Wednesday, November 05, 2025

More polar ocean turbulence due to planetary warming

Institute for Basic Science

image:
Comparison of Arctic Ocean FSLE snapshots from the two simulations during March. Brighter regions (high FSLE) indicate more vigorous horizontal stirring. Left panel: present-day conditions; right panel: future conditions representing a quadrupling of atmospheric CO2. Basemap credit: NASA Blue Marble. Figure credit: YI Gyuseok.
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Credit: Institute for Basic Science

A study published in the journal Nature Climate Change by an international team of scientists, from the IBS Center for Climate Physics (ICCP) at Pusan National University in South Korea, presents new evidence that ocean turbulence and a process known as “horizontal stirring” will increase dramatically in the Arctic and Southern Oceans due to human-induced Global Warming and decreasing sea ice coverage.

“Shaken, not stirred” - it is widely known how James Bond prefers his Martinis. Stirring works by stretching a fluid into thin streaks, which eventually helps to create turbulence, contributing to the mixing of water properties. In the ocean, a similar stirring process of seawater occurs due to the action of winds and other sources of energy. When it happens horizontally over scales of tens to hundreds of kilometers, it is referred to as mesoscale horizontal stirring (MHS). MHS transports and redistributes heat and nutrients, thereby determining the distribution of plankton in the upper ocean. Moreover, the stretching, rotation, and spatial separation of nearby fluid parcels over time also control the dispersal of fish eggs and larvae, but also of pollutants, such as microplastics.

Due to the remoteness of polar regions, it has remained challenging to study the impact of global warming on small-scale ocean currents and marine ecosystems using ship-based observations and satellite data. Instead, climate scientists have relied heavily on climate computer models. However, the current generation of such models lacks the spatial resolution to resolve small-scale ocean processes relevant to MHS and the production of turbulence and horizontal mixing.

To overcome this shortcoming, the South Korean research team analyzed results from ultra-high-resolution simulations conducted with the Community Earth System Model version 1.2.2 (CESM-UHR), on the Aleph supercomputer at the Institute for Basic Science in Daejeon. This fully-coupled model integrates atmosphere, sea ice, and ocean components to realistically represent their interactions within the climate system, using a horizontal resolution of 0.25° for the atmosphere and 0.1° for the ocean. The team focused on simulations under present-day (PD), CO2 doubling (2xCO2), and quadrupling (4xCO2) conditions to investigate how MHS responds to human-induced warming.

To characterize the stretching of fluids into elongated filament-like structures (Fig. 1), the research team employed a technique known as finite-size Lyapunov exponents (FSLE), which tracks how quickly neighboring fluid parcels separate over time due to mesoscale ocean eddies (swirling currents with scales of tens to hundreds of kilometers), meandering flows, and ocean fronts. Using daily data from 10 years of simulation, the computationally demanding FSLE calculations show a pronounced future intensification of MHS across the Arctic Ocean and along the Antarctic coastal region (Fig. 1, 2), which can be attributed primarily to the dramatic decline in sea ice in a warming world. The researchers found that the mechanisms linking sea ice loss to enhanced MHS differed between the two regions.

In the Arctic Ocean, the disappearance of sea ice increases the mechanical energy input into the ocean. Uninterrupted by sea ice, a clockwise wind forcing can strengthen both the mean ocean flow and enhance the generation of upper ocean eddies, ultimately leading to intensified MHS (Fig. 1) and turbulence.

By contrast, in the Antarctic coastal region, the projected future strengthening of MHS around Antarctica arises from near-shore freshening due to sea ice decline, which enhances the north-south density gradient. This, in turn, reinforces the mean ocean currents, such as the Antarctic Slope Current, enhancing eddy activity and MHS (Fig. 2). Given that such intensification of MHS is expected to induce major changes in ocean ecosystems as well as in the dispersal of marine pollutants, further research is urgently needed.

“The contrast between the Arctic Ocean, which is enclosed by surrounding continents, and the Southern Ocean, where the continent is encircled by ocean, creates different physical conditions for ocean stirring. But the outcome for ocean stirring under warming is quite similar,” said lead author YI Gyuseok, a doctoral researcher at the ICCP and Pusan National University.

“Horizontal stirring is a crucial factor for fish larval transport across the ocean. For moderate values, this process connects populations and habitats geographically, increasing their genetic exchange. However, for increasing stirring in the future, larvae can be transported into unsuitable areas where they may not survive,” remarks Prof. LEE June-Yi from the ICCP and co-corresponding author of the study.

Understanding the ecological implications of the author’s main findings requires additional earth system modeling experiments at high spatial resolution, including computer models of plankton and fish.

“Currently, at the IBS Center for Climate Physics in South Korea, we are developing a new generation of earth system models that better integrates the interactions between climate and life. This will deepen our understanding of how polar ecosystems respond to Global Warming”, said Prof. Axel TIMMERMANN, co-author of the study and Director of the ICCP.

