They looked to an unusual phenomenon that had been observed from satellite images taken between 1974 and 1976. The images revealed a large ice-free area within the Weddell Sea. Called a polynya, this opening in the sea ice forms when warm water of North Atlantic origin is pushed up toward the Southern Ocean's surface. In a separate process, brine released during the sea-ice formation process produces a reservoir of cold, salty waters at the surface of the Weddell Sea. Because this situation is not stable, the heavy surface waters mix with the warmer, lighter waters underneath in a process called open-sea convection.
Polynyas were not observed again in the Weddell Sea after 1976, leading researchers to believe they - and hence open-sea convection - were rare events.
In the new study, however, the team suggests that polynyas were likely more common in the pre-industrial era, before anthropogenic climate change took hold.
The reason has to do with the fact that climate change has led to more precipitation around the Antarctic continent, both of which lead to greater levels of fresh water at the surface. Fresh water is more buoyant than saltwater and thus doesn't sink through the layers of the ocean as saltier water does, leading to fewer polynyas and less open-sea convection in the Southern Ocean.
"This is important because this process of deep convection that happens in polynyas is a big contribution to the Antarctic Bottom Waters, these deep currents that feed the rest of the ocean," Marinov noted.
Examining 20,000 data points, the researchers showed that the Southern Ocean surface has freshened during the last 60 years. They also found that vertical gradients of salinity and density have increased in the Southern Ocean, suggesting that mixing has been reduced.
Using the latest generation of climate models, 36 finely tuned and complex models that simulate climate change patt
|Contact: Katherine Unger Baillie|
University of Pennsylvania