Posted on 7 December 2021 by Guest Author

This is a re-post from Yale Climate Connections by Kristen Pope

Ocean eddies have an important role in moving and mixing the ocean’s waters, including things like nutrients, carbon, salt, and heat, and therefore an important role on climate change.

By moving heat within ocean waters – including transporting heat between different areas and vertically within the ocean – eddies, often referred to as “the weather of the ocean,” help regulate the climate. They vary in size, with some just a handful of miles across, others far larger, and NOAA estimates they account for over half of the ocean’s kinetic energy.

“[Eddies are] pretty much everywhere in the ocean,” says Josué Martínez-Moreno, oceanographer and PhD candidate at Australian National University, in Canberra. “They’re really important because they can transport energy, transport heat, and transport carbon across the ocean. A really good example for that is the Southern Ocean, where it is believed that eddies do play a crucial role in how the ocean captures heat and carbon.”

Important significance for heat and carbon exchange

Martínez-Moreno and colleagues published a 2021 article in Nature Climate Change examining how ocean eddy energy is changing, and pointing to “important implications for the exchange of heat and carbon between the ocean and atmosphere.”

Using altimetry data from satellites dating back to 1993, the researchers found “a global statistically significant increase of ocean eddy activity,” with variations among regions. “Eddy-rich regions showing a significant increase in mesoscale variability of 2-5% per decade,” they wrote, while finding that other areas, especially in tropical regions, showed a decrease in this variability. “The trend that we’re observing in tropical oceans is basically a decrease in eddy activity due to more homogenous temperatures in the ocean surface,” Martínez-Moreno says.

Martínez-Moreno says the researchers are working to learn more about these findings and about impacts on the climate. “We don’t really know the impacts yet,” he says. “But it is important to know that this is changing and try and capture it in future climate models to have better predictions of how our oceans and how the Earth system is evolving.”

Vast and with parts remote, oceans can be challenging to study, but learning about them and their energy is vital to better understanding climate change. Gaining knowledge will also help scientists create more accurate models for planning for the future.

Complicating that better understanding now is that many modeling scales used do not capture sufficient detail to include most eddies, which can be just a few miles wide. Martínez-Moreno says modeling scales generally use grids capturing 1°, around 68 miles wide, and as a result can include only the very largest eddies. Seeking to overcome those computational limitations, he says, many oceanographers are working to transition to using more detailed modeling scales – such as ¼° or 1/10° grids, which are around 18 and 7 miles wide, respectively.

Looking to the western boundary currents

Tal Ezer, professor of ocean and earth sciences at Old Dominion University in Norfolk, Va., studies sea-level rise and coastal changes and how they relate to ocean circulation and currents. In a 2021 paper in Climate Dynamics, Ezer and co-author Sönke Dangendorf used global sea-level reconstruction from 1900 to 2015 to study patterns in ocean kinetic energy, comparing these observations to wind patterns and wind kinetic energy. They studied five western boundary currents – the western parts of circular currents around some of the world’s major oceans – including the Gulf Stream and Kuroshio, Brazil, Agulhas, and East Australian currents. Those are located in the North Atlantic, North Pacific, South Atlantic, Southern Indian Ocean, and South Pacific Ocean, respectively.

They found “a coherent upward trend in [ocean kinetic energy] “(+ 24% ± 3 increase per century)” near western boundary currents. However, wind kinetic energy was more variable. Over the Gulf Stream, they found an 11% decrease, while other areas, such as the Brazil Current, had a significant uptake, with a 28% increase there.

Challenges in predicting local sea-level rise

“Global warming is not evenly distributed, so you have ocean waters getting warmer, [but] they’re not getting warmer at same rate everywhere, so this creates large differences from one place to another, and the result is ocean currents are changing and sea level is very uneven in recent years,” Ezer says. “So it’s much more difficult to predict local sea-level rise because we see much larger differences from one place to another.”

He points out that these changes affect western boundary currents. “Uneven warming of the ocean,” he says, “increases the variations and increases the surface ocean currents of all these strong western boundary currents, so we see these currents getting more variable and stronger” because of the uneven ocean warming, Ezer says.

These western boundary currents, like the Gulf Stream, have a vital role in mixing the world’s waters by transporting warm water from lower to higher latitudes. “The Atlantic Ocean would be completely different in the climate and weather – everything would be completely different without the Gulf Stream,” Ezer says.

He and his research colleagues found kinetic energy increases in the western boundary currents were similar to one another despite the varied conditions. “I was quite surprised that all these western boundary currents’ increasing kinetic energy was very similar to each other despite the different conditions,” Ezer says.

The authors cite a few different factors that might impact ocean kinetic energy, including wind stress curl increases in subtropical areas, wind pattern changes in some areas, and uneven warming near the western boundary currents.

While they work to learn more about these changes, Ezer underscores the need to gain deeper knowledge about the ocean, and its important role in climate change. “The ocean plays a very important role in climate change because much of the heat in the atmosphere is absorbed by the ocean,” Ezer says, adding that warmer seas likely will also lead to more intense hurricanes, more ice melt, and more sea-level rise.

Keeping an eye on ocean patterns and changes, especially with the western boundary currents, is vital, according to Ezer. “Any changes in these huge currents [are] very critical to predicting the future of climate change and the effect on extreme weather and other components,” Ezer says.