It’s no secret that big changes are happening in the Arctic Ocean. As temperatures have warmed, over 2 million square kilometers of sea ice have been lost.
A recent study led by researchers at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., has shed new light on the effects of this sea ice loss on the Beaufort Gyre, an important ocean current in the Arctic.
For the past few decades, the Beaufort Gyre has rotated in a clockwise direction, driven by the wind. When ice covers the surface of the ocean, it’s harder for the wind to push the ocean around. But as this protective barrier of sea ice shrinks (and the remaining ice becomes thinner and more mobile), wind is able to put more energy into the rotation of the Beaufort Gyre.
Thomas Armitage, a polar remote sensing specialist at the Jet Propulsion Laboratory and the lead author of the study, said, “the clockwise Beaufort Gyre current system tends to corral and retain fresh water at the surface, making the Beaufort Gyre a major reservoir of fresh water.… Since the 1990s, the Beaufort Gyre has accumulated around 8,000 cubic kilometers of additional fresh water, enough to cover all of California in 60 feet [18 meters] of water.”
Eddies and Dissipating Energy
In the Northern Hemisphere, when ocean currents spin in a clockwise direction, they draw surface waters toward the center of the current. Once the surface level fresh water from ice melt, river runoff, and precipitation reaches the middle of the gyre, it gets forced downward. As more fresh water moves toward the center of the gyre, the interface (called the halocline) between the surface fresh water and the salty water beneath should get deeper.
But something strange is happening in the Beaufort Gyre—fresh water keeps getting forced down, but the halocline isn’t getting much deeper. Some other process must be acting to help dissipate all the fresh water, thus balancing the freshwater budget.
Thanks to the loss of sea ice, winds have been adding extra energy to the Beaufort Gyre. One way that extra energy can be dissipated is by a mechanism called the ice-ocean governor. This means that the ice and the ocean beneath it are moving at different speeds, producing drag and helping dissipate the extra energy added by wind.
But the researchers calculated that since 2007, the energy dissipated by the ice-ocean governor has not been able to keep up with the extra energy added by the wind.
So what was going on?
The answer was eddy activity. Researchers figured out that an increase in eddy activity could account for the discrepancies in both the freshwater budget and the energy budget. In other words, eddies could help release the extra fresh water as well as dissipate energy from the gyre.
Armitage said that this increased eddy activity has important implications for conditions in the Arctic Ocean: “More eddy activity means more mixing of water properties like heat, salt, and nutrients.… The Arctic Ocean has enough warm water at depth to melt the ice pack multiple times, but it is isolated from the surface by cold and fresh—more buoyant—surface waters. Enhanced upwards mixing of this heat could lead to further ice pack melt. Changes in mixing of nutrients [have] potential impacts for biological systems, in terms of the amounts of nutrients available near the surface and at what times of year.”
Mark Johnson, a professor of physical oceanography at the University of Alaska Fairbanks not involved in the study, said that changes in Arctic Ocean currents have the ability to alter climate in other parts of the Northern Hemisphere. For example, he said, cold water off the coast of Greenland sinks and must be replaced by warmer surface water from the south. This convection current brings warm equatorial water north, warming Europe by several degrees.
Changes in Arctic Ocean currents—or in the Beaufort Gyre specifically—could alter the amount of cold, relatively fresh water coming down from the Arctic. This reduction of cool fresh water could disrupt the warm Atlantic current and cool Europe substantially.
Improved Models Needed
Both Armitage and Johnson said that this study also highlights the need for oceanographic models with increased resolution. Many of the current models are not able to resolve features like eddies, and because eddies play an important role in the dynamics of the Beaufort Gyre, higher-resolution models are necessary for accuracy. And having accurate models of the Arctic Ocean—and how it will be affected by climate change—may be very important for future global climate predictions.
The study was published in February in Nature Communications.
—Hannah Thomasy (@HannahThomasy), Freelance Science Writer