120
Walter Munk
capture some of the spirit underlying the developments and discoveries of the last
half-century.
THE MESOSCALE REVOLUTION
Ocean acoustic monitoring was introduced in the 1970s in direct response to the
demonstration by the Mid-Ocean Dynamics Experiment (MODE) that the kinetic
energy of the ocean circulation is mostly associated with variability on a relatively
small scale, the mesoscale (order 100 km). The general circulation (time average)
of the major ocean gyres contains only a small fraction, perhaps 1%, of the kinetic
energy.
The classical physical oceanographers cast their Nansen bottles and contoured
dynamic heights, so that these would be available for computing geostrophic currents, which are then published on permanent charts. Oceanographic vessels operating singly in the expedition mode were unable to cope with the spatial and temporal
sampling requirements imposed by the newly discovered mesoscale variability. Expedition ships were unwilling (those under sail were unable) to stand still; the tradition
“never to occupy a station twice” prevailed for over a hundred years. Textbooks by
Kr¨ ummel
2 (1911), Sverdrup
3 et al. (1942), and Defant
4 (1945, 1961) all reflected the
picture of a steady-state ocean. I was brought up in this tradition.
Satellites came just in time to provide the required sampling strategy for the
mesoscale variability: global sampling with good spatial and adequate temporal resolution. These are three quite separate considerations, equally fundamental to the
subsequent development of physical oceanography. But the interior ocean is opaque
to electromagnetic radiation, yet transparent to sound. In the mid-1970s Carl Wunsch
and I started thinking in terms of an acoustic system to fill this gap (Figure 8.1).
EARLY ACOUSTIC RUMBLINGS
The SOFAR channel (Sound Fixing And Ranging) was discovered by Ewing and
Worzel in 1944. Soundspeed increases with both temperature and pressure, thus maintaining a minimum in soundspeed (and attendant waveguide) between the warm surface waters and the high-pressure abyss. Ewing and Worzel worked out the expected
characteristics of the sound propagation, and then set about to prove the theory.
Saluda departed Woods Hole with a deep hydrophone hung over the side. A second
ship dropped 4-pound charges at distances up to 900 nautical miles. In the words of
Ewing and Worzel, “the end of the . . . transmission was so sharp that it was impossible for the most unskilled observer to miss it.” They spoke even then of transmission
over 10,000 miles.
5
Two years later the Russian acoustician Leonid Brekhovskikh independently
discovered the ocean acoustic waveguide (personal communication): “Some work had
Walter Munk
capture some of the spirit underlying the developments and discoveries of the last
half-century.
THE MESOSCALE REVOLUTION
Ocean acoustic monitoring was introduced in the 1970s in direct response to the
demonstration by the Mid-Ocean Dynamics Experiment (MODE) that the kinetic
energy of the ocean circulation is mostly associated with variability on a relatively
small scale, the mesoscale (order 100 km). The general circulation (time average)
of the major ocean gyres contains only a small fraction, perhaps 1%, of the kinetic
energy.
The classical physical oceanographers cast their Nansen bottles and contoured
dynamic heights, so that these would be available for computing geostrophic currents, which are then published on permanent charts. Oceanographic vessels operating singly in the expedition mode were unable to cope with the spatial and temporal
sampling requirements imposed by the newly discovered mesoscale variability. Expedition ships were unwilling (those under sail were unable) to stand still; the tradition
“never to occupy a station twice” prevailed for over a hundred years. Textbooks by
Kr¨ ummel
2 (1911), Sverdrup
3 et al. (1942), and Defant
4 (1945, 1961) all reflected the
picture of a steady-state ocean. I was brought up in this tradition.
Satellites came just in time to provide the required sampling strategy for the
mesoscale variability: global sampling with good spatial and adequate temporal resolution. These are three quite separate considerations, equally fundamental to the
subsequent development of physical oceanography. But the interior ocean is opaque
to electromagnetic radiation, yet transparent to sound. In the mid-1970s Carl Wunsch
and I started thinking in terms of an acoustic system to fill this gap (Figure 8.1).
EARLY ACOUSTIC RUMBLINGS
The SOFAR channel (Sound Fixing And Ranging) was discovered by Ewing and
Worzel in 1944. Soundspeed increases with both temperature and pressure, thus maintaining a minimum in soundspeed (and attendant waveguide) between the warm surface waters and the high-pressure abyss. Ewing and Worzel worked out the expected
characteristics of the sound propagation, and then set about to prove the theory.
Saluda departed Woods Hole with a deep hydrophone hung over the side. A second
ship dropped 4-pound charges at distances up to 900 nautical miles. In the words of
Ewing and Worzel, “the end of the . . . transmission was so sharp that it was impossible for the most unskilled observer to miss it.” They spoke even then of transmission
over 10,000 miles.
5
Two years later the Russian acoustician Leonid Brekhovskikh independently
discovered the ocean acoustic waveguide (personal communication): “Some work had