When it comes to the ocean, deeper usually means colder. But there are exceptions. In the Arctic Ocean, for example, a warm layer of water is trapped 150 feet under the ocean’s chunky ice floes by a ceiling of cold fresh water near the surface. The cold layer insulates the sea ice from the warmer waters below.
In a 2018 study, scientists have found that the amount of heat in the trapped warm layer in the Beaufort Gyre, a major Arctic Ocean circulation system north of Alaska, has doubled over the past 30 years. And, if the temperatures continue to spike, it could eventually spell trouble for the ice above.
“At some point, the heat from this layer is going to have to come up to the surface, and it’s going to impact the ice,” said John Toole, a scientist at Woods Hole Oceanographic Institution (WHOI) and co-author of the study. “It’s a ticking time bomb.”
The study, by Mary-Louise Timmermans of Yale University, Toole, and Richard Krishfield of WHOI, was published in Science Advances.
The warm water layer forms in the summer, when warmer air temperatures cause ice in the adjoining North Chukchi Sea to melt, exposing the seawater to sunlight. The sun’s rays heat up the water, and then winds sweep the warm meltwater toward the ice-covered waters in the Arctic Ocean’s interior.
Warmer water typically floats since it’s lighter, but the higher salt content of this water makes it slightly denser than the more buoyant fresher ice-covered waters. The warmer, saltier waters sink and get trapped below the colder surface layer.
Toole and colleagues from several other research institutions have been using instruments developed at WHOI called Ice-Tethered Profilers (ITPs) to measure seawater temperatures and salt content in the region since 2004. It starts with a large, bright-yellow surface buoy that sits atop ice floes like construction barrels on a snow-covered highway. Hanging from each buoy is a weighted cable that dangles through an 11-inch-diameter hole in the ice and into the ocean. A motorized sensor package travels continuously up and down the tether through various layers of the ocean in yoyo-like fashion, taking continuous measurements of temperature, salinity, and other seawater properties along the way.
The ITPs, together with ship based observations, gave the scientists the ability to quantify changes over the past 30 years and revealed the unexpectedly large and rapid warming in the ocean. “Due to the northward retreat of the summer sea ice edge in recent years, the ocean has been soaking up a great deal of solar energy during the summers,” said Toole, “leading to this surprising increase in heat content of the warm layer.”
The study is yet another case in point of why melting Arctic sea ice is a problem: When ice melts, it can no longer play the important role of reflecting sunlight. Instead, the melting gives the sun unfettered access to the open water, allowing the ocean to absorb more heat.
As that heat diffuses upward over time, it stalls the growth of the sea ice during the winter in the region, said Toole. “And, if the deeper, warmer water starts mixing more vigorously with the cold water above it, we will see even more sea ice thinning during the summer.”
This research is funded by the National Science Foundation’s Arctic Observing Network program with additional contributions from the Office of Naval Research.