The melting rate of certain glaciers in Antarctica is unmistakably accelerating and the most direct link to global warming is in the unknown adjustment of the surrounding Southern Ocean. That leads to not only an uncertain future stability of the icy continent but also raises questions about the pace of sea level rise around the world, according to work by a Texas A&M University researcher.
Alejandro Orsi, associate professor of oceanography, and colleagues from the University of Gothenburg in Sweden, have reported their findings in the current issue of Nature Geosciences magazine. The project was funded by the National Science Foundation.
The team examined the entry to one of three cross-shelf passages in the Amundsen Sea that allow warm oceanic water to reach beneath the ice shelves in front of the Thwaites and Pine Island Glaciers, which show the fastest thinning. They found a year-long persistent inflow of warm bottom water along this trough, which has been dramatically underestimated by numerical models simulations that showed oceanic inputs in response to regional winds changing with the seasons.
“The first reaction was to look for direct atmospheric anomalies in Antarctica, since air temperature would be easiest to be blamed, but we found that is not necessarily the case here,” Orsi explains. “These changes are being driven mainly by the ocean, which in turn interacts with the atmosphere in complex ways.”
Led by Texas A&M graduate student Christina Wiederwohl and senior technician Paul Clark, the first-ever deployment of a fully instrumented mooring on the Swedish icebreaker Oden was done in early 2010. Every half hour for a year, a series of instruments measured ocean currents, temperature and salinity from 600 feet below the sea surface, to avoid damages from passing icebergs, down to the ocean floor itself.
The team discovered that bottom waters steadily flowing southward have temperatures as much as 2.5 degrees warmer than what would be required to melt the base of the ice-shelf found in the region farther downstream.
“There are at least a dozen other known locations around the Antarctic continental margins where relatively warm waters are channeled under ice shelves and glaciers,” he adds.
Oceanographers around the world are trying to figure out the ultimate oceanic response to stronger westerly winds around Antarctica, but specifically how these will contribute to the current rate of melting, “and this is a concern because the mighty westerly winds in this southern polar region have intensified over the past decades,” says Orsi.
“We have also placed moorings in front of the Ross Ice Shelf, the largest in the world, because we know it has been stable for at least 400 years,” Orsi explains, adding that, “our year-long measurements showed a warm inflow lasting for a couple of months, immediately followed by outflow of ‘super-cooled’ waters emanating from underneath the ice shelf.
“What we have confirmed is that the warm oceanic inflows are not due to one or two single episodes, but rather occurring over long periods of time.”
Orsi says the team’s findings raise new questions about continental ice stability in the region.
“What will the rates of Antarctic glacial and ice-shelf melt be in the future, considering that we still don’t know exactly where and how warmer waters come in and contribute to that melting?” he asks.
“Also, what is the ultimate role played by the changing regional winds in both the sea ice and continental ice melt? We know that the average wintertime extent of Antarctic sea ice has increased slowly in recent decades, only about 1 percent per decade, but we also know that there are vast regions such as the Ross Sea with larger and even opposite trends.
“We desperately need adequate data from long-term measurements to begin to understand these ocean-air-ice interactions, or to answer some of these questions, and ultimately make projections to the next century. There is no other place in the world like Antarctica, and it is the most likely area to give us answers to many critical climate-related questions.”