The ice crust that forms over the Weddell Sea of Antarctica every winter holds a mystery. From time to time, a huge hole appears, exposing the dark, icy water below.
Why do the holes appear in a few years and not in others? Why do they even appear? It has never been really clear – but now, with a combination of floating robots, satellite imagery and seals wearing small hats, scientists are starting to figure it out. These holes are called Polynyas, and they are pretty well known and widespread. Polynyas are actually very useful for Antarctic animals like seals, whales and penguins, who swim around under the sea ice and must come to the surface to breathe and rest.
But the Weddell Sea Polynya is weird. It was first discovered in 1
Then it reappeared in 1975 and 1976, although the air temperatures in the region were well below freezing. After 1976, however, it almost seemed to have disappeared.
"We thought that this large hole in the sea ice was something rare, perhaps a process that was extinct," Oceanographer Ethan Campbell of the University of Washington said.
But in 2016 it reappeared, a bit smaller – about the size of Maine – and yet unmistakable. Then it reappeared in 2017. And a study earlier this year linked the Polynya 2017 with cyclone activity; However, as with many global phenomena, it may not be as easy as a single cause.
"This study shows that this polynomy is actually caused by a number of factors that must all be in line for them to occur," she said oceanographer Stephen Riser of the University of Washington. "Several of these things could happen in a given year, but if you do not get them all, you will not get Polynya."
Our technology is significantly better than it was in the 1970s, and Antarctica has been doing it's area of interest for some time, both because of the impact that climate change will have on the region, and because of its possible impact the rest of the world decades past.
There was also data from the Southern Ocean Carbon and Climate Observing and Modeling (SOCCOM), a system of floating instruments that monitor temperature, salinity and current in the Southern Ocean to a depth of 2,000 meters. This started in 2014, so there are some years data.
For more than a decade, scientists have equipped Antarctic elephant seals with Argos Systems instruments containing GPS and temperature and salinity sensors. This provided a third stream of data.
Taken together, these allowed the team to compile a picture of what is causing the mysterious holes.
"The recent Polynyas were born from a combination of factors – not only unusual marine conditions, but also a series of very intense storms that swirled across the Weddell Sea with almost hurricane force," Campbell said.
The team found that strong ocean winds blowing closer to the coast of the Antarctic intensify the mixing in the Weddell Sea near Maud Rise. Maud Rise herself then squeezes dense seawater around, causing a whirl over him. Two of the floating instruments were trapped in this vortex for years.
If, as in 2016, the sea is particularly salty, winter storms can trigger a circulation feedback loop, where warmer water rises from below to the surface and is cooled to the surface by contact with the air. As a result, the water becomes denser, falls down again and is replaced by more warm water, which is then cooled and sinks back down, etc.
All these cycles again prevent the formation of sea ice.
And this could have further effects on the climate, the researchers said. This is because there is a lot of carbon at the bottom of the Antarctic Ocean.
Usually, this gradually circulates in the oceans, but as the winter breeze becomes stronger and more frequent, as predicted A collapse of the climate could lead to more polynyas circulating more groundwater to the surface.
"This deep carbon reservoir has been trapped for hundreds of years, and in a Polynya it could be really aerated on the surface of violent mixing," Campbell said.
"A major event of carbon outgassing could really shake the climate system when it happens several years in a row."
The study was published in Nature . 19659003]