Climate change felt in deep waters of Antarctica – A surge in freshwater at the surface may have shut down mixing of water layers in the Weddell Sea
By Sarah Zielinski
3 March 2014 (smithsonianmag.com) – In 1974, just a couple years after the launch of the first Landsat satellite, scientists noticed something odd in the Weddell Sea near Antarctica. There was a large ice-free area, called a polynya, in the middle of the ice pack. The polynya, which covered an area as large as New Zealand, reappeared in the winters of 1975 and 1976 but has not been seen since. Scientists interpreted the polynya’s disappearance as a sign that its formation was a naturally rare event. But researchers reporting in Nature Climate Change disagree, saying that the polynya’s appearance used to be far more common and that climate change is now suppressing its formation. What’s more, the polynya’s absence could have implications for the vast conveyor belt of ocean currents that move heat around the globe. Surface seawater around the poles tends to be relatively fresh due to precipitation and the fact that sea ice melts into it, which makes it very cold. As a result, below the surface is a layer of slightly warmer and more saline water not infiltrated by melting ice and precipitation. This higher salinity makes it denser than water at the surface. […] To find out what has been going on in the Weddell Sea, Casimir de Lavergne of McGill University in Montreal and colleagues began by analyzing temperature and salinity measurements collected by ships and robotic floats in this region since 1956—tens of thousands of data points. The researchers could see that the surface layer of water at the site of the Weddell polynya has been getting less salty since the 1950s. Freshwater is less dense than saltwater, and it acts as a lid on the Weddell system, trapping the subsurface warm waters and preventing them from reaching the surface. That in turn, stops the mixing that produces Antarctic Bottom Water at that site. That increase in freshwater is coming from two sources: Climate change has amplified the global water cycle, increasing both evaporation and precipitation. And Antarctic glaciers have been calving and melting at a greater rate. Both of these sources end up contributing more freshwater to the Weddell Sea than what the area experienced in the past, the researchers note. [more]
