Why the weirdest sea level changes on Earth are happening off the coast of Japan

The 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)

Bathtubs and pools mislead us about the ocean: Its surface is anything but flat.

Seas 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 — veins of warm water — race toward the poles, causing additional slopes at the surface.

The 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.

It’s the fingerprint of one of those surface currents changing its location, an event that has had dramatic repercussions. The Kuroshio, or “Black Current,” is one of the largest streams of water anywhere in the world, and its recent movement has triggered record-warm ocean temperatures and upended fisheries, an indelible staple of Japanese culture. Scientists say the warm waters have even amplified heatwaves on land and driven extreme rainfall.

And while there are signs some of the changes are now waning, fishing communities say they aren’t yet back to normal. Meanwhile, scientists worry it could be a sign of more volatility to come.

The position of the current could keep fluctuating, said Bo Qiu, a leading Kuroshio expert at the University of Hawaiʻi at Mānoa. “It’s hard to predict the future, but given the data we have so far, I can only see the intensity becoming larger and larger,” he said.

A river in the ocean

The 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’s Boso peninsula, near Tokyo. Here, it becomes known as the Kuroshio Extension as it heads into the open Pacific.

But 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’s coast. Its northern edge shifted as much as 300 miles farther poleward, leading to unprecedented warm waters in the surrounding region.

“I was so surprised I don’t even know if ‘surprised’ is the right word,” said Shusaku Sugimoto, an associate professor at Tohoku University in Sendai, Japan, a northern coastal city.

Sugimoto led a study analyzing ocean temperatures off the coast in locations the Extension didn’t historically reach, but has in recent years. “The fact that the temperature rose 6 degrees (Celsius) off the Sanriku coast, and the elevated temperature persisted for two years, represents a level of water temperature rise we’ve never seen before,” he said.

It’s not the only change.

In August of 2017, the Kuroshio current south of Japan settled into a “large meander” 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.

Large meanders themselves are a well-known recurring feature of the current, explained Shinichiro Kida, an oceanographer at Kyushu University. Records of these events 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’s main island of Honshu. The anchovy were replaced by sardines, which favor the warmer water the current brought to the region.

But no Kuroshio large meander that we know of has lasted as long as this one. In August, the Japan Meteorological Agency finally declared 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.

The meander and the Extension shift are connected, Qiu said. He co-authored a new paper in the Journal of Climate arguing as much, and calling the configuration a “new dynamic regime.”

“I’ve been working on the Kuroshio Extension for more than three decades,” Qiu said. “I never expected this.”

As 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.

But because of these factors, any new movement of the current can have dramatic effects.

The large meander, for example, triggered a sea level fall in one region and pronounced sea level rise — by as much as half a foot — 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.

It 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.

The big question is how much the recent phenomena are part of a natural cycle, and how much it’s influenced by something else, like climate change. (And how much it’s both, combined.)

The 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.

When the tropics travel to you

To understand the recent changes in the Kuroshio is to understand how water moves through the world’s oceans, especially the large rotating “gyres” found in the world’s major ocean basins.

Five of these basins — North and South Pacific, North and South Atlantic, and Indian — 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 “western boundary currents,” and they include the Kuroshio and its four famed cousins: the Gulf Stream, the Brazil, the East Australian, and the Agulhas.

Scientists are now seeing most of these currents changing in a similar way — getting warmer and pushing even farther poleward.

This change is rooted in a phenomenon known as the Hadley Cell, a global zone of warm, rising air across the tropics. It’s now expanding due to climate change.

“This expansion shifts not just rainfall patterns but also the zones of sinking air that anchor high-pressure systems, such as the Pacific High,” explains Brown University climate scientist Emanuele Di Lorenzo.

In 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.

Both models and data suggest the Kuroshio Extension, in particular, has been moving north partly because of the atmospheric shift, Di Lorenzo explained.

Between 1993 and 2021, driven by wind changes, the northern edge of the Kuroshio Extension shifted north by about 130 miles, a recent study found. This was before an even larger shift in 2023 and 2024.

The Kuroshio Extension’s leap in 2023 and 2024 was an extreme event. The northern edge reached nearly to the northern tip of Honshu, Japan’s largest island. In a different study, Sugimoto and colleagues with the Japan Meteorological Agency and the Meteorological Research Institute took oceanographic measurements in the Extension’s new location. They found water as much as 18 degrees Fahrenheit warmer than usual, extending to a depth of around 400 meters, or 1,300 feet.

The authors note that for an entire year and a half between April 2023 and August 2024, the region saw “intense marine heatwave conditions almost every day.”

And it was not just in the water: the Japan Meteorological Agency also found that the extreme ocean conditions contributed to record summer heat over land in northern Japan in 2023. Another group of scientists, meanwhile, linked the warm offshore current to extreme rainfall over Japan’s Chiba Prefecture, near Tokyo, in September of 2023.

An uncertain future for fisheries

These ocean changes have altered the distributions of fish populations along Japan’s Pacific Coast, creating extreme impacts on Japan’s iconic fisheries.

In central Japan, for instance, a major mackerel fishery was upended, and fishers say that even though the large meander has now ended, that doesn’t mean things can instantly go back to the way they were.

“Immediate recovery is not a reality, and while conditions may gradually improve, at present, there has been no recovery in catches,” said Osamu Nagai, executive director of the Mie Gaiwan (Outer Bay) Fisheries Cooperative Association.

“The catch has fallen to less than half of what it was 10 years ago — we’re now only catching about 20 to 30 percent of the mackerel. This is a major blow,” Nagai said.

Northeastern Japan’s 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.

And it is not just fish species, like Pacific salmon and saury, that became harder to catch.

“What matters most is the foundation of Japan’s most important food culture, which is the flavor made out of Kombu seaweed — it can only be harvested in Hokkaido, near Japan,” said Yoshihiro Tachibana, a professor who specializes in climate dynamics at Mie University.

“Kombu stocks are declining dramatically. Dashi (the fundamental Japanese soup stock) culture might collapse. It’s declining. We can’t get any at all. So I believe this has a tremendous impact on our food culture as well,” he said.

Recently, 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’s still a high location historically, but hardly as extreme as before.

But the questions remain: what do these extreme ocean events mean, how they are linked to climate change? For at least one researcher, they’re an early sign of things to come.

“It’s a great opportunity to learn what the oceans will be like 100 years from now,” said Sugimoto, of Tohoku University in the northeastern region of Honshu, Japan’s largest island.

“An unprecedented ocean phenomenon is now occurring by chance in Tohoku,” he continued. “Understanding how this has altered the seas of Tohoku offers a chance to understand how the world’s oceans will change in the future.”

Elisabeth Doty contributed to this report.

By Chris Mooney and Yumi Asada