This could be yet another casualty of global warming.
A huge ice-free region--the size of New Zealand--that existed within the ice blanket of the southern ocean surrounding Antarctica has disappeared from the region recently.
A "freshwater lid" has probably caused this ice-free region, or polynya, to close, according to the researchers who published their findings in a paper titled "Cessation of deep convection in the open Southern Ocean under anthropogenic climate change" in Nature Climate Change.
The polynya was first noticed in satellite images of Antarctica taken during the polar winter season in 1974.
The opening was the result of mixing of relatively warm deep-ocean waters with surface-ocean waters that "opened a conduit between the surface and the abyssal ocean, and had maintained the polynya through the massive release of heat from the deep sea," say researchers.
The polynya stayed open for the next two years, before it disappeared.
To study what caused its disappearance, scientists examined climate models which indicate that as levels of carbon dioxide in the atmosphere increase, precipitation changes. "This agrees with the observations, and fits with a well-accepted principle that a warming planet will see drier regions become drier and wetter regions become wetter," Jaime Palter, a professor in McGill University's Department of Atmospheric and Oceanic Sciences and co-author of the study said in a press release.
And "the polar Southern Ocean - as a wet place - will see an increase in precipitation," Eric Galbraith, a professor in McGill's Department of Earth and Planetary Sciences and a fellow of the Canadian Institute for Advanced Research and co-author of the paper told the Monitor.
In addition to precipitation, melting of glaciers is also responsible for adding additional freshwater to the southern ocean surface.
Over a period of time, the Antarctic ocean was covered with a "freshwater lid" thus impeding the convection process, Dr. Galbraith says.
After looking at data collected by ships and robotic floats in the ocean around Antarctica over a 60-year period, researchers found that the southern ocean's surface has been slowly getting less salty since the 1950s.
The density of saline water is higher than fresh water. This prevents water from deep below the ocean's surface to mix with less-saline water which is present on top of the ocean, thus trapping warm water in ocean depths. "Imagine a bottle of Italian salad dressing with oil on top and vinegar at the bottom," Galbraith says.
Since "deep ocean waters only mix directly to the surface in a few small regions of the global ocean, so this has effectively shut one of the main conduits for deep ocean heat to escape," says Casimir de Lavergne, a recent graduate of McGill's Master's program in Atmospheric and Oceanic Sciences and lead author of the paper.
Closing of the polyna could also explain why Antarctic Bottom Water, a mile-thick layer of water that was last at the surface near Antarctica, is getting thinner. It is possible that "with the ocean capped by the freshwater lid, the source of water has been cut off," Galbraith adds.
Researchers suggest that it is unlikely that the polynya will reappear any time soon, but "if it does, it will release decades-worth of heat and carbon from the deep ocean to the atmosphere in a pulse of warming," Galbraith adds. Such ocean aerators also help in supplying oxygen to the marine life, he adds.