Mysterious giant holes spotted in 2016 and 2017 in the icy winter surface of an Antarctic sea, one bigger than the state of Maryland, fascinated scientists. Though even bigger gaps had formed in the area's sea ice decades before, this time oceanographers closely monitored the gaps using real-time data.
Thanks to new research combining satellite images, data from floating robots and elephant seals swimming under the ice outfitted with sensors on their heads, the mystery of the giant ice holes may be solved. The study, conducted by scientists across the US and Canada, has enabled a better understanding of why such large holes in the sea ice, known as polynyas, form; why they show up only some years; and what they could mean for global ocean circulation -- and the atmosphere. It appears in Monday's issue of the journal Nature.
"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," said Ethan Campbell, a University of Washington doctoral student in oceanography and lead author of the study.
Those storms have a dramatic effect, chipping away at the icy surface and disturbing the water beneath it, causing warmer, saltier water to rise up from deep in the ocean. It had long been suspected that deep ocean heat sustains Antarctic sea-ice openings like those seen in 2016 and 2017, but this had never been observed directly. "It was exciting and surprising to see that vertical mixing reached over a mile deep into the ocean during the polynyas," Campbell said.
However, a polynya (that's Russian for ice hole) is more than just a quirky force of nature that overturns the ocean from top to bottom.
"Winds and sea ice around Antarctica are changing. It follows that the prevalence and importance of polynyas may also be changing," said biological oceanographer Pete Strutton, an associate professor at the University of Tasmania's Institute for Marine and Antarctic Studies, who was not involved with the study. "Using these observations together with computer simulations we can begin to understand what the future holds for Antarctica, and what this means for global climate."
The same heat that wells up from deep in the ocean could melt ice shelves, the study notes. Plus, when the gaps are large and long-lasting, they can affect the atmosphere. That's because deep water contains carbon from lifeforms that have sunk over time and dissolved on their way down. When this water reaches the surface, as it does when storms contribute to a polynya, that carbon could be released.
"This deep reservoir of carbon has been locked away for hundreds of years, and in a polynya it might get ventilated at the surface through this really violent mixing," Campbell said. "A large carbon outgassing event could really whack the climate system if it happened multiple years in a row."
Climate models show that this carbon release in a polynya much larger than the 2016/2017 events "could be substantial, contributing somewhat to global warming," Campell said, adding that it's unknown how much carbon was expelled during the recent polynyas, but that it was no doubt far less than humans put into the atmosphere each year.
The new research relied on observations from the Southern Ocean Carbon and Climate Observations and Modeling project, or Soccom, which sends autonomous robotic instruments adrift with the currents to monitor Antarctic conditions and their impact on climate change.
The study also tapped data collected by seals that swam under the sea ice with temporary satellite tags showing normal water conditions in the years that did not see large polynyas form. They then beamed the data back to shore.
"One of the best features of this work … is the way it brings together observations from many different platforms -- satellites, robotic floats and tagged animals -- to produce a picture of polynyas that wouldn't have been possible even a few years ago," said Strutton, who's also affiliated with the Centre of Excellence for Climate Extremes.
Up next for the researchers: studying what impact the polynya had on biology in the region. Data from the robotic floats show large phytoplankton blooms developed after both of the recent polynyas, and the scientists want to better understand how they form and influence the ocean ecosystem.