Tracing CFCs and greenhouse gases

National Center for Atmospheric Research oceanographer Synte Peacock studies “the distribution of various tracers – something that tags a water mass and is carried around by ocean currents – to learn more about ocean circulation in the past and present.”

These tracers include carbon and radiocarbon isotopes, paleotracers (fossils from the sea, in sediments and shells) and theoretical tracers called transit time distributions, which get at how long it’s been since water at the ocean’s interior was lost to the surface – useful for tracking the behavior of greenhouse gases carbon dioxide and methane and the long-banned chlorofluorcarbons (CFCs).

Add passive dye to that list of tracers. In fact, a key reason she and her Los Alamos National Laboratory collaborator Mathew Maltrud could start simulations so quickly during the Deepwater Horizon incident was that passive dye already was part of their modeling repertoire and the code had been worked out for previous simulations.

“It was easy to set this model up,” Peacock says, “like flipping a switch and letting the model go. You specify the latitude and longitude for where you want to inject (the dye), the depth, and how long you want to inject for.”

Earlier in 2010, Peacock and Maltrud had reported on their simulation of CFCs and numerous other tracers in a 100-year ocean circulation model. The model’s extreme high resolution of .1 degree let the researchers follow the movements of eddies, which previous CFC distribution studies characterized poorly because their resolutions were coarse in comparison. Though the eddying model did not directly lead to the Deepwater Horizon incident study, it laid the groundwork for the researchers to act and implement similar theory.

The model the researchers ran is considered to be one of the most realistic global fine-resolution eddying models, and the only one to simulate such a large set of tracer distributions, thanks to the power of the Jaguar supercomputer at Oak Ridge National Laboratory.

CFC modeling is vital in understanding how the ocean can store chemicals for up to thousands of years and globally circulate them at various depths from the surface to hundreds of meters down, a process called ventilation that has multiple effects on climate.

Maltrud wants “to know how gases get transferred from the surface of the ocean down into the depths, and CFCs are a really good way to do this.”

Adds Peacock: “It was the first time that CFCs had been carried for many decades in a model of one-tenth degree resolution. So, the grid was small enough that we could resolve these eddies.”

Peacock refers to eddies as “the weather of the ocean. They happen on small spatial and time scales, are about 1 to 10 kilometers in diameter, with circular features that rotate. You get very large temperature gradients in relationship with surrounding water and very different properties within the core of an eddy. They’re important in transporting, for example, heat and mixing properties in the ocean. One of the reasons we run the high resolution models is to see how accurate we are in parameterizing them.”

CFCs are directly measured in real-time on ocean-going ships with devices called CTD sensors (for conductivity, temperature, depth) from water samples captured in bottles dunked to various depths. Peacock says many measurements have been made over the past 20 years. She and Maltrud used the real-time measurements to test their simulated CFC distributions.

“We didn’t use the real-time observations to push the model in any way, just for the validation,” Peacock says. “This was a very powerful tool that gives us a snapshot of what is going on and clues to how the ocean is being ventilated over the past couple of decades.”