Tagged seals help scientists understand Antarctic currents

Tagged seals help scientists understand Antarctic currents
Elephant seals are tagged as part of a French research program called SO-MEMO (Observing System--Mammals as Samplers of the Ocean Environment), operated by the French National Center for Scientific Research (CNRS). (Jet Propulsion Laboratory)

Jet Propulsion Laboratory scientists used a seal tagged with specialized sensors to understand the current and climatic conditions surrounding the Antarctic Circumpolar Current, which flows in a loop around Antarctica and connects the Atlantic, Pacific and Indian oceans. The ocean current is considered a key heat exchanger among the oceans it links, but has been difficult for scientists to study because of its turbulence and length.

Lia Siegelman, a visiting scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, found that the rough seas posed no challenge for her scientific sidekick: a tagged southern elephant seal. Equipped with a specialized sensor reminiscent of a small hat, the seal swam more than 3,000 miles (4,800 kilometers) on a three-month voyage, much of it through the turbulent, eddy-rich waters of the Antarctic Circumpolar Current.

The seal made around 80 dives at depths ranging from 550 to 1,090 yards (500 to 1,000 meters) per day during this time. all the while collecting a continuous stream of data that has provided new insight into how heat moves vertically between ocean layers in this volatile region.

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For a paper published recently in Nature Geoscience, Siegelman and her co-authors combined the seal's data with satellite altimetry data. The satellite data of the ocean surface showed where the swirling eddies were within the current and which eddies the seal was swimming through. Analyzing the combined dataset, the scientists paid particular attention to the role smaller ocean features played in vertical heat transport. Siegelman was surprised by the results.

"These medium-sized eddies are known to drive the production of small-scale fronts—sudden changes in water density similar to cold and warm fronts in the atmosphere," she said. "We found that these fronts were evident some 500 meters [550 yards] into the ocean interior, not just in the surface layer like many studies suggest, and that they played an active role in vertical heat transport." 

Siegelman added their analysis showed that these fronts act like ducts that carry a lot of heat from the ocean interior back to the surface. "Most current modeling studies indicate that the heat would move from the surface to the ocean interior in these cases, but with the new observational data provided by the seal, we found that that's not the case," she said.

The ocean surface layer can absorb only a finite amount of heat before natural processes, like evaporation and precipitation, cool it down. When deep ocean fronts send heat to the surface, that heat warms the surface layer and pushes it closer to its heat threshold. Thus, in the areas where this dynamic is present the ocean isn't able to absorb as much heat from the Sun as it otherwise could.

The scientists consider this significant, because current climate models used to estimate Earth's heat transfer capability don't factor in the effects of these small-scale ocean fronts.

"Inaccurate representation of these small-scale fronts could considerably underestimate the amount of heat transferred from the ocean interior back to the surface and, as a consequence, potentially overestimate the amount of heat the ocean can absorb," Siegelman said. "This could be an important implication for our climate and the ocean's role in offsetting the effects of global warming by absorbing most of the heat."

The scientists added this phenomenon is also likely present in other turbulent areas of the ocean where eddies are common, including the Gulf Stream in the Atlantic Ocean and the Kuroshio Extension in the North Pacific Ocean.

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