Antarctica is keeping the ocean alive…for now
Water in the ocean is constantly in motion. Scientists can identify distinct “water masses” in the ocean from different sources, and keep track of them by measuring dissolved salts, human pollutants, and the different isotopes of hydrogen and oxygen in the water molecules themselves. This is how oceanographers have mapped the world’s currents, like the Gulf Stream that brings warm water north from the equator and keeps Europe warmer than Siberia.
Of all the water masses in the ocean, one is particularly responsible for keeping the ocean currents flowing. It’s the deepest and densest water in the ocean, and it only forms in a few specific sites off the coast of Antarctica. And it might collapse in the next few decades.
Image by “Fred the Oyster”
This water, called “Antarctic Bottom Water” or AABW, is so dense thanks to its high concentration of salt and extremely cold temperatures. It’s formed on the surface of the ocean in the waters around Antarctica itself.
As seawater hits the cold winds pouring off the Antarctic ice sheet, some of it freezes into a ring of sea ice surrounding the Antarctic continent. Salt ions can’t fit into the crystal structure of ice, so as the sea surface freezes the remaining unfrozen water gets saltier and saltier, in a process called “brine rejection”. This extra-salty sea water collects in open areas of the ocean surface within the sea ice, called “polynyas”.
A polynya in Hudson Bay in Canada (by Mike Beauregard)
The brine in Antarctic polynyas gets chilled even further by the ice around it and the frigid air, further increasing its density. This water then sinks down and slides off Antarctica’s continental shelf, forming a layer of Antarctic Bottom Water on much of Earth’s ocean floor.
Only a few areas around Antarctica form large enough polynyas to create AABW by this process. The main sites are in the Weddell Sea, the Ross Sea, the Adélie Coast, and Cape Darnley, though oceanographers are starting to find evidence of other sites with smaller contributions.
So why is this dense water so important? The two factors that make AABW unique, temperature and salinity, are the two main factors that drive ocean currents in general. Appropriately, it’s called “thermohaline circulation”. Essentially, when cold and salty water sinks down near the poles it pulls warmer and fresher surface water from the equator to replace it. This happens in two main places. One is Antarctica’s polynyas, like we’ve just discussed, which power the “Southern Meridional Overturning Circulation” or SMOC. The other is the North Atlantic, where strong winds evaporate and chill surface water, powering the “Atlantic Meridional Overturning Circulation” or AMOC. And while the AMOC (which includes the Gulf Stream) is better studied, it seems like the SMOC may contribute more to the ocean current system overall, just because more of the ocean happens to lie in the Southern Hemisphere.
Ocean currents move heat from the equator to the poles, moderating our planet’s climate, but they also move oxygen and nutrients around the ocean, creating the conditions for life to thrive. The fish we eat and the algae that make the oxygen we breathe rely on the thermohaline circulation driven by salty polynyas in Antarctica’s sea ice.
So what would happen if, say, a bunch of fresh water from melting glaciers was suddenly dumped right into those polynyas, diluting their brine? Of course, this isn’t just a theoretical question. Human-caused climate change is melting portions of Antarctica’s ice sheet, especially West Antarctica.
A lot of attention has been paid to the AMOC potentially shutting down. This last happened about 12,900 years ago, leading to a cold period in the Northern Hemisphere called the Younger Dryas. One leading hypothesis is that a massive river of glacial meltwater that was draining southward shifted east instead, adding fresh water to the North Atlantic and shutting down the circulation. So there’s some scientific worry that Greenland’s melting ice could cause another Gulf Stream shutdown.
There’s less data for Antarctica, but there are indications that fresh water could disrupt the formation of AABW too, and thermohaline circulation along with it. We’ve got plenty more to understand about the climate and our impact on it, but learning more about bottom water just might be a top priority.
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