Research

After World War II Munk helped to analyze the currents, diffusion, and water exchanges at Bikini Atoll in the South Pacific, where the United States was testing nuclear weapons.

Wind driven gyres
Munk pioneered research on the relationship between winds and ocean circulation, coining the now widely used term “wind-driven gyres.”[8]

Rotation of the earth
Munk was the first to show rigorously why one side of the moon always faces the earth (Munk and McDonald, 1960; and later papers up to 1975), a phenomenon known as tidal locking. Lord Kelvin had also considered this question, and had fashioned a non-quantitative answer being roughly correct. The moon does not have a molten liquid core, so cannot rotate through the egg-shaped distortion caused by the Earth’s gravitational pull. Rotation through this shape requires internal shearing, and only fluids are capable of such rotation with small frictional losses. Thus, the pointy end of the “egg” is gravitationally locked to always point directly towards the earth, with some small librations, or wobbles. Large objects may strike the moon from time to time, causing it to rotate about some axis, but it will quickly stop rotating. All frictional effects from such events will also cause the moon to regress further away from the earth.

In the 1950s, Munk investigated irregularities in the Earth’s rotation, such as the Chandler wobble and annual and long-term changes in the length of day (rate of the Earth’s rotation), to see how these were related to geophysical processes such as the changes in the atmosphere, ocean, and core, and the energy dissipated by tidal acceleration. He also investigated how western boundary currents, such as the Gulf Stream, dissipated planetary vorticity. His inviscid theory of these currents did not have a time invariant solution; no simple solution to this problem has ever been found.[citation needed]

Project Mohole
In 1957, Munk and Harry Hess suggested the idea behind Project Mohole: to drill into the Mohorovicic Discontinuity and obtain a sample of the Earth’s mantle. While such a project was not feasible on land, drilling in the open ocean would be more feasible, because the mantle is much closer to the sea floor. Initially led by the informal group of scientists known as the American Miscellaneous Society (AMSOC, including Hess, Maurice Ewing, and Roger Revelle),[3] the project was eventually taken over by the National Science Foundation (NSF). Initial test drillings into the sea floor led by Willard Bascom occurred off Guadalupe Island, Mexico in March and April 1961.[9] The project became mismanaged and increasingly expensive after Brown and Root won the contract to continue the effort, however, and Congress discontinued the project toward the end of 1966.[10] While Project Mohole was not successful, the idea led to projects such as NSF’s Deep Sea Drilling Program.[11]

Ocean swell
Starting in the late 1950s Munk returned to the study of ocean waves, and, thanks to his acquaintance with John Tukey, he pioneered the use of power spectra in describing wave behavior. This work culminated with an experiment that he led in 1963 to observe waves generated by winter storms in the Southern Hemisphere and traveling thousands of miles throughout the Pacific ocean. To trace the path and decay of waves as they propagated northward, he established stations to measure waves from islands and at sea (on R/P FLIP) along a great circle from New Zealand to Alaska. The results showed little decay of wave energy with distance traveled.[12] This work, together with the wartime work on wave forecasting, led to the science of surf forecasting, one of Munk’s best-known accomplishments.[5] Munk’s pioneering research into surf forecasting was acknowledged in 2007 with an award from the Groundswell Society, a surfing advocacy organization.[13][14]

Ocean tides
Between 1965 and 1975 Munk turned to investigations of ocean tides, being partially motivated by their effects on the Earth’s rotation. Modern methods of time-series and spectral analysis were brought to bear on tidal analysis, leading to the development of the “response method” of tidal analysis.[15] With Frank Snodgrass, Munk developed deep-ocean pressure sensors that could be used to provide tidal data far from any land.[2][16] One highlight of this work was the discovery of the semidiurnal amphidrome midway between California and Hawaii.[17]

Ocean acoustic tomography
Main article: Ocean acoustic tomography
Beginning in 1975, Munk and Carl Wunsch of the Massachusetts Institute of Technology pioneered the development of acoustic tomography of the ocean.[18] With Peter Worcester, Munk developed the use of sound propagation, particularly sound arrival patterns and travel times, to infer important information about the ocean’s large-scale temperature and current. This work, together with the work of other groups,[19] eventually motivated the 1991 “Heard Island Feasibility Test”, to determine if man-made acoustic signals could be transmitted over antipodal distances to measure the ocean’s climate. The experiment came to be called “the sound heard around the world.” During six days in January 1991, acoustic signals were transmitted by sound sources lowered from the M/V Cory Chouest near Heard Island in the southern Indian Ocean. These signals traveled half-way around the globe to be received on the east and west coasts of the United States, as well as at many other stations around the world.[20] The follow-up to this experiment was the 1996–2006 Acoustic Thermometry of Ocean Climate (ATOC) project in the North Pacific Ocean,[21][22] which engendered considerable public controversy concerning the effects of man-made sounds on marine mammals.[23][24][25]

Tomography has come to be a valuable method of ocean observation,[26] exploiting the characteristics of long-range acoustic propagation to obtain synoptic measurements of average ocean temperature or current. Applications have included the measurement of deep-water formation in the Greenland Sea in 1989, [27] measurement of ocean tides, [28] [29] and the estimation of ocean mesoscale dynamics by combining tomography, satellite altimetry, and in situ data with ocean dynamical models.[30] In addition to the decade-long measurements obtained in the North Pacific, acoustic thermometry has been employed to measure temperature changes of the upper layers of the Arctic ocean basins,[31] which continues to be an area of active interest. [32] Acoustic thermometry was also recently been used to determine changes to global-scale ocean temperatures using data from acoustic pulses sent from one end of the earth to the other.[33] [34]

Tides and mixing
In recent years Munk has returned to the work on the role of tides in producing mixing in the ocean.[35] In a 1966 paper “Abyssal Recipes”, Munk was one of the first to quantitatively assess the rate of mixing in the abyssal ocean in maintaining oceanic stratification.[36] According to Sandström’s theorem (1908), without the occurrence of mixing in the abyssal ocean, such as may be driven by internal tides, most of the ocean would become cold and stagnant, capped by a thin, warm surface layer.

Munk has also recently focused on the relation between changes in ocean temperature, sea level, and the transfer of mass between continental ice and the ocean.