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Research > Science Highlights > What Goes Down Must Come Up

What Goes Down Must Come Up

From space, the ocean may look like a big, calm bathtub. But below its surface lies the ultimate dynamic environment—328 million cubic miles of water flowing, sinking, mixing, and rising.

It’s as turbulent as another, more familiar fluid environment—the atmosphere—but the action is hidden from view.

Data collected on five cruises between 1993 and 1999 (above: yellow, orange, and red tracks) showed evidence of intense turbulent mixing in the Southern Ocean. The mixing helps bring deep, dense waters back toward the surface. Oceanographers now hypothesize that much more mixing (below: red and yellow areas) may occur when the powerful Antarctic Circumpolar Current is disrupted as it flows over rough seafloor topography. (Illustration by Jayne Doucette)

“We can observe the atmosphere because we live in it, rather than just venturing on top of it,’’ said Bernadette Sloyan of the WHOI Physical Oceanography Department.

“But we can’t look at ocean dynamics directly to get information about what is happening over space and time,” said her colleague, Kurt Polzin. Instead, physical oceanographers work with measurements of thin slices of the ocean at brief moments in time. They piece together snapshots, envisioning the epic movie.

Small wonder, then, that one of the most basic questions about ocean circulation remains a mystery. The world’s oceans circulate like a conveyor belt of sinking and rising waters: oceanographers know why cold waters sink to the depths, but what makes them rise back to the surface completing the loop?

It’s not an academic question, because the oceans and atmosphere act as equal partners in a planetary heating and ventilation system that transfers equatorial heat to the poles. Together, they regulate Earth’s climate and keep the planet habitable.

To understand and forecast changes in Earth’s climate, scientists construct computer simulations of its complex dynamics, said WHOI physical oceanographer Steven Jayne. But the climate models have a fundamental blank spot: the “up” component of the Ocean Conveyor.

Sinking ocean waters were relatively easy to find and measure, because they are concentrated in a few locations. Only in the North Atlantic and the Southern Ocean around Antarctica do waters become both cold and salty (and therefore dense) enough to plunge swiftly in a pipeline to the deep.

But there’s no obvious pipeline coming back up. The cold water spreads through the ocean depths, bounded by overlying layers of lighter waters. It takes energy to break the boundaries between layers, mix waters of different densities, and create a way for denser waters to escape back upward.

“Mixing is dispersed over areas thousands of times larger (than sinking) and happens a thousand times more slowly,” Jayne said, which makes mixing tough to locate, and harder still to measure.

Data from Alberto C. Naveira Garabato, Kurt L. Polzin, Brian A. King, Karen J. Heywood, and Martin Visbeck in Science, Vol. 303, Jan. 9, 2004.
Sloyan launched a search to find where mixing might be occurring. She analyzed salinity and temperature data from the World Ocean Circulation Experiment, a multi-institution ambitious effort during the 1990s to collect global ocean measurements. She detected subtle signs of mixing in the Southern Ocean—especially in regions where the seafloor is marked by saw-toothed mountains, steep slopes, and narrow chasms.

The Antarctic Circumpolar Current whips around the continent at great speed and depth, Sloyan said. It may bump into rough topography, like air currents hitting the Rockies, generating waves within the ocean’s interior. These internal waves can ripple away, overturning density gradients and causing mixing. Currents surging through a constrained passage, like a river through a narrow gorge, may also force mixing.

Tides may also play a role, providing energy to move water back and forth across seafloor topography. Jayne is exploring models simulating how tides create internal waves that spur mixing (see Doing the Wave).

Polzin narrows his investigations down to the microscale level, often using a High-Resolution Profiler (HRP), an instrument developed by WHOI scientists John Toole and Ray Schmitt and equipped with sensors developed by former WHOI engineers Sandy Williams, Neil Brown, and Tom Sanford. The HRP accurately measures changes in water temperature, velocity, and conductivity (i.e., salinity) on scales ranging from tens of meters down to centimeters.

“We can reveal turbulence as small as this,” Polzin said, holding his thumb and forefinger nearly together. “The HRP gives us an ability to see and understand the ocean in new ways.”

Polzin is trying to find links between flows at disparate spatial scales. “Small changes may not be recognized as important,” he said “but they can initiate nonlinear, but coupled, changes that are important.”

Today, many scientists see evidence that ocean conditions may be approaching a tipping point that would disrupt the delicately balanced ocean circulation system—a phenomenon that has abruptly rearranged global ocean currents several times in Earth’s history and caused rapid, widespread climate changes. Like a tiny cog in a colossal machine, centimeter-scale mixing may prove a small but important piece that helps determine whether the Ocean Conveyor shuts down or keeps running.

—Lonny Lippsett (llippsett@whoi.edu)