Oceanographers have an explanation for the mysterious ocean turbulence in the Arctic

Eddies are often thought of as the weather of the ocean. Like large-scale circulations in the atmosphere, eddies swirl through the ocean like slow-moving sea cyclones, sucking in nutrients and heat and transporting them around the world.

In most oceans, eddies are seen at any depth and are stronger at the surface. But since the 1970s, researchers have observed a peculiar pattern in the Arctic: In summer, Arctic eddies resemble their counterparts in other oceans, popping up all over the water column. However, with the return of the winter ice, the Arctic waters become quiet and eddies are nowhere to be found within the first 50 meters below the ice. Meanwhile, deeper layers continue to cause eddies unaffected by the abrupt change in shallower water.

This seasonal turn in Arctic vortex activity has puzzled scientists for decades. Now an MIT team has a statement. In an article published today in the Journal of Physical Oceanography, the researchers show that the key ingredients for controlling Arctic vortex behavior are ice friction and ocean stratification.

By modeling the physics of the ocean, they found that in winter, ice acts as a friction brake, slowing down surface water and preventing it from entering turbulent eddies. This effect is only so deep; between 50 and 300 meters deep, the researchers found, the ocean’s salty, denser layers work to insulate water from frictional effects, allowing eddies to swirl year-round.

The results highlight a new relationship between vortex activity, polar ice and ocean stratification, which can now be incorporated into climate models to make more accurate predictions of Arctic evolution with climate change.

“As the Arctic warms, this vortex dissipation mechanism, ie the presence of ice, will disappear because the ice will not be there in the summer and will be more mobile in the winter,” said John Marshall, professor of oceanography. at MIT. “So what we expect to see moving in the future is an Arctic that is much more powerfully unstable, and that has implications for the large-scale dynamics of the Arctic system.”

Marshall’s co-authors on the paper include lead author Gianluca Meneghello, a research scientist with MIT’s Department of Earth, Atmospheric and Planetary Sciences, along with Camille Lique, Pal Erik Isachsen, Edward Doddridge, Jean-Michel Campin, Healther Regan and Claude Talandier.

Below the surface

For their study, the researchers collected data on the activity of the Arctic Ocean that was made available by the Woods Hole Oceanographic Institution. The data was collected between 2003 and 2018, from sensors that measure the speed of the water at different depths in the water column.

The team averaged the data to produce a time series to produce a typical year of Arctic Ocean speeds with depth. These observations revealed a clear seasonal trend: during the summer months with very little ice cover, they saw high speeds and more vortex activity at all depths of the ocean. In winter, as the ice grew and increased in thickness, shallow waters came to a halt and eddies disappeared, while deeper waters continued to move at high speeds.

“In most of the ocean, these eddies extend all the way to the surface,” says Marshall. “But in the Arctic winter, we see eddies that kind of live below the surface, like submarines hanging at depth, and they don’t come all the way to the surface.”

To see what could be causing this curious seasonal change in vertebral activity, the researchers performed a “baroque clinical instability analysis.” This model uses a series of equations that describe the physics of the ocean and determines how instabilities, such as atmospheric weather systems and ocean eddies, evolve under certain conditions.

An icy friction

The researchers put different conditions in the model and for each condition they introduced small perturbations, similar to ripples from surface winds or a passing boat, at different depths of the ocean. They then pre-run the model to see if the perturbations would evolve into larger, faster vortices.

The researchers found that when they plugged in both the frictional effect of sea ice and the effect of stratification, such as in the different density layers of the Arctic waters, the model produced water speeds that matched what the researchers initially saw in actual observations. That is, they saw that without friction from ice, eddies formed freely at all depths of the ocean. With increasing friction and ice thickness, the water slowed and eddies disappeared in the first 50 meters of the ocean. Below this boundary, where the density of the water, that is, its stratification, changes dramatically, eddies continued to swirl.

When they installed other initial conditions, such as a stratification that was less representative of the true Arctic Ocean, the results of the model were less consistent with the observations.

“We are the first to provide a simple explanation for what we see, namely that underground eddies remain powerful year-round and that once there is ice nearby, eddies are rubbed off by frictional effects,” Marshall explains. .

Now that they have confirmed that ice friction and stratification have an effect on Arctic eddies, the researchers speculate that this relationship will have a major impact on shaping the Arctic in the coming decades. There are other studies showing that the polar ice in summer, which is already receding more rapidly year after year, will disappear completely by the year 2050. With less ice, the water will be free to swirl in eddies, both at the surface and at depth. . Increased swirl activity in the summer could bring in heat from other parts of the world, further warming the Arctic.

At the same time, the Arctic will be covered with ice in winter for the foreseeable future, notes Meneghello. Whether a warming Arctic will result in more ocean turbulence throughout the year or greater variability across the seasons depends on the strength of the sea ice.

Regardless, “if we go to a world where there is no ice at all in the summer and weaker ice in the winter, the vortex activity will increase,” Meneghello says. “That has important implications for things that move in the water, such as tracers and nutrients and heat, and feedback on the ice itself.”