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Physical oceanography

12,000 feet under the sea, from space

Source: Mazloff, Matthew R., and Carmen Boening. “Rapid variability of Antarctic Bottom Water transport into the Pacific Ocean inferred from GRACE.”Geophysical Research Letters (2016). http://dx.doi.org/10.1002/2016GL068474

Making measurements of the deep ocean is challenging, as it means braving rough conditions in ships or deploying expensive instruments on the seafloor. Two scientists have taken a different approach, using satellite gravity measurements to track deep ocean currents from their desks in California.

The global conveyer belt

The ocean controls Earth’s climate by picking up heat and carbon from the atmosphere and delivering it to the deep ocean by the global conveyer belt of ocean circulation. At the surface, currents like the Gulf Stream are the express delivery service bringing heat from the tropics toward the poles. Near the poles, very salty dense  water sinks to the seafloor and fills the ocean abyss where it can stay for up to 1000 years, eventually returning to the ocean surface by mixing upward and upwelling in the Southern Ocean. The speed of this overturning circulation and how much heat and carbon it can ship depends on how much dense water is formed in the North Atlantic and around Antarctica and exported to the deep ocean.

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The global ocean overturning circulation with Antarctic Bottom Water (blue) forming in Antarctica and spreading into the Atlantic, Indian and Pacific oceans. From Talley at el. 2011 (http://booksite.elsevier.com/DPO/index.php)

Sinking to the bottom

Around the fringes of Antarctica, when seawater freezes to form sea ice, it leaves the salt behind. As the water gets saltier it becomes denser and sinks and accumulates on the continental shelves. It then spills over the shelf, flowing in currents along deep channels away from Antarctica, and filling the bottom of the global oceans with dense, salty water, known as Antarctic Bottom Water, or AABW. Measurements of deep ocean temperature show that excess heat from the atmosphere is absorbed by AABW near Antarctica and is spreading throughout the global deep ocean. However, because of the challenging conditions and cost of making measurements of the ocean abyss, particularly near Antarctica, there is a lot scientists don’t know about  AABW and how it is changing over time. Because this is such an important component of ocean circulation and can carry heat and carbon to the deep ocean to be stored for centuries, scientists are thinking creatively about how to use satellite data to study variations in AABW.

Ocean currents from gravity

The strength of gravity between two objects depends on their mass and the distance between them. On Earth, the strength of gravity in a particular location depends on the amount of mass below it, which depends on the density of the Earth’s core and crust and the density and height of the column of ocean, land, or ice above it. Mountains and continents don’t move very quickly, but the water on Earth is always moving. Because the water contains a lot of mass, movement of water between ice, oceans, land, and the atmosphere over days, months, and years causes small changes in the gravity field. Above the ocean, the local gravity field depends on the mass of water below, indirectly measuring the total pressure at the bottom of the ocean. Up until recently, these little variations in gravity have been too small to detect from space, but a NASA satellite mission called GRACE (Gravity Recovery and Climate Experiment) is now able to map Earth’s gravity field once a month.

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An illustration of NASA’s GRACE satellite orbiting Earth. From NASA JPL.

So the question is, can tiny changes in Earth’s gravity field measured by GRACE be used to see changes in AABW flowing away from Antarctica at the bottom of the ocean? A scientist working on the GRACE mission at NASA’s Jet Propulsion Laboratory in Pasadena, CA teamed up with a researcher from Scripps Institution of Oceanography in San Diego, CA to answer that question.

Before trying to estimate deep ocean flows, the researchers searched for a location where they could compare what GRACE measured from space with measurements from the ocean floor. They chose a spot East of New Zealand where measurements of pressure at the seafloor were collected from 2010 to 2012. The comparison found good news, GRACE and the pressure recorder were very similar, so the researchers could move on to the next step to use bottom pressure to estimate the flow of AABW.

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Figure 1 from Mazloff and Boening (2016) showing on top the location of the bottom pressure measurements (red) which were compared to the bottom pressure from GRACE in the black square at the same location. The other two black squares show the GRACE ocean bottom pressure used to estimate the northward flow of AABW between them. The bottom shows closely matching bottom pressure from a seafloor pressure recorder (red) and from GRACE (blue) for 2010-2012.

Rapidly changing flow

Using the difference in pressure from East of New Zealand to the middle of the Pacific, the scientists used a state-of-the-art climate model to relate the pressure difference to the flow of AABW between the two locations. Using the numbers they got from the model, they estimated the northward flow of AABW from GRACE pressure data for 2003-2014. They found that the amount of AABW flowing north changed rapidly from month to month, but there was no long term trend. Where the ocean model overlapped with GRACE the changes in AABW flow were very similar and the results show that gravity measured from space is a viable way to estimate flows at the bottom of the ocean.

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Figure 3 from Mazloff and Boening 2016. The red line shows the northward flow of AABW from 2003-2014 and the blue line shows the AABW flow in the ocean model from 2005-2010.

To the seafloor, and beyond!

Now that scientists have proved that AABW can be tracked from space in one location, the next challenge is to expand and use GRACE to follow all the paths of AABW that spread out from Antarctica to fill the deep ocean. The rapid changes in the strength of the flow of AABW mean that measurements taken from ships, often decades apart, are not sufficient for showing long term changes in the deep ocean. GRACE may be able to monitor AABW from space, helping to fill the gaps between ship surveys. As well as monitoring changes, the new information from GRACE could help scientists better understand the role of AABW in the global ocean circulation and how quickly it can transport heat and carbon to the ocean abyss.

Veronica Tamsitt
I’m a PhD student at Scripps Institution of Oceanography in La Jolla California. My research is focused on the Southern Ocean circulation and it’s role in climate. For my research I sometimes spend months at sea on ice breakers collecting data, and at other times spend months analyzing computer models.

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