Comparison of Arctic Ocean FSLE snapshots from the two simulations during September. Brighter regions (high FSLE) indicate more vigorous horizontal stirring. Left panel: present-day conditions; right panel: future conditions representing a quadrupling of atmospheric CO2. Basemap credit: NASA Blue Marble. Figure credit: YI Gyuseok.
Credit
Institute for Basic Science

Journal

Nature Climate Change

DOI

10.1038/s41558-025-02471-2

Method of Research

Computational simulation/modeling

Subject of Research

Not applicable

Article Title

Future mesoscale horizontal stirring in polar oceans intensified by sea ice decline

Article Publication Date

5-Nov-2025

Fishes, young and old, are shrinking in Michigan's inland lakes

University of Michigan

Lake trout museum specimen — image:
Researchers will also include museum samples, like this lake trout, in their future work studying the impacts of climate change on fishes in Michigan's inland lakes. Image credit: Peter Flood
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Credit: Peter Flood

A new study led by the University of Michigan shows that changes in climate are also changing the size of fishes in Michigan's inland lakes. Using data that covered 75 years and nearly 1,500 lakes, researchers have shown that, for several species, old and young fish in 2020 were significantly smaller than their typical size in 1945.

"Climate change is altering the size of different organisms around the world, including fishes in lakes here in Michigan," said Peter Flood, a postdoctoral research fellow at the U-M School for Environment and Sustainability, or SEAS. "And most of those changes we're seeing in Michigan fishes are declines in size through time."

Flood is also the lead author of a new study published in the journal Global Change Biology and was supported by the U.S. National Science Foundation and Michigan Institute for Data and AI in Society, or MIDAS. The report is the latest to make use of data amassed by a community science project that digitized decades worth of observation cards that characterize fishes over time in 1,497 inland lakes in Michigan.

This allowed Flood and his colleagues not only to track the sizes of 13 different species, but also of different age groups within those species. The team found that, out of 125 species-age class combinations, 58 had changed size. Of those 58 that changed, 46 were smaller.

"The largest decreases in length over time were found in the youngest and oldest fishes," Flood said. "Both of those groups have outsized roles in maintaining healthy fish populations and ecosystem functions and services."

The predators of these fishes are largely gape limited, Flood said, which means they can only eat what fits in their mouths. When younger fishes are smaller, then, they're easier prey, which can limit the population of not only the current generation, but future ones as well. And while older fishes don't have the same impact on population dynamics, they still have important roles in their communities, he said.

"We don't often think about culture with fishes, but they're more social than we realize. They are learning from each other to some extent," Flood said. "And these old individuals, they have this outsized influence on keeping the ecosystems healthy, for a variety of reasons."

Beyond the ecological implications, changes in the size of individual fishes and their populations are important considerations for those who fish and manage Michigan's fisheries. The DNR sets limits on the size and number of fish that anglers can keep to maintain healthy populations. With climate change putting key characteristics in flux, understanding the nature of the changes could help managers stay ahead of the curve.

"Our study is showing that there are differential responses of certain age ranges, so this is a tool in the toolbox managers can use to try to mitigate some of these climate change effects," Flood said.

For those who might be wondering how you determine the age of a fish, the answer is in its scales. As the scales grow, they form ring-like patterns, almost like a tree, that can be similarly analyzed to deduce an age.

The data behind the study were collected and maintained by a collaboration between the university and what would become the Michigan Department of Natural Resources, or DNR. Today that collaboration is known as the Institute for Fisheries Research. But an online platform called Zooniverse invited contributors from around the world to help digitize these records so researchers could more easily reveal trends and insights hidden in the ocean of data.

Unearthing such findings in historical and contemporary data is one of the broader goals of work led by the research team of Karen Alofs, a senior author of the study and an associate professor at SEAS. For example, Alofs and her team have also shown that largemouth bass, which are adapted for warmer temperatures, have become more abundant as Michigan's lakes have warmed. They've also found that mass mortality events for fish in these lakes, which typically happened as ice thawed in the spring, are happening later in the year as lakes experience less ice.

"Each of these changes, whether size, abundance or mortality, has important implications for these ecosystems," Alofs said.

The DNR is still collecting data on the fish, which means researchers like Alofs, Flood and their colleagues can keep an eye on these trends moving forward. But the data also has more dimensions to probe, enabling researchers to ask more questions that could help address new questions, such as how climate change is driving the size changes for different species.

"There's still so much more that can still be done with this data set," Flood said. "There aren't many out there in the world like this one because of the crowdsourced part of it."

The team is also starting to incorporate fish specimens from the U-M Museum of Zoology into their study. The museum's Division of Fishes has about 3.5 million specimens from around the world, letting the scientists look even further back in time and examine more species. That includes more fish native to Michigan that haven't been studied as much because they aren't fisheries targets, Alofs said.

Katelyn King, a fisheries research biologist, and Kevin Wehrly, research station manager, with the DNR also contributed to the study. U-M team members also included undergraduate researcher Katilin Schiller and Andrew Runyon, who worked on the project as an aquatic biology lab manager.

Largemouth bass today aged 0 through 11 years are statistically smaller today than they were 45 years ago, according to new research from the University of MIchigan.

Credit

Andrew Runyon