Scientists using robotic ocean gliders to wander frigid Antarctic waters say they may have discovered a mechanism behind the melting of polar ice shelves – miniature submarine "storms" that are lobbing packets of warmer water toward the continent.
The findings, published in the journal Nature Geoscience, shed light on the complicated currents that could potentially be contributing to the loss of West Antarctic ice.
Thawing ice in Antarctica has contributed to the rising ocean levels that are a signature of climate change, and it's thought that warm water reaching the ice shelf has played a key role in melting it. But it's not clear how warm water has managed to get past the continental shelf break to start the process, and an international team of researchers wanted to find out.
"Polar regions are areas where changes are being amplified," said lead author Andrew Thompson, a Caltech physical oceanographer. "They also tend to be regions where we don't have as many observations, because they're very difficult to get to."
Expeditions to the Antarctic can run about $30,000 or more per day, Thompson pointed out – and it costs precious time as well as money.
"One of the challenges of using ship-based oceanography is that it's difficult to stay out for long periods of time," Thompson said. "Life gets in the way, sometimes."
Caltech scientists had a workaround: Deploy underwater robots to explore the Weddell Sea. These drones consume very little energy; rather than using a propeller, they move up and down through the water using a pump that adjusts their internal density relative to the water around them, causing the vehicles to either rise like a balloon or to sink like a rock. A pair of wings converts this vertical movement into horizontal movement – much in the same way that a gliders' wings help it travel forward.
As the roughly 6-foot-long, bullet-shaped glider changes its internal density and shifts its center of mass to help angle its nose upward or downward, it makes a repeated V-pattern through the water. The rovers completed a total of 750 dives, often descending to 1,000 meters below the surface, and took measurements every five seconds (roughly every 0.5 meters). Each time a rover came to the surface, it would place a satellite "call" home, typically around five or six times a day, Thompson said.
Given that one of these calls costs about a dollar a minute, the cost of operating each robot was around $100 per day, Thompson said – a fraction of what it cost to run a human-staffed seaborne experiment. Spaced between 2 to 5 kilometers apart, the gliders covered an area of ocean with a resolution that was roughly 10 times better than traditional ship surveys, the scientists said.
Of course, it also meant that the scientists had to check on the robots a couple of times a night to make sure they were on course and not about to, say, run into a giant iceberg, Thompson said – which was a little like being on baby-time.
"I often compare it to having a small child – it needs your attention every few hours," Thompson said.
For about 10 weeks, the rovers sampled a number of features of the water, including temperature, salinity, oxygen levels and fluorescence. Taking these measurements at different depths was key, because in Antarctica, the warmest water doesn't always end up on top. Instead, it can get stuck between two colder layers, because the combined effects of salinity and temperature affect the density of the water in complex ways.
Scientists often refer to ocean currents driving water around the globe as a "conveyor belt" – which implies that the flows are smooth, Thompson said.
But as it turned out, the flows the robots were measuring were anything but smooth. The scientists discovered that warm water was reaching the ice shelf because of eddies – swirling vortexes of water with heat trapped in their centers.
These eddies, which can range in size from around a kilometer to 10 kilometers, look like miniature, liquid versions of the storm systems in our atmosphere, Thompson added. But, just like the weather, they're the result of complicated processes and thus far, hard to predict.
"This is our first glimpse at just how variable these currents are," Thompson said.
Next up? In a couple of weeks, the team will be heading back to Antarctica, to deploy their gliders in the Drake Passage.