Warm Eddies Contribute to Polar Ice Melt
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Warm Eddies Contribute to Polar Ice Melt

The melting ice sheets in West Antarctica are a potential major contributor to rising ocean levels worldwide. Although warm water offshore is thought to be the main factor causing the ice to melt, the process by which this water ends up near the cold continent is not well understood. Using robotic ocean gliders, Caltech researchers have now found that swirling ocean eddies, similar to atmospheric storms, play an important role in transporting these warm waters to the West Antarctic. This discovery will help the scientific community determine how rapidly the ice is melting and, as a result, predict how quickly ocean levels will rise.

Their findings were published online on 10 November 2014 in the journal Nature Geoscience. When there is a melting slab of ice, it can either melt from above because the atmosphere is getting warmer or it can melt from below because the ocean is warm, according to lead author Andrew Thompson, assistant professor of environmental science and engineering. All of research evidence points to ocean warming as the most important factor affecting these ice shelves, so they wanted to understand the physics of how the heat gets there.

Ordinarily when oceanographers like Thompson want to investigate such questions, they use ships to lower instruments through the water or they collect ocean temperature data from above with satellites. These techniques are problematic in the Southern Ocean. Because the gliders are small—only about six feet long—and are very energy efficient, they can sample the ocean for much longer periods than large ships can. When the glider surfaces every few hours, it "calls" the researchers via a mobile phone–like device located on the tail. This communication allows the researchers to almost immediately access the information the glider has collected.

Three robotic underwater gliders were deployed to explore waters on the Antarctic continental shelf to see how warm water is reaching the coast. This warmer water is normally found hundreds of metres down throughout the Southern Ocean, but how it reaches the shallow water around Antarctica has not been observed before.

The gliders were remotely controlled from Norwich, more than 10,000 miles away, and they sent data back via satellite mobile phone technology every few hours for two months.

Prof Karen Heywood, from UEA’s Centre for Ocean and Atmospheric Sciences, said that the robots help them to build up a picture of underwater conditions by collecting data on water salinity, temperature, and oxygen levels. The results have identified ocean features that could not feasibly have been studied by any other means. 

The research team found that warm salty water was managing to reach the Antarctic continental shelf by being transported by eddies – swirling underwater storms that are caused by ocean currents. 

In Antarctica the combined effects of temperature and salinity create an interesting situation, in which the warmest water is not on top, but actually sandwiched in the middle layers of the water column. The researchers are taking a look at that very warm temperature layer, which happens to sit in the middle of the water column. That's the layer that is actually moving toward the ice shelf.

Because the gliders could dive and surface every few hours and remain at sea for months, they were able to see the eddies in action—something that ships and satellites had previously been unable to capture.

In future work, Thompson plans to couple meteorological data with the data collected from his gliders. In December, the team will use ocean gliders to study a rough patch of ocean between the southern tip of South America and Antarctica, called the Drake Passage, as a surface robot, called a Waveglider, collects information from the surface of the water. With the Waveglider, the researchers can measure the ocean properties, and atmospheric properties as well, such as wind speed and wind direction. So we'll get to actually see what's happening at the air-sea interface.

In the Drake Passage, deep waters from the Southern Ocean are "ventilated"—or emerge at the surface—a phenomenon specific to this region of the ocean. That makes the location important for understanding the exchange of carbon dioxide between the atmosphere and the ocean.

The work with ocean gliders was published in a paper titled "Eddy transport as a key component of the Antarctic overturning circulation." Other authors on the paper include Karen J. Heywood of the University of East Anglia, Sunke Schmidtko of GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany, and Andrew Stewart, a former postdoctoral scholar at Caltech who is now at UCLA. Thompson's glider work was supported by an award from the National Science Foundation and the UK's Natural Environment Research Council; Stewart was supported by the President's and Director's Fund program at Caltech.

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