Climate Change Affects Even The Deepest Waters Of The Ocean
At one point, ocean currents or the temperatures of surface water would create areas of open water in the middle of large ice packs. Now, a new study has revealed that the occurrence of these open bodies of water, called polynyas, which most frequently appeared in the Antarctic, has reduced due to climate change.
In the mid-1970s, satellite images of Antarctica showed a huge polynya the size of New Zealand within the ice pack of the Weddell Sea. It stayed open for multiple winters, caused by warm waters from the depths of the ocean traveling all the way up to the surface and melting the surrounding ice. The polynya in the Weddell Sea, however, has not reappeared in nearly 40 years, and scientists have now labeled it a rare natural occurrence.
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Researchers from McGill University believe that they may not have appeared for so long because of the effects of climate change on ocean salinity. For their study, published in Nature Climate Change, they studied ocean temperatures and salinity around Antarctica collected by ships and robotic floats over the past 60 years.
Their study, published in Nature Climate Change, shows that the ocean's surface has been gradually losing its salinity since the 1950s creating a lid of fresh water on the surface of the ocean. This top layer of fresh water does not mix with the warm waters underneath. As a result, the deep ocean heat has been unable to reach the top and melt the ice. "Deep ocean waters only mix directly to the surface in a few small regions of the global ocean, so this has effectively shut one of the main conduits for deep ocean heat to escape," said lead author Casimir de Lavergne, a recent graduate of McGill's Master's program in Atmospheric and Oceanic Sciences, in a press release.
The scientists also studied several state-of-the-art climate models, and predicted an increase in the amount of precipitation over the Southern Ocean, as atmospheric carbon dioxide increases. "This agrees with the observations, and fits with a well-accepted principle that a warming planet will see dryer regions become dryer and wetter regions become wetter," said co-author of the study Professor Jaime Palter in the press release.
"True to form, the polar Southern Ocean - as a wet place - has indeed become wetter. And in response to the surface ocean freshening, the polynyas simulated by the models also disappeared." Although the models didn't account for them, melting glaciers also contribute to more freshwater in the ocean.
Their research may also explain why Antarctic Bottom Water, the water mass that surrounds Antarctica and fills the deepest layer of the world ocean, has been shrinking over the last few decades.
"The waters exposed in the Weddell polynya became very cold, making them very dense, so that they sunk down to become Antarctic Bottom Water that spread throughout the global ocean. This source of dense water was equal to at least twice the flow of all the rivers of the world combined, but with the surface capped by freshwater, it has been cut off," Eric Galbraith, a fellow of the Canadian Institute for Advanced Research, said in the press release. The likeliness that a "giant" polynya will appear in the next century is small, Galbraith said, but if it does, then it "will release decade's worth of heat and carbon from the deep ocean to the atmosphere in a pulse of warming."
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