On the trail of ocean bubbles.

On the trial of ocean bubbles

Although the atmosphere is an obviousforce in weather and climate, another vital participant in global dynamics is the interior of the ocean. It has a mass that is 260 times that of the atmosphere and a significantly greater capacity to store heat. Through the uppermost layer of the ocean, these two huge reservoirs exchange heat, momentum and gases--a transference that drives the ocean currents, influences short-term weather and even affects the gradual shifts in the global climate.

While scientists know that the subsurfacemixing of water provides this important link between the atmosphere and deep ocean, "the processes of mixing in the upper few meters of the ocean are poorly known and largely unquantified,' say S.A. Thorpe of the University in Southampton and A.J. Hall of the Institute of Oceanographic Sciences in Surrey, England.

In the July 2 NATURE, Thorpe and Hallreport on a new means of monitoring this mixing process by using sonar and thermistors (electric thermometers) to track the movement of clouds of bubbles. A trial of this technique revealed a downward movement of relatively warm nearsurface water--a process that transported bubbles into colder, deeper water, which is ordinarily deficient of bubbles, say the British researchers.

As they descend and encounter increasedpressure, the bubbles shrink until they disappear, transferring their gas content to the ocean. In this way, the ocean provides a sink for important gases such as carbon dioxide, which can contribute to the gradual heating of the earth via the greenhouse effect. Although the mechanism behind this downward transport is unknown, Thorpe and Hall suggest three possible mechanisms and say they will test for these in future experiments.

Until now, scientists had lacked thetools to measure these slow, vertical flows, which are easily obscured by the turbulence of ocean waves, says Robert A. Weller of the Woods Hole (Mass.) Oceanographic Institute. "This is a whole new tracking tool and way of looking at the important physical [subsurface] processes,' he says. Previous, unsuccessful methods for measuring the vertical water movement near the surface had involved mechanical instruments such as propellers.

Thorpe and Hall made their observationsby towing a catamaran that had sonar and thermistors hung at varied depths beneath the vessel. Because the catamaran rides the waves without disturbing the subsurface waters, the researchers could monitor the top 8 1/2 meters of the ocean during these trials in the seas east of Iceland.

Bubble clouds, created at the oceansurface by breaking waves, proved to be a good target for the sonar, which emits and receives sound waves. These waves reflect off variations in the water, including bubbles, and are recorded when they return to the sonar.

The new method, says Weller, willcontribute to a better understanding of these ocean-atmosphere interactions and will help scientists who are devising models of the global climate.