Allow me to introduce my very first Science + Comics interviewee:
Name: Andrew Shao
Job: 3rd year PhD student – Physical Oceanography (University of Washington)
Research: Tracer oceanography
Have you ever gone to the beach, stared out at the crashing waves, and wondered, “where have you been, you parcels of water”? Okay, probably not. But in a way, that’s the kind of question that Andrew Shao asks, every day. (not usually while watching the sun set at the beach…)
Andrew uses “tracers” to try to answer questions about ocean circulation and climate change. A tracer is a sort of marker that sticks with a particular parcel of water, no matter where it goes in the ocean. You can think of it like this: if you dump food coloring into a swimming pool, you could see where the water moved by looking at where the dye went.
Although it is certainly possible to dump dye into the ocean and see where it goes, it’s tough to do that on a large enough scale to see what’s happening across ocean basins. As it turns out, there are certain “chemical signatures” that you can use as tracers. One of the ones that Andrew uses is CFC concentration.
You’ve probably heard of CFCs (chlorofluorocarbons). They were used since the early 20th century in refrigerants, propellants, and solvents. Eventually people started to realize that they were really bad for the environment, and in the late 1980s, the international community decided to phase them out. The concentration of CFCs in the atmosphere is well know, and is shaped (roughly) like this sketch:
The CFC concentration in the atmosphere at any given time makes an “imprint” on the surface ocean, and that water caries that imprint wherever it goes. The next drawing shows a simplified example of how this might happen. Let’s say the CFC concentration in 1980 is 180 ppt (parts per trillion). The surface water pulls in the CFCs until the concentration matches the atmosphere. Depending on the circulation patterns at that location, the water will get moved around. In the picture, the water sinks down and then moves horizontally over a total time span of 20 years.
Most recently, Andrew has been working on modeling how CFCs move through the ocean. When he builds a model, he starts with what he knows about the physics and chemistry (things like conservation of mass, fluid dynamics, gas exchange). Then he adds effects like: wind, temperature and salinity of the sea surface, depth of the mixed layer… I haven’t even listed everything here! It’s really complicated.
In his model, he sets what we know about atmospheric CFC concentrations, and lets the model run, stepping through and re-calculating a day or a month at a time. The model predicts where water in the ocean will go.
One of the important parts of an oceanographic model like this is model validation. In Andrew’s case, this means comparing the output of his model with actual measurements when and where possible. To get the type of measurements used to validate this type of model, ships go out and move in a more or less straight line across different oceans. They stop several times along that line and lower sampling equipment all the way to the bottom, sampling the water along the way. They measure all sorts of things, CFCs being one of them. Two major long-term experiments of this type that have been done:
- WOCE (World Ocean Circulation Experiment) – between 1990 and 2002
- CLIVAR (Climate Variability and Predictability) – started in 1995 and ongoing
The next couple of images show an example of data collected during a WOCE survey. (See more examples here). The first image is a map showing a transect in the North Atlantic Ocean. The dots along the red line are locations where the ship stopped to take measurements.
The next figure shows the results of the CFC measurements along that transect. The x-axis shows distance along the transect, and the y-axis shows pressure (you can interpret it as depth). The north end of the transect is at the right and the south end is at the left. The colors indicate CFC concentration, with cooler colors indicating lower concentrations (older water). Looking at the colors, you can see how the water has moved.
Andrew can combine what his model predicted with what is measured during ship surveys to try to answer questions about ocean circulation, and how ocean currents are changing. This in turn allows him, and other scientists, to address questions like, “How much carbon dioxide generated by humans ends up in the ocean?”. And that gives us another piece of the puzzle to better understand climate change and global warming, and what we might expect to see in the future.