El Niño Helped Slow Global Warming: Pine Island Glacier In The Antarctic Melted Half As Slowly In 2012 As It Did In 2010
The rate at which Antarctic glacial ice melts is "highly dependent" on local wind direction associated with El Niño — the prolonged warming of the Pacific Ocean surface — geologists reported Thursday in a paper published in the advance online version of the journal Science. The study, led by geologist Pierre Dutrieux at the British Antarctic Survey, demonstrates how winds influence how much warm water reaches the ice shelf. "The new observations show that the amount of warm Circumpolar Deep
Water reaching the Antarctic ice sheet -- and how much melting occurs as a consequence -- can be tied quite strongly to local wind conditions," co-author Eric Steig, a University of Washington professor of earth and space sciences told the International Science Times. "Those conditions, in turn, depend on what is happening elsewhere, in this case, far away in the tropical Pacific." Those climactic conditions include La Nina, a prolonged cooling of portions of the surface of the Pacific Ocean, which in turn hinge on the El Niño event, which warms Pacific surfaces.
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The research was based on data collected on the Pine Island Glacier, which is a major thoroughfare for ice flowing from Antarctica into the sea. The floating ice shelf at the glacier's tip has been melting for the past 40 years. But British Antarctic Survey researchers and colleagues from Germany and Korea revisiting the area in January of 2012 found the layer of warm water under the glacier was much thinner than it had been two years earlier. It was topped by an unusually thick layer of cold water that surrounded and protected the ice shelf from further melting.
In fact, about half as much melt-water was being produced from the glacier compared to 2010, making 2012 the year with the lowest summer melting of the Pine Island Glacier on record. This was uncharacteristically good news. In the past 20 years, there has always been a thick layer of warm water around the ice shelf. This indicated to scientists that warming global oceans were gradually melting the underside of the ice sheet. But the trend, at least for now, has reversed.
Measurements of water temperature and ocean circulation indicated that the reduced melting in 2012 was because cooler, deep water was able to flow across an underwater ridge separating Pine Island Glacier from the Southern Ocean, helped along with persistently easterly winds for most of the preceding year. Winds in this region are normally westerly. Steig was co-author of a 2011 study in Nature Geoscience, which showed that winds in the Pine Island Glacier area are related to changes in the tropical Pacific tied to El Niño events. Though Steig had thought that wind variation played a small role, the new data shows that it has a strong effect, Steig said.
Specifically, the change is due to a shift that occurred in 2012, when the El Niño tropical system switched to a La Niña - a prolonged cooling of the surface temperature of the Pacific Ocean, which also changed the wind directions. "The wind field in late 2011 and early 2012 had changed dramatically compared to previous years - the dominant westerly winds in the surrounding area were easterly almost all through late 2011 and early 2012, and those changes were related to the very large 2011 La Niña event," said Steig.
If that trend were to continue, the authors write, it would have profound implications for the Pine Island ice shelf, reversing the current thinning trend, potentially allowing the glacier edge to rebuild. But that's unlikely, Steig said. "The 2011 to 2012 La Nina was a rather exceptional La Nina. It was very strong, and very centered in the part of the Pacific that is particularly important for affecting Antarctica. I am not saying 'we will never see an event like this again,' but I don't think it reflects the average La Nina conditions, and certainly not the long term average. Hence a return visit to the Pine Island glacier area won't likely see the same conditions, next time around."
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