Off the coast of Japan, sea levels are rising … and falling. No one knows why

We often think of the ocean as having a flat surface, but that's far from reality. The ocean's surface is uneven, influenced by factors such as trade winds and the gravitational pull of massive ice sheets, which cause water to pile up in certain areas. Fast-moving surface currents, like warm water veins heading toward the poles, also contribute to these variations.

This unevenness is not static; it's constantly changing. Recent maps reveal a complex pattern of peaks and valleys in the ocean's surface. One area of particular interest is off the coast of Japan, where a section of the ocean is rising by almost an inch per year, while an adjacent area is falling at an even faster rate. The reason? A shift in the location of a major surface current.

The Kuroshio Current, also known as the "Black Current," is one of the world's largest water streams. Its recent movements have had significant consequences, including record-breaking ocean temperatures and disruptions to Japan's vital fisheries. According to scientists, the warmer waters have even intensified heatwaves on land and led to extreme rainfall.

While there are indications that some of these changes are subsiding, fishing communities report that conditions have not yet returned to normal. This has scientists concerned that it could signal increased instability in the future.

Bo Qiu, a leading expert on the Kuroshio at the University of Hawaiʻi at Mānoa, suggests that the current's position could continue to fluctuate. He notes that predicting the future is difficult, but based on current data, the intensity of these fluctuations could increase.

A River in the Ocean

The Kuroshio Current is a deep, warm current that transports more than 200 times the water volume of the Amazon River. It flows north from the equator and typically turns eastward around Japan's Boso Peninsula, near Tokyo. As it enters the open Pacific, it becomes known as the Kuroshio Extension.

That said, the reality is a bit more complicated. in recent years, the current's behavior has been unusual. The Kuroshio Extension, in particular, has experienced a significant divergence along the Japanese coast. Its northern edge has shifted as far as 300 miles poleward, leading to unprecedented warm water temperatures in the surrounding region.

Shusaku Sugimoto, an associate professor at Tohoku University in Sendai, Japan, was taken by surprise. His study analyzed ocean temperatures off the coast in areas the Extension historically didn't reach but has in recent years. The temperature rose 6 degrees Celsius off the Sanriku coast, and this elevated temperature persisted for two years, marking an unprecedented level of water temperature rise.

Meanders and Shifts

Another change occurred in August 2017 when the Kuroshio Current south of Japan entered a "large meander" pattern, moving away from the coastline and looping southward, taking its warm waters with it. This shift in water temperatures altered the distribution of fish species offshore.

Shinichiro Kida, an oceanographer at Kyushu University, explains that large meanders are a recurring feature of the current, with records dating back to the 1960s. During a long meander event from 1975 to 1980, anchovy populations in the Enshunada Sea, a major fishing area south of Honshu, declined sharply and were replaced by sardines, which prefer warmer waters.

The Japan Meteorological Agency declared the end of the most recent large meander after nearly eight years in August. Qiu and colleagues argue that the meander and the Extension shift are connected, describing the configuration as a "new dynamic regime" in a paper published in the Journal of Climate.

As the Kuroshio flows, it carries warmer water and influences sea levels. The current's warmth and speed can create a significant difference in ocean height on either side. Therefore, any movement of the current can have dramatic effects.

The large meander, for instance, caused a sea level drop in one area and a rise of up to half a foot along the coast of Honshu, south of Tokyo. The Japan Meteorological Agency reported that this higher sea level amplified the damage when Typhoon Lan struck the Shizuoka Prefecture in October 2017.

The Kuroshio's impact extends beyond sea height; as a warm current moving into cooler waters, its arrival in new areas significantly affects ocean temperatures.

Climate Change Connection

The key question is determining the extent to which these recent phenomena are part of a natural cycle versus being influenced by climate change, or a combination of both.

While the impact of climate change on the large meander is unclear due to its long history, there's growing evidence implicating greenhouse gases and a changing climate in the Extension shift.

Understanding the Kuroshio's recent changes requires understanding how water moves through the world's oceans, especially the large rotating "gyres" in major ocean basins.

Five basins—North and South Pacific, North and South Atlantic, and Indian—share a similar pattern: warm water flows west along the equator before turning toward the poles. These currents, which carry warm water north or south, are known as "western boundary currents" and include the Kuroshio, the Gulf Stream, the Brazil Current, the East Australian Current, and the Agulhas Current.

Scientists are observing similar changes in most of these currents, including warming temperatures and a push farther poleward.

This change is linked to the Hadley Cell, a global zone of warm, rising air across the tropics that is expanding due to climate change.

Emanuele Di Lorenzo, a climate scientist at Brown University, explains that this expansion affects rainfall patterns and the zones of sinking air that anchor high-pressure systems, such as the Pacific High.

These high-pressure systems drive currents like the Kuroshio in the middle latitudes. Consequently, when the winds shift, so do the currents.

Models and data suggest that the Kuroshio Extension has been moving north partly due to this atmospheric shift, according to Di Lorenzo.

A recent study found that between 1993 and 2021, wind changes caused the northern edge of the Kuroshio Extension to shift north by approximately 130 miles. This occurred before an even larger shift in 2023 and 2024.

The Kuroshio Extension's leap in 2023 and 2024 was an extreme event, with its northern edge reaching near the northern tip of Honshu, Japan's largest island. Sugimoto and colleagues from the Japan Meteorological Agency and the Meteorological Research Institute measured oceanographic conditions in the Extension's new location and found water up to 18 degrees Fahrenheit warmer than usual, extending to a depth of about 1,300 feet.

They noted that the region experienced "intense marine heatwave conditions almost every day" for a year and a half between April 2023 and August 2024.

The Japan Meteorological Agency also found that these extreme ocean conditions contributed to record summer heat over land in northern Japan in 2023. Other scientists linked the warm offshore current to extreme rainfall over Japan's Chiba Prefecture, near Tokyo, in September 2023.

Impact on Fisheries

These ocean changes have altered fish populations along Japan's Pacific Coast, significantly impacting Japanese fisheries.

For example, a major mackerel fishery in central Japan was disrupted. Fishermen report that even with the end of the large meander, conditions haven't returned to normal.

Osamu Nagai, executive director of the Mie Gaiwan (Outer Bay) Fisheries Cooperative Association, stated that immediate recovery is not possible. Catches have fallen to less than half of what they were 10 years ago, with only about 20 to 30 percent of the mackerel being caught, which is a major setback.

The Sanriku coast in northeastern Japan, known for its rich fisheries, faces a different but equally challenging situation. The southward-flowing Oyashio Current traditionally brings cool waters that support abundant fisheries. That said, the reality is a bit more complicated. the northward movement of the Kuroshio Extension displaced the Oyashio, causing a significant change in ocean temperature.

The impact extends beyond fish species like Pacific salmon and saury.

Yoshihiro Tachibana, a professor specializing in climate dynamics at Mie University, emphasizes the impact on Kombu seaweed, a foundation of Japanese food culture harvested only in Hokkaido. Kombu stocks are declining dramatically, threatening dashi, the fundamental Japanese soup stock. He believes this has a tremendous impact on Japanese food culture.

The northern edge of the Kuroshio Extension has recently retreated slightly to around 37 degrees North Latitude, according to Qiu. While still historically high, it's less extreme than before.

That said, the reality is a bit more complicated. questions remain about the meaning of these extreme ocean events and their connection to climate change. Sugimoto sees them as an early indication of what's to come.

He believes that the unprecedented ocean phenomenon occurring in Tohoku offers a chance to understand how the world's oceans will change in the future.