In stark contrast to the rapid warming of the Arctic Ocean, the waters surrounding Antarctica have remained at roughly the same temperature over the past few decades, and now a team of researchers from the University of Washington believe they have found out why.
Using a combination of observations and climate models, Kyle Armour, an assistant professor of oceanography and atmospheric sciences at the university learned that the Southern Ocean has a unique current that continually forces older water to the surface.
This seawater can be up to several centuries old, they explained Monday in the journal Nature Geosciences, meaning that the last time that it was exposed to the Earth’s atmosphere was prior to the machine age and the beginning of fossil fuel-related climate change.
“With rising carbon dioxide you would expect more warming at both poles, but we only see it at one of the poles, so something else must be going on,” Armour explained in a press release. “We show that it’s for really simple reasons, and ocean currents are the hero here.”
“The Southern Ocean is unique because it’s bringing water up from several thousand meters,” or as much as two miles below surface level, he added. “It’s really deep, old water that’s coming up to the surface, all around the continent. You have a lot of water coming to the surface, and that water hasn’t seen the atmosphere for hundreds of years.”
Some of the water hasn’t been seen in hundreds of years. Credit: Thinkstock
Ocean currents responsible for shaping regional climate change
Known as meridional overturning circulation, this phenomenon is the result of gale-force winds travelling in a westerly direction causing surface water to be pushed northward, and unmodified deep water to be drawn up from below. The subducted water takes stored heat with it as it travels north, and it is replaced by water that will take centuries to fully experience warming.
“These findings,” Armour and colleagues from UW and the Massachusetts Institute of Technology (MIT) wrote, “suggest the Southern Ocean responds to greenhouse gas forcing on the centennial, or longer, timescale over which the deep ocean waters that are upwelled to the surface are warmed themselves. It is against this background of gradual warming that multidecadal Southern Ocean temperature trends must be understood.”
“The oceans are acting to enhance warming in the Arctic while damping warming around Antarctica. You can’t directly compare warming at the poles, because it’s occurring on top of very different ocean circulations,” he added. “When we hear the term ‘global warming,’ we think of warming everywhere at the same rate. We are moving away from this idea of global warming and more toward the idea of regional patterns of warming, which are strongly shaped by ocean currents.”
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Image credit: Kyle Armour/University of Washington
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