmon and saury that are becoming harder to catch.
“The most important thing is the flavor made from kelp, which is the basis of Japan’s most important food culture – it can only be harvested in Hokkaido, near Japan,” said Yoshihiro Tachibana, a professor at Mie University who specializes in climate dynamics.
On July 15, 2012, kelp was dried in the sun in Wakkanai, Hokkaido. This seaweed is an important part of Japan’s food culture and economy. But harvesting is becoming increasingly difficult. – Hiroaki Murata/The Yomiuri Shimbun/Associated Press
“Kombu stocks have dropped dramatically. Dashi (Japan’s basic soup stock) culture may be collapsing. It’s declining. We can’t buy anything at all. So I believe this has a huge impact on our food culture as well,” he said.
Recently, the northern edge of the Kuroshio extension has retreated. It has returned to about 37 degrees north latitude, Qiu said. Historically, this is still a high position, but not as extreme as it once was.
But questions remain: What do these extreme ocean events mean, and how are they connected to climate change? For at least one researcher, they’re an early sign of things to come.
“This is a great opportunity to see what the ocean will look like in 100 years,” said Sugimoto of Tohoku University in the northeastern region of Honshu, Japan’s largest island.
“The Northeast is now stumbled upon by unprecedented marine phenomena,” he continued. “Understanding how this changed the ocean in the Northeast provides opportunities to understand how the world’s oceans will change in the future.”
Elizabeth Doty contributed to this report.
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