Every now and then, gaping holes would suddenly materialize in the middle of the Antarctic sea ice, opening up to the dark waters below.
For decades, scientists have been baffled by these strange holes known as polynyas, which have been observed in both the Antarctic and Arctic.
In the winters of 2016 and 2017, two of the largest ones that have been seen in decades appeared in the Weddell Sea with the first one measuring at 13,000 square miles and the second one at 19,000 square miles. By monitoring these recent polynyas and the conditions surrounding them, scientists were able to figure out what's likely causing this strange phenomenon — with a little help from robots and elephant seals.
Findings published in the journal Nature revealed that the combination of particularly salty water and powerful storms spurred the creation of these enormous gaps in the sea ice.
"Observations show that the recent polynyas opened from a combination of factors — one being the unusual ocean conditions, and the other being a series of very intense storms that swirled over the Weddell Sea with almost hurricane-force winds," lead author and University of Washington oceanography doctoral student Ethan Campbell said in a statement.
While polynyas can appear near the sea ice's shore, they're also known to occur far from the coast. Various animals such as seals, penguins, and whales use these polynyas in the middle of sea ice as an oasis where they can occasionally pop up for a quick breather.
In the recent study, the researchers analyzed information from Antarctic elephant seals wearing hat-like instruments with GPS and sensors measuring the ocean's temperature and salinity.
Data from the Southern Ocean Carbon and Climate Observations and Modeling project also factored in the findings. This project is known for sending out floating robotic instruments that drift with the currents and monitor the conditions of the Southern Ocean.
Finally, the authors also monitored satellite data going back decades to see the patterns that might be related to polynyas.
'The Perfect Storm' Produces Polynyas
Findings show that when the ocean winds draw closer to the Antarctica shore, they cause stronger upward mixing in the eastern Weddell Sea and the nearby underwater mountain Maud Rise forces dense seawater around it. This results in a spinning vortex above, where two SOCCOM instruments were trapped for years collecting valuable information.
In years when the ocean is particularly salty, such as in 2016, strong winter storms can trigger an overturning circulation. Warmer, saltier water rise to the surface, where it cools and becomes denser than the water found deeper. So, it sinks and gets replaced by the warmer deep water — and the same events occur again in a feedback loop that ensures that ice doesn't form on the surface.
"This study shows that this polynya is actually caused by a number of factors that all have to line up for it to happen," explained coauthor Stephen Riser, who is a UW professor of oceanography. "In any given year you could have several of these things happen, but unless you get them all, then you don't get a polynya."
Polynyas could have significant implications for the climate, since there are carbon deposits found at the bottom of the Antarctic.
Campbell pointed out that the carbon in the ocean depths have been preserved there for centuries, but the violent mixing of a polynya could get it ventilated at the surface. Such a large carbon outgassing event could have a huge impact on the climate system if it occurs in multiple years consecutively.