Page 40 - 2016Spring

Walter Munk – 75 Years in Oceanography
Walter has modestly written about himself: “During my ca- reer I have worked on rather too many topics to have done a thorough job on any one of them; most of my papers have been superseded by subsequent work. But ‘definitive papers’ are usually written when a subject is no longer interesting. If one wishes to have a maximum impact on the rate of learn- ing, then one needs to stick out one’s neck at an earlier time. Surely those who first pose a pertinent problem should be given some credit, not just criticized for having failed to pro- vide a final answer.”
The work on waves conducted during the war years was fol- lowed by theoretical studies of wind-driven ocean gyres and then on Earth’s wobble and spin in the 1950s. Irregularities in the Earth’s rotation are an elegant remote sensing tool from which one can infer information about Earth’s core, its air and water masses, and global winds. The notion of re- mote sensing has been fundamental to Walter’s science, and it is no surprise that remote sensing is the basis for Walter’s major contributions to ocean acoustics—the use of under- water sound to measure physical ocean parameters such as tides, currents, and heat content (Munk, 2006).
Walter spent a decade working on ocean swell (1955-65) and another (1965-75) on ocean tides. His work on swell is remi- niscent of his later work on ocean acoustics in that both are of global dimensions, both involved global instrument ar- rays, and both involved close collaboration with engineers. Aerial photographs taken off the California coast during the war had revealed regular swells that Walter calculated (taking into account the change of direction caused by wave refraction in shallow water) were coming from the south- southwest (SSW) and, incredibly (at that time), must have been the result of winter storms in the southwest Pacific. Furthermore, it appeared that the waves were coming from a source either in or beyond the window between Antarc- tica and New Zealand, some 5,000 miles away. He and Frank Snodgrass, a talented engineer with whom Walter collabo- rated for 25 years, installed a global array of six wave stations between New Zealand and Alaska to track the waves. The data confirmed that waves off the California coast can in- deed originate in Southern Ocean storms and that some may even start in the Indian Ocean some 10,000 miles distant, halfway around the world.
In the mid-1970s, as a result of serving on the JASON com- mittee (an independent scientific advisory group to the Department of Defense named after a hero in Greek my- thology), which was working an anti-submarine warfare
problems, it was our luck that Walter was lured into the world of underwater sound. The Navy was concerned about the stability of underwater sound transmissions, which ex- hibited fluctuations in amplitude and phase that negatively affected the performance of sonar systems. Walter’s work on the effects of internal waves on ocean microprocesses nat- urally led to considering their effect on fluctuations in the speed of sound. It was perfect for Walter—a new problem, an important one, and one that no one had tackled before.
Characteristically, Walter entered what some had thought was a mature field—after all, ocean acoustics had been stud- ied to death since WWII—turned it upside down, complete- ly upsetting much conventional wisdom (perhaps because he doesn’t like to read and wasn’t encumbered by prevail- ing ideas?), and generated an excitement that persists today. Through two major books (Flatté et al., 1979; Munk et al., 1995), numerous publications in the Journal of the Acous- tical Society of America (JASA) and other journals, invited lectures worldwide, and the education of a generation of students, Walter has fundamentally changed the field by the rigorous introduction of oceanography into underwa- ter acoustics. More than anyone, he has revolutionized our understanding of the intrinsic role oceanography plays in ocean acoustics. His name is associated with the invention of ocean acoustic tomography and ocean acoustic thermom- etry, with the eponymous Munk sound-speed profile, and with the Garrett and Munk internal wave spectrum.
In 1976, as part of a forerunner experiment to ocean acous- tic tomography, one of us (PW), then Walter’s graduate stu- dent, deployed two acoustic transceivers about 25 km apart and measured the differential travel time of acoustic pulses transmitted in opposite directions, thus providing a measure of the average current between the instruments (Worcester, 1977). In this case, transmission in one direction is aided by the current; in the opposite direction it is opposed by the current. This led Walter, together with Carl Wunsch of MIT, to propose the idea of ocean acoustic tomography (Figure 2) wherein acoustic transmissions between multiple acous- tic sources and receivers, or transceivers, would effectively produce a tomographic cross-section of the enclosed ocean volume (Munk and Wunsch, 1979).
The spatial distribution of sources and receivers provides horizontal resolution, while the up-down cycling of sound paths in the sea provides vertical resolution. The initial idea was to measure the ocean mesoscale, features of roughly 100 km dimension that account for most of the kinetic energy
38 | Acoustics Today | Spring 2016