MIT scientists and colleagues have moved a step closer to uncovering some of the mysteries of the ocean's impact on climate. Using sophisticated computer modeling technology developed at MIT, they combined satellite measurements of sea-surface undulations with measurements of how sound travels through the Earth's great bodies of water to describe circulation in the Western Mediterranean.
Oceans store and transport enormous amounts of heat, and their circulation plays an important role in determining the Earth's climate. Measuring circulation on a global basis has, however, proven a difficult and expensive task. Only recently have the required technologies started to catch up with theory in measuring ocean circulation.
In a paper that appeared in the February 13 issue of Nature, MIT scientists and colleagues in Germany and Australia present a picture of circulation in the Western Mediterranean. Their work is based on a systematic combination of sophisticated computer models, highly accurate satellite altimeter measurements of the sea surface shape, and subsurface acoustic tomographic data of heat content. A much larger-scale version is now operating over much of the Pacific Ocean, and the intention is eventually to make the system fully global.
"Climate change is inevitable, and the ocean is a major factor in that change. Unless you understand what the ocean is doing today, you won't be able to predict how it might behave in the future," said Carl I. Wunsch, Cecil and Ida Green Professor of Physical Oceanography in the Department of Earth, Atmospheric and Planetary Sciences (EAPS). "However, our immediate goal is not to predict the ocean, but to determine to what degree it is predictable."
Professor Wunsch is co-author of the paper along with research scientist Dimitris Menemenlis and research engineer Chris N. Hill, both of EAPS; Tony Webb, senior lecturer at University College, Canberra, Australia; and Uwe Send, associate professor at the Institut fur Meereskunde in Kiel, Germany.
In a related Nature article in the same issue, Professor Send and colleagues present acoustic tomography data for the Western Mediterranean. The data were gathered during an experiment on the feasibility of acoustically monitoring seasonal changes in the Mediterranean.
PAVING THE WAY
Professor Wunsch said the work shows the capability which is emerging from a 20-year effort to develop observation and modeling techniques to the point where oceanographers can determine the motion of the ocean in time and three dimensions. In 1978, he and Professor Walter Munk of the Scripps Institution of Oceanography proposed the use of long-range acoustics for measuring the ocean and later pointed out the natural complementarity of acoustic tomography, satellite altimetry and numerical models for observing the oceans.
"This is the first time we have been able to demonstrate their ideas in practice with real altimeter and acoustic data," said Dr. Menemenlis. "The technology simply wasn't mature enough earlier."
A new computer model of ocean circulation developed at MIT plays a central role in this work. The model, which exploits advanced parallel computing technology, was developed by John C. Marshall, professor of atmospheric and oceanic sciences in EAPS, in collaboration with Dr. Arvind, the Charles W. and Jennifer C. Johnson Professor of Computer Science and Engineering, and their respective groups (Professor Arvind is also affiliated with the Laboratory for Computer Science). "With the ever-growing power of parallel computers and languages, a synthesis of ocean observations on a truly global scale is within reach," said Professor Marshall.
Satellite altimeters flying 1,300 kilometers above the ocean use radar to measure the shape of the sea surface to an accuracy of a few centimeters. The shape of the sea surface gives scientists an accurate measurement of large-scale currents.
Because the ocean is salty, it is a good electrical conductor, and therefore it is not possible to use light or any kind of radio wave to penetrate it. That is why satellites can only measure properties of the sea surface, and acoustic measurements are required to sample the interior ocean.
Scientists have known since at least 1944 that it is possible to send sound over long distances through the ocean. But measuring those sounds accurately is technically demanding: undersea loudspeakers must be able to function at depths of 1,000 meters or more and at about 100 atmospheres of pressure.
The acoustic tomography system works by sending a series of coded signals from an acoustic source, or underwater loudspeaker. "The primary information we are looking for is how warm or how cold the water is along the propagation path and how the water flows," said Dr. Menemenlis. "The warmer the ocean, the faster the sound propagates."
The sound that the speakers make is less than that of large breaking waves or big ships. It is comparable to the intensity of a stereo system playing at a depth of one kilometer. The signals are picked up by receivers thousands of kilometers away by making use of signal-processing technology similar to that used for detecting faint signals from distant spacecraft.
"We are concentrating on how one can bring the acoustic measurements of the interior ocean together with satellite measurements of the sea surface to produce a consistent estimate of what is going on in the ocean," said Dr. Menemenlis. "Through these innovative technologies we can recover a picture of the state of the ocean without disturbing its rich, fascinating, and precious environment."
The new measuring systems will enable scientists to begin to track shifts in ocean circulation, Professor Wunsch said. Virtually all climate models suggest there will be major shifts in climate over the next several decades--perhaps in the form of global warming. However, there are not enough data so far on the oceans to reach any reliable conclusions.
"For example, rising sea levels are an immediate threat to enormous human populations," Professor Wunsch said of the dangers of major climatic shifts. "One has to understand what is going on out there."
A version of this article appeared in MIT Tech Talk on February 26, 1997.