Magazine article Oceanus

The Ocean-Captured in a Box: An "Experimentalist" Encapsulates Ocean Fluid Dynamics in the Lab

Magazine article Oceanus

The Ocean-Captured in a Box: An "Experimentalist" Encapsulates Ocean Fluid Dynamics in the Lab

Article excerpt

Claudia Cenedese prides herself on thinking inside the box. Her boxes are made of Plexiglass, and they contain the oceans--but on a miniature scale.

Most oceanographers make tiny observations in a vast ocean to piece together the big picture of how it moves and works. Theoreticians start with the big picture, weaving the laws of physics into equations that can never contain all the intricacies of the real ocean. Somewhere in between is Cenedese, a physical oceanographer at Woods Hole Oceanographic Institution, who describes herself as an experimentalist.

In the WHOI Geophysics Fluid Dynamics (GFD) Laboratory, she telescopes real-ocean scenarios into her boxes. The same basic laws that govern fluid dynamics in the boxes also apply in the real ocean, and Cenedese's experiments reveal on a small scale what would be impossible to see and encompass at sea.

Her oceans-in-a-box are not bathtubs filled with stagnant, homogenous water. Like the ocean, they contain distinct masses of water with different salinities and densities, which tend to remain separate but flow in interweaving patterns. The box sits on a spinning table that mimics Earth's rotation, simulating the Coriolis effect on the ocean.

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The different water masses are color-coded with food dye to observe more easily how they move and mix. An overhanging camera rotates in synchrony with the table, providing a detailed visual record of the waters' colorful, swirling motions.

"It's not the real ocean, but it's a real fluid; it's not on paper," said Cenedese's colleague Karl Helfrich, who is also an oceanographic "experimentalist" at WHOI. "It's hard to do a controlled experiment out there in the real ocean."

In her miniature oceans, Cenedese can manipulate the variables. She can change water densities, for example, or add plastic islands, continents, or underwater mountains, then shift their geometry.

"Claudia comes up with an idea, and we carry it out," said Keith Bradley, a research associate in the GFD Lab. "She says, 'Make me a seamount,' and I say, 'Aaaal-right. We can do that.' It's all one-of-a-kind. I try to use stuff that is readily available and that doesn't cost a lot of money."

In one experiment, Cenedese and Rachel Bueno de Mesquita, a visiting undergraduate from the University of Rome, inserted a 12-centimeter-high plastic mound in the center of their box to investigate how water flows around a seamount sticking 1,000 meters (3,281 feet) up from the seafloor. It did not flow randomly, but organized itself into a large eddy, a self-contained spinning parcel of water, that whirled around the base of the seamount.

Next, the scientists tested what would happen if the seamount's topography were smooth as a desert, or bumpy like the Alps. They repeated the experiments, covering the plastic seamount with chicken wire, plastic grass, sandpaper, and insulating foam--each with a texture that produced a quantifiable drag. The rougher the "topography," the slower the eddy rotated.

In the same way that scientists use wind tunnels to examine aerodynamics, "there is a long tradition of studying fluid mechanics in the lab," Helfrich said. "Sometimes we have a general idea about a phenomenon or process--like the formation of eddies spinning off a current as it goes around a corner of land, for example--but we really want to explore the intimate physics of it" Does it always happen? Under what conditions?"

In another of her experiments, Cenedese recreated a similar scenario: Into a box, she inserted a block of plastic representing Canada. …

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