Tracer Oceanography with Andrew Shao

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…)

* This drawing is not based on reality

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.

CFCparcels 2013-02-08 (07.48.55-870 PM)
This is a completely hypothetical drawing to illustrate how scientists (like Andrew!) use CFC concentrations to figure out how the water in the ocean moves around. So cool!

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.

WOCE transect in the North Atlantic Ocean

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.

WOCE transect – CFC concentrations.

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.


5 thoughts on “Tracer Oceanography with Andrew Shao

  1. So CFC’s were basically zero on the graph prior to 1930? Looks like this stuff takes a long time to dissipate. What are the ramifications of continued elevated levels, and can the model predict when the levels would once again be back at pre-1930 levels?

    Also is there any way to trace agricultural runoff? Do any pesticides, herbicides or excess fertilizers leave chemical signatures? As a clothing company trying to promote organic clothing I would like to find data that shows the impact of these agricultural chemicals. We know they’re hazardous to the local ecosystems, causing algae blooms and dead zones, deformation of frogs, and kill some marine life, but do the chemicals then travel away in the open ocean and cause more havoc, or are do they decompose or degrade close to their origin? Interesting questions…

    1. Hi Noel,
      I’ll step in here to answer at least some of your questions.

      One of the reasons why CFCs are such good tracers in the ocean is that they are inert once they dissolve into the ocean and do not change because of chemistry or biological activity. As such, there’s no real idea of them being an oceanic “pollutant” (compared to CO2 which does affect ocean biogeochemistry). CFCs in the atmosphere however are chemically active since when hit by UV radiation, they become unstable. A number of papers have attempted to determine the lifetime of CFCs in the atmosphere with most estimates being 100-200 years.

      In regards to the second, it’s a much more difficult question to trace such agricultural runoff for a couple of reasons: 1) The amount of runoff versus the size of the ocean means that any concentrations that reach the open ocean are going to be diluted fairly quickly 2) Most agricultural products are chemically and biologically active which complicates the question of where/when it came from 3) From a practical standpoint, measurements would likely have to be done from a ship using specialized lab equipment; oceanographers are still trying to make sense of very broad biogeochemical related like how are the distributions of nutrients, carbon, and oxygen related? From a cursory glance, I only found one study that specifically tried to measure pesticides in the open ocean coming up with a mean value of about 1 nanongram/liter (10^-6% less than the level that has been shown to have an effect on algae). However, without other corroborating studies, additional knowledge about pesticide impacts on marine organisms, and additional , it’s a bit impossible to even speculate that terrestrial runoff of pesticides, etc. have any impact at all on open-ocean ecosystems. As you’ve pointed out, most studies have been confined to estuaries and the coastline where the runoff has yet to be diluted.

      Hope that helps and let me know if you have any other questions or would you like any further clarification.


  2. Thanks Andrew! It must be fun for you and Michelle to be in a field with so many unanswered and complicated questions.

    Could you use plastic particles to answer the question? When man made plastic particles from the open ocean are analyzed they’re found to have absorbed man made pollutants.

    Now that there is so much plastic in the sea water are they now the repository’s for these pollutants? As opposed to being able to measure pesticides in the open ocean could we use the plastic particles to measure the density of agricultural pollutants and then trace those particles throughout the oceans?

    Here’s one of the articles I was looking at in regards to this:

    “Plastic particles can absorb chemicals that are “hydrophobic,” that is they don’t mix with water, but are “lipophilic,” that is they are attracted to oily substances, such as petroleum-based plastics,” he explains.

    “Pollutants like PCBs, DDT, flame retardants, pesticides, and other hydrophobic persistent organic pollutants can stick to plastic particles at high concentrations – up to a million times higher concentration than the seawater around them,” Eriksen says.

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