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Climate Change

Melting ice shelves could be slowing down ocean circulation: Elephant seals lend a flipper to find out

Williams, G. D. et al. (2016), The suppression of Antarctic bottom water formation by melting ice shelves in Prydz Bay, Nature Communications, 7, 12577, doi:10.1038/ncomms12577.

(Featured Image Credit: Joachim Ploetz, Alfred Wegener Institute)

An unlikely group of researchers has been working tirelessly to collect data in the frigid Antarctic waters. Carrying instruments on their heads that measure water temperature, salinity, and depth, they spend their days diving deep to the seafloor and swimming back to the surface, over and over and over. Dive, climb, dive, climb. They float at the surface only for a few moments, then let out an offensive snort, and somersault back over, down through the water, rolls of fat rippling behind them. Elephant seals. They have been an invaluableasset to scientists seeking to gain a better understanding of what goes on in the remote Antarctic seas.

A pair of elelphant seals, one with a satellite tag glued to its head that measures temperature, salinity, and depth. (http://www.antarctica.gov.au/)

A pair of elelphant seals, one with a satellite tag glued to its head that measures temperature, salinity, and depth. (http://www.antarctica.gov.au/)

A “conveyor belt” system connects all the world’s oceans. At the surface, it moves heat away from the equator and, when it gets to the poles, it sinks down and slithers along the seafloor, carrying trapped carbon dioxide and other atmospheric gases with it. The whole cycle takes about a thousand years. It’s kept in motion because water gets cold and salty enough at high latitudes to become denser than the surrounding water and sink to the bottom of the ocean. This important process occurs in only a few known locations near Greenland and Antarctica.

Deep water is often formed in polynyas, which are areas of the ocean that stay open despite being surrounded by sea ice. Polynyas in Antarctica usually form at certain regions of the coast where very strong winds, called katabatic winds, flow down high mountain slopes and blow off the continent, pushing sea ice away. The open spot of ocean is left exposed to extremely cold atmospheric temperatures, which cause new ice to form. This ice also gets blown away and the whole process continues indefinitely. As new ice grows, an interesting thing happens to the water below it. Salt is left behind as water crystalizes and so the surface ocean becomes incredibly cold and salty. The cold, salty water sinks because it is denser than the water around it. If it is really dense, it joins the global ocean’s deep water.

Deep water formation at a coastal polynya. Strong winds blow cold air over the open area of ocean, allowing water to freeze and form ice. The ice is blown away from the continent so that new ice can form, but salty water is left behind and sinks. (Open University)

Deep water formation at a coastal polynya. Strong winds blow cold air over the open area of ocean, allowing water to freeze and form ice. The ice is blown away from the continent so that new ice can form, but salty water is left behind and sinks. (Open University)

Satellite images show a large amount of sea ice being formed at a coastal polynya called Cape Darnley, on the east side of the continent. For several years, it was hypothesized that deep water was forming here, but getting good data throughout the depth of the ocean there was difficult.

A common practice for obtaining a long time-series of data in the ocean is to drop a string of instruments with an anchor, called a mooring. In the Antarctic, thick winter sea ice and the unpredictable passage of deep-hulled icebergs requires that the top of the mooring be tens of meters below the surface of the ocean, so you need to be confident of its precise location, and have a little bit of luck on your side, to get the instruments back. In 2007 and 2008, Japanese researchers had deployed some moorings in the Cape Darnley area, but none of them were recovered, probably, at least in part, because there were no good, detailed maps of ocean depth there.

We want to be able to measure water temperature, salinity, and density at the surface of the ocean, where salt is extruded from ice, and throughout the depth of the water below to determine how far down the salty stuff sinks. We need something that will measure the full depth of the ocean and be guaranteed to surface again with the data. Elephant  seals are the perfect fit for the job.

Elephant seals spend most of their time under water, diving down hundreds of meters to feed on octopi, squid, and fish, and stay at the surface just long enough for the satellite transmitters on their heads to relay all their data back to scientists sipping coffee in front of their computers thousands of miles away. When they molt, the instruments just shed off with the seals’ skin, if they haven’t already been recovered by researchers in the field.

While they were out swimming with instruments on their heads in 2011, 2012, and 2013, elephant seals confirmed the existence of a previously unknown location of deep water formation in Antarctica. These hard workers discovered not one, but a whole series of coastal polynyas that form deep water through a sort of production line in Prydz Bay, which feeds into the Cape Darnley polynya. Satellite images show three open polynyas in Prydz Bay, but none of them produce water quite dense enough to become true deep water. They do, however, prime the Cape Darnley polynya with water dense enough that it only needs a small push to make it all the way to the bottom. Water cycles through Prydz Bay getting a little bit denser at each polynya. Then it flows into Cape Darnley where it gets the final burst of salt to form the “Deep Shelf Water” that sinks to the seafloor.

The deep water factory of Prydz Bay, Antarctica. The Barrier Bay polynya kicks off the production line making dense water that circulates clockwise along the coast. It meets up with modified Circumpolar Deep Water (mCDW), a relatively warm but salty water mass that has come in from offshore. The Davis and MacKenzie Bay polynyas each take their turn to make the water saltier. From there it takes one of two paths: out to the shelf break or up and over a sill and into the Cape Darnley polynya, out of which emerges the densest deep water. (Figure 3c and 5 in the paper)

The deep water factory of Prydz Bay, Antarctica. The Barrier Bay polynya kicks off the production line making dense water that circulates clockwise along the coast. It meets up with modified Circumpolar Deep Water (mCDW), a relatively warm but salty water mass that has come in from offshore. The Davis and MacKenzie Bay polynyas each take their turn to make the water saltier. From there it takes one of two paths: out to the shelf break or up and over a sill and into the Cape Darnley polynya, out of which emerges the densest deep water. (Figure 3c and 5 in the paper)

The seals also made another important discovery. The data they collected showed freshwater coming into the bay under ice shelves. Since deep water production relies on the salinity of water formed at polynyas, too much freshwater could prevent polynyas from creating water dense enough to sink to the bottom and feed into the ocean conveyor system. If deep water production were to stop, it would restrict the ocean’s ability to redistribute heat around the planet. It would also affect the carbon cycle by limiting the amount of carbon dioxide that gets trapped at the surface and sequestered far from the atmosphere for hundreds of years.

We now know that there is at least one additional area of deep water formation initiating the ocean conyeor system, but we have also learned that these production zones are at risk of slowing down. Warming ocean and atmospheric temperatures are melting ice shelves all around Antarctica, and the freshwater they are adding to the system could supress the sinking of deep water that drives the whole ocean circulation system.

We’ll have to keep up underwater observations to understand how quickly the slowdown could be happening. To do this, we’ll have to continue exploring creative solutions. There are limitations to using wild animals to collect your data: they don’t always agree with you on the most interesting places to swim! Although it would have been really informative to collect data under one of the ice shelves in Prydz Bay all throughout the year, the elephant seals seem to avoid that region entirely between April and October. But we can’t ask for much more. They’ve helped us out a lot already.

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