Melting and Freshening in the Amundsen and Ross Seas
Large changes have appeared during recent decades in the salinity of shelf waters on the Ross Sea continental shelf. Substantial interannual variability was initially identified in 1985 near Ross Island, but resembled early measurements of the annual salinity cycle in McMurdo Sound. Since then, a significant negative trend has emerged, and was shown in 1998 to extend throughout the deep continental shelf region. Reasons for the salinity drift were unclear, but possible mechanisms included increased air temperature, reduced sea ice formation, increased current velocity and increased melting of glacial ice. Some of those premises were suspect, since regional air temperatures have changed little over the last three decades, during which time sea ice extent has increased in the Ross Sea. Given such anomalies, we widened the study area, finding a stronger salinity decline in the much larger Ross Gyre north of the continental shelf. This indicates that much of the observed signal on the continental shelf has an external origin, and results from the import of relatively fresher waters. One implication is that its origins could also include an increase in precipitation as part of a stronger hydrological cycle, perhaps connected with a decadal-scale shift in the El-Nino Southern Oscillation pattern. In addition, the extensive salinity decrease suggested changes upstream of the Ross Sea, such as less sea ice production and accelerated glacial ice melting in the Amundsen and Bellingshausen Seas. The freshening appears too large to be accounted for by a single process, but is consistent with geochemical tracer measurements, ocean warming near the deep water temperature maximum, and thinning in grounded and floating parts of the West Antarctic Ice Sheet.