Just this week, a new study has appeared which describes a clever method for measuring the flows of ocean currents and their impacts on ice shelves. This study has identified a major mechanism for melting ice in the Southern Hemisphere.
The paper, co-authored by Andrew Thompson, Karen Heywood, and colleagues is very novel. The scientists used sea gliders to identify water flows that bring warm waters to the base of ice shelves in Antarctica. As I’ve written before, ocean currents are complex; you cannot neglect their impact on the Earth’s climate.
In some parts of the ocean, dense waters near the surface fall to the ocean floor and spread across the globe. In other regions, waters from the deep rise to the surface. Similarly, waters move horizontally and carry their heat with them. In some cases the surface waters and the mid-depth waters flow in different directions.
But regardless of the direction of flow, these waters carry energy with them. This process, often called “advection,” results in a major redistribution of heat across the globe. Sometimes, warm waters flow into cold regions, transferring heat, and melting ice. It is this phenomenon that was at the center of the current paper.
Fluids sometimes move as large directional masses (sometimes called bulk motion) caused by some agent of motion, for instance winds that blow over the surface and drag waters. Other motions are characterized by swirls and eddies – not unidirectional flow. This type of motions is called eddy-induced transport. A determination of which type of transport dominates and where they dominate is important to understanding Antarctic ice melt.
The Caltech and UEA scientists used sea gliders to make their measurements. Sea gliders are devices that move up and down in the ocean waters, taking samples of water properties, while they glide. Sea gliders are a relatively new measurement tool and they provide high-resolution, high-quality data.
Three such gliders were deployed in the Weddell Sea in early 2012. Together, the gliders made 750 dives to depths of 1000 meters or to within 20 meters of the sea floor. They completed their dives every four hours, making measurements of temperature, salinity, oxygen levels, and optical properties every five seconds. The data was then transmitted wirelessly to a data collection station.
The data showed that eddy-transport and surface-wind-caused motion are comparable in their contribution to water circulation. They showed however, that the eddy motion is largely confined to the warm intermediate water layers. The penetration of the warm waters to the ice shelves is believed to be responsible for the dramatic ice loss that has been observed in the Antarctic.
I asked Dr. Heywood about her research and she told me,
It helps us know how the heat gets to the continental shelf – something that is not well understood at present. Imagine that you have a swimming pool with a deep end and a shallow end. There is a lot of warm water in the deep end (this is the Southern Ocean). In the shallow end, there is a wall of ice (this represents Antarctica). How do you get the warm water from the deep end to the shallow end to melt the ice? One way is to stir the water so that little eddies and swirls take the warm water and push it into the shallow water so it contacts the ice. This is the process we were able to observe and quantify for the first time using our glider data at the edge of Antarctica. The next step will be to incorporate this process into climate models so that scientists and policy makers can get a better idea of how much the ice might melt in the future.
I asked her how they came up with this novel sea-glider concept. She responded,
Back in 2007, Andy Thompson was working with us on a research project to understand ocean currents at the tip of the Antarctic Peninsula. In those days, we could only make measurements from ships, stopping every few kilometers. We made these measurements as close together as we could for two weeks. But then we thought that what was really needed was a high-resolution observing tool because the ocean changes over really short distances – sometimes a kilometer or less. So, in 2012, we were able to return with our sea gliders. They are autonomous underwater vehicles that move up and down in the ocean and send their measurements of temperature and saltiness back by satellite. We realized that we could demonstrate the important role that these eddies have played transporting heat towards Antarctica.
In 2013, I was one author among many on a major oceanography paper that extolled the need for instruments like the sea gliders. It is great to see these devices in action – gathering information that was not previously available. Melting from below is a major issue in Antarctica. I have a paper in press with Dr. Ted Scambos that identifies the potential for rapid sea level rise based on Antarctic melting. I hope to share our findings with you in the next few weeks.
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