National Geographic : 2000 Oct
painstakingly by research vessels like the Thompson and the Knorr. The problem is this: Oceans are big, deep, and moving. Ships are slow, bound to the surface, and narrow in their tracks. "Afew ships chasing around the sea can't see things that shift in time and space,"' explains Walter Munk of the Scripps Institution. At best, ship research can create only piecemeal images, scattered and hard to link up. On the wall of my study hangs another map, a brilliantly colored image of the whole global ocean, showing in tones of yellow, orange, red, blue, green, and purple the slight mounds and depressions of the sea surface. The image was created with data from the satellite TOPEX/ Poseidon. A project shepherded into being by Carl Wunsch, TOPEX/Poseidon has been in orbit since 1992, circling Earth 4,700 times a year, mapping the subtle, fluctuating hills and valleys of the ocean's surface. One thinks of the vast ocean as flat, but it WAVE ACTION INWAR AND PEACE Amphibious landings of Allied troops during World War II,including the D day invasion of Normandy (below), were possible because scientists such as Walter Munk learned how to predict wave conditions. Now at Scripps, Munk (left) tests global warming theories by documenting ocean temperatures. Sound from the underwater speakers travels faster through warm water than through cold. actually dips and bulges, in part because of converging and diverging currents that cause water to pile up or move apart. So sensitive are the radar altimeters of TOPEX/Poseidon that even from its height 830 miles above the planet, it can detect elevation to within an inch or so across hundreds of miles of ocean surface. Because the satellite measures along the same track every ten days, scientists can see how the ocean changes over time. From the resulting maps they can calculate the speed and direction of surface currents. "With satellites we can take a steady stream of measurements nearly everywhere at once:' says Wunsch. "A sampling of data that would take a research ship years to complete a satellite can do in days." Ocean-observing satellites now circling Earth can measure sea-surface temper ature and height, ocean winds, and chloro phyll, an indication of biological activity on the sea surface. Satellite images reveal features of surface currents, including fluctuations in the Gulf Stream and the birth, evolution, and death of eddies, vortices of water that spin off major currents, often creating islandlike ecosystems for marine organisms. Some eddies are enor mous, more than a hundred miles in diameter and thousands of feet deep and can last for years before dissipating; when they sweep into an area, they can change the weather. But satellites see (Continued on page 108) Itr~ : *rr "" 1I .,r ; '~r .