Researcher finds small scales aren’t just drops in the ocean

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Rotation and stratification

It’s often assumed that phenomena smaller than 10 kilometers are irrelevant in ocean dynamics, Kurien says.  At around that specific length, called the Rossby deformation radius, two important factors in ocean movement – deformation caused by swirling due to the earth’s rotation, and the propensity of gravity to flatten the ocean – tend to balance each other.

At longer distances, the effect of the earth’s rotation tends to dominate the effect of gravity.  Today’s best-resolved ocean models can just about capture dynamics up to the Rossby deformation radius, but no smaller.

Kurien and fellow researchers Beth Wingate of Los Alamos and Leslie Smith and Zhengyu Liu of the University of Wisconsin at Madison believe models must span multiple scales to include the effects of even smaller phenomena.

There’s evidence to support them, now that some computers are powerful enough to simulate ocean activity at smaller scales.  A prime example is the Gulf Stream, which pulls warm water from the Gulf of Mexico north to the west coast of Europe.  The current has a huge impact on climate.

Models have had difficulty capturing just where and how the Gulf Stream divides as it passes the U.S. Atlantic coast, Kurien says.  Capturing that separation is crucial to accurately modeling climate over centuries-long time scales.

When scientists’ models became detailed enough to resolve the Rossby deformation radius, however, all these eddies, all this activity appeared, Kurien says, and the stream separated at the correct location.

The Rossby deformation radius is large enough near the equator to be captured by a model resolving activity on a scale of 10 kilometers.  But the radius shrinks at higher latitudes, and is barely captured by the best current models as the Gulf Stream moves north.

“It starts to gain errors as you get to the higher latitudes,” Kurien says.  “So the next step is to look at even smaller scales.  It doesn’t seem intuitive that the small scales should matter this much, but they do, in the long term.”

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