In recent years, environmental scientists have warily watched as the ice sheet on the coast of West Antarctica has begun melting at unprecedented rates. In May, NASA glaciologists declared that the catastrophic melt of the ice sheet to be “unstoppable.” Researchers have long believed that warm waters were somehow whittling away at the ice sheet, but little was understood about how such warm water made it to one of the coldest regions of the world.
With the help of some underwater robots, researchers at California Institute of Technology have pieced together the most detailed picture to date of what might be happening beneath the surface of the Southern Ocean. The team published their findings online Monday in the journal Nature Geoscience.
Typically, oceanic temperature measurements are gathered via satellite or from temperature buoys towed by ships. Neither are particularly viable methods around Antarctica.
“Observationally, it’s a very hard place to get to with ships,” lead author Andrew Thompson said in a press release. “Also, the warm water is not at the surface, making satellite observations ineffective.”
That’s where robotic ocean gliders come in.
These six-foot-long torpedo shaped drones, which resemble robotic dolphins, use very little energy and can traverse the ocean depths for long periods of time, bobbing to the surface every few hours to “call home” and transmit data to researchers.
In recent years, corporations, research institutions, and even the military have used similar underwater drones to explore the ocean. Fossil fuel companies use autonomous subs to map the ocean floor in search of hidden oil and gas reserves. Researchers at the University of Delaware and Delaware State University have employed remote-controlled, underwater gliders to study elusive sand tiger sharks in the Delaware Bay. And earlier this year, the US Navy sent an American-made underwater drone to aid in the search for the missing Malaysian airliner that is believed to have crashed into the Indian Ocean in March.
Professor Thompson’s gliders spent two months traversing the coast of the Antarctic Peninsula collecting temperature and salinity data beginning in January 2012. While the ultimate goal of the mission was to study thermal transport, salinity levels play a major role in the way waters of differing temperatures flow, the Caltech professor of environmental science and engineering explained.
In fresh water, warm water always rises to the top of the water column, just as heat rises in the atmosphere. However, the presence of salt in the water can alter the stratification of the water column, especially along the coast where fresh melt water mixes with saline ocean water. The melt water coming off the coastal shelf is very cold, just above freezing, but has a much lower salt content than the ocean water so it remains suspended at the surface.
In this region, the warmest waters actually float suspended in a middle layer of the water column. “That’s the layer that is actually moving toward the ice shelf,” Thompson said.
By analyzing data gathered by the gliders, the researchers determined that warm layer was being transported from warmer latitudes by swirling eddies. Eddies behave similarly to atmospheric storms, but whip water instead of winds. They serve a vital function of helping to churn nutrients from the bottom of the ocean up to the surface. In this case, they also deliver warm water from warm latitudes toward the pole.
While ships frequently observe eddies – oceanographers actually name each one just as meteorologists name hurricanes – they typically only glimpse them briefly because the Southern Ocean is treacherous: The weather is fierce and expeditions are extremely costly. However, Caltech’s gliders were able to spend much longer monitoring the eddies at work than a ship typically could.
“Ocean currents are variable, and so if you go just one time, what you measure might not be what the current looks like a day later. It’s sort of like the weather – you know it’s going to be warm in the summer and cold in the winter, but on a day to day basis it could be cold in the summer just because a storm came in,” Thompson said. “Eddies do the same thing in the ocean, so unless you understand how the temperature of currents is changing from day to day – information we can actually collect with the gliders – then you can’t understand what the long-term heat transport is.”
Thompson hopes that future missions that gather both oceanic and atmospheric data will provide clues into the transfer of carbon dioxide between the sea and air.