The National Institute for Computational Sciences

Going with the Flow

Kraken Allowed New Views of the Ocean that Could Transform Climate Models

This video shows particle trajectories in the Southern Ocean, south of Australia. The different colors represent the varying depths of the particles; you can see how they veer around subsea topography and circle around eddies, or whirlpools. The model used in this video is from the Los Alamos National Laboratory Popular Ocean Program (POP). [Video courtesy: Alexa Griesel, University of Hamburg, Germany.]

Like so much of the English language, the word ocean can be traced to the ancient Greeks. It is derived from their idea of oceans, or the sea beyond the Mediterranean. The Greeks believed an outer stream of water encircled the earth and actually deified the stream as a god (also named Okeanos).

Thousands of years later, scientists have used the Kraken supercomputer (decommissioned last year) at the National Institute for Computational Sciences (NICS) to explore the effects of ocean streams on Earth’s climate and future.

One of those effects has a different name—eddies. They are the weather systems of the ocean, and they might have a major effect on our warming climate, particularly by influencing sea level changes and heat redistribution.

The ocean-equivalent of the cyclones at Earth’s surface, eddies can span from tens to hundreds of kilometers and occur across the globe. They typically happen when an oceanic current pinches off from its normal route and begins to circulate around one region of water. The swirling motion of eddies is important for the transport and mixing of heat, nutrients, and other elements in the ocean.

Alexa Griesel, a researcher at the University of Hamburg in Germany, studies these eddies and recently published a paper on the subject titled “Eulerian and Lagrangian Isopycnal Eddy Diffusivities in the Southern Ocean of an Eddying Model” in the Journal of Physical Oceanography.

She says that most current climate models fail to resolve the effects of eddies because they are much smaller than their atmospheric model counterparts. The climate models that do resolve eddies respond quite differently (from those that do not) to changes in surface heat, freshwater, and wind.

Scientists indirectly account for the effects of eddies through “eddy mixing parameters,” or the numerical data that explain how the energies of eddies mix and swirl beneath the waters. But her research shows that future analysis must represent the effects of eddies through more refined parameters.

“Climate models, especially ones that simulate past and present climate changes over hundreds or even thousands of years, will have to parameterize the effects of eddies for some time to come,” she says.

Griesel’s research did just that, using Kraken to generate a global eddy-permitting ocean model of the Southern Ocean with one million numerical particles (called “floats”). The code required around 4,000 processors, a calculation load that only supercomputers such as the Kraken can compute.

Her project incorporated two different perspectives on ocean flow. The first—known as the Eulerian perspective–observed ocean flow from a fixed point in space. The second—known as the Lagrangian perspective—observed flow by moving with it, like the numerical particles did. By collecting data from both perspectives, Griesel and her team hoped to determine more accurate climate models.

What they found contrasts with the traditional explanation for the ways eddies mix together in waters of constant density (so-called “isopycnals”). Scientists usually apply the “eddy diffusion hypothesis,” which assumes eddy-mixing works like molecular diffusion.

Simply put, diffusion occurs when molecules move from high concentrations to low concentrations, forming constant densities. Eddies were thought to work in the same way, but Griesel’s research shows that eddy-mixing parameters are actually highly variable in space.

“The eddy diffusion hypothesis may not be appropriate, or at least only when averaging over large spatial scales that are much larger than the eddies themselves,” she says.

The next step is to extend the research from the Southern Ocean to the global ocean, examining eddy-mixing parameters as well as the “non-diffusive” eddy effects that challenge the eddy diffusion hypothesis.

R. J. Vogt, science writer, NICS, JICS

Article posting date: 21 January 2015

About JICS and NICS: The Joint Institute for Computational Sciences (JICS) was established by the University of Tennessee and Oak Ridge National Laboratory (ORNL) to advance scientific discovery and state-of-the-art engineering, and to further knowledge of computational modeling and simulation. JICS realizes its vision by taking full advantage of petascale-and-beyond computers housed at ORNL and by educating a new generation of scientists and engineers well versed in the application of computational modeling and simulation for solving the most challenging scientific and engineering problems. JICS runs the National Institute for Computational Sciences (NICS), which had the distinction of deploying and managing the Kraken supercomputer. NICS is a leading academic supercomputing center and a major partner in the National Science Foundation's eXtreme Science and Engineering Discovery Environment, known as XSEDE. In November 2012, JICS sited the Beacon system, which set a record for power efficiency and captured the number one position on the Green500 list of the most energy-efficient computers.