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Previously the Saltiest Sea in the Southern Ocean
New Research Shows That Ross Sea Waters Are Freshening
Columbia University scientists are reporting in the journal Science that salinity levels in the Ross Sea have experienced a large decline in recent decades. Once the saltiest body of water in the Antarctic, this distinction now goes to its distant neighbor, the Weddell Sea.
Stan Jacobs, an oceanographer with Columbia's Lamont-Doherty Earth Observatory led the study, which documents a 40-year record of salinity in the Ross Sea. The freshening was initially observed on the Ross Sea continental shelf and then found to extend north and east over the much larger Ross Gyre.
"We have been surprised by both the magnitude and duration of the freshening signal, and its apparent causes. Had anyone predicted such large changes would occur after we first sampled the Ross Sea in 1967, I might have considered it a case of model drift, or wild speculation," said Jacobs.
Jacobs and his co-authors Giulivi and Mele attribute the freshening to a combination of factors including reduced sea ice production, increased precipitation and accelerated melting of the West Antarctic Ice Sheet. Less sea ice formation results in less brine drainage (salt saturated water) into the seawater below. Oxygen isotope measurements, which can be used to identify the source of freshwater in a seawater sample, also show the presence of more glacial meltwater in 1994 and 2000 than in 1977-79.
Their research also indicates that fresher and/or more surface water than the saltier deep water has recently been flowing onto the Ross Sea continental shelf. This less-saline water could have resulted in part from increased precipitation related to an altered El Nino-Southern Oscillation pattern in the atmosphere.
The salinity decrease in the Ross Sea is larger than the 'Great Salinity Anomaly' of the North Atlantic Ocean, but occurs mostly at shallower levels. Nonetheless, there are indications that the Ross Sea salinity decline has influenced the properties of bottom waters south of Australia.
Along with colleagues at the Lamont-Doherty Earth Observatory and other institutions, Jacobs will begin a year-long NSF-sponsored experiment in the northwest Ross Sea later this year. One of their objectives is to better understand the annual transport of mass, heat, salt and freshwater on and off the Ross Sea continental shelf. Also under consideration by NSF is a proposal by Lamont investigators to study the current mass balance of Antarctic Ice Shelves, which have recently experienced several major iceberg calving events.
Stan Jacobs is a Senior Research Scientist with the Lamont-Doherty Earth Observatory, a research unit of the Earth Institute of Columbia University. The work reported in Science was supported by NSF, NASA and NOAA, with oxygen isotope measurements by Rick Mortlock in the Lamont stable isotope laboratory.
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