For years, scientists have observed that Arctic waters are much calmer in the wintertime when covered in a layer of ice.
In the summer, Arctic eddies whirl throughout the water column. With the return of winter ice, however, Arctic waters go eerily quiet, with no eddies to be found in the first 50 meters beneath the ice.
Now, an MIT team has an explanation for why Arctic waters are so much calmer in the winter. Their paper, published in the Journal of Physical Oceanography, shows that less Arctic ice means more water turbulence in the region.
Turbulent waters ahead
The MIT team showed that the main factors in driving eddy behavior in the Arctic are ice friction and ocean stratification.
By modeling the physics of the ocean, the team found that wintertime ice essentially acts as a frictional brake, slowing surface waters and preventing them from speeding up and turning into turbulent eddies.
This only goes so far though: between 50 and 300 meters deep, the researchers found that the ocean's salty denser layers act to insulate the water from frictional effects. This effect, known as stratification, allows eddies to swirl year-round in the deeper Arctic waters.
"As the Arctic warms up, this dissipation mechanism for eddies, i.e. the presence of ice, will go away, because the ice won’t be there in summer and will be more mobile in the winter," John Marshall, professor of oceanography at MIT, explains in a press release.
"So what we expect to see moving into the future is an Arctic that is much more vigorously unstable, and that has implications for the large-scale dynamics of the Arctic system," he continued.
Predicting the impact of climate change
The MIT team used a "baroclinic instability analysis" to see what might be causing the seasonal change in eddy activity. They used a set of equations describing the physics of the ocean that help determine how instabilities in the ocean, such as eddies, evolve under certain conditions.
When the researchers keyed the frictional effect of sea ice and the effect of stratification into the system, the model produced water velocities that matched the scientists' observations.
“We’re the first to put forward a simple explanation for what we’re seeing, which is that subsurface eddies remain vigorous all year round, and surface eddies, as soon as ice is around, get rubbed out because of frictional effects,” Marshall explains.
Aside from warning of a turbulent future for Arctic waters, the team's findings highlight a new connection between eddy activity, Arctic ice, and ocean stratification. Their work can now be factored into climate models to help us gain a stronger understanding of the impact of climate change on our planet.