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The Role of Acoustics in Ocean Observing Systems

Peter Worcester* and Walter Munk*


In the summer of 1940, Walter Munk asked Harald Sverdrup, then the Director of Scripps Institution of Oceanography, if he could “… be admitted for study towards the Ph.D. Degree in Oceanography.” Munk reports that “Harald Sverdrup gave my request his silent attention for an interminable minute, and then said that he could not think of a single job in oceanography that would become available in the next ten years.” World War II (WWII) and the Cold War changed all that.

“Business as usual” is not, from the historical perspective, necessarily what is now considered “business as usual.” This observation is not meant to imply that a return to the prewar environment in which basic oceanographic research was conducted in the U.S. is likely. Two forces have been at work that are leading to significant changes in how oceanographic research is and will be conducted, however.

Since the end of the Cold War nearly 20 years ago, the Congress and President have not had an easy metric to decide how much funding to provide basic science in general and oceanography in particular. As a result, institutional and organizational arrangements that were developed during the Cold War are unlikely to be able to continue unchanged. It is becoming more and more obvious, for example, that oceanography will be unable to continue to support the numbers of individuals funded solely by grants to perform basic research that has been the norm in the

*

Scripps Institution of Oceanography, University of California, San Diego



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The Role of Acoustics in Ocean Observing Systems Peter Worcester* and Walter Munk* In the summer of 1940, Walter Munk asked Harald Sverdrup, then the Director of Scripps Institution of Oceanography, if he could “. . . be admitted for study towards the Ph.D. Degree in Oceanography.” Munk reports that “Harald Sverdrup gave my request his silent attention for an interminable minute, and then said that he could not think of a single job in oceanography that would become available in the next ten years.” World War II (WWII) and the Cold War changed all that. “Business as usual” is not, from the historical perspective, necessar- ily what is now considered “business as usual.” This observation is not meant to imply that a return to the prewar environment in which basic oceanographic research was conducted in the U.S. is likely. Two forces have been at work that are leading to significant changes in how oceano- graphic research is and will be conducted, however. Since the end of the Cold War nearly 20 years ago, the Congress and President have not had an easy metric to decide how much funding to provide basic science in general and oceanography in particular. As a result, institutional and organizational arrangements that were developed during the Cold War are unlikely to be able to continue unchanged. It is becoming more and more obvious, for example, that oceanography will be unable to continue to support the numbers of individuals funded solely by grants to perform basic research that has been the norm in the * Scripps Institution of Oceanography, University of California, San Diego 5

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59 PETER WORCESTER AND WALTER MUNK years since WWII. One implication is that the size of oceanographic insti- tutions performing basic research will shrink. Another implication is that oceanography will have to become ever more involved in undergraduate teaching to ensure hard-money positions. The second force at work is technology. Oceanography is an observa- tional science. Many, if not most, of the advances in our understanding of the ocean depended on the development of the tools needed to make measurements in the ocean environment. A triad of observational tools —satellite altimetry, hydrography performed by profiling floats and glid- ers, and acoustic remote sensing—combined with the development of ever more realistic ocean modeling and data assimilation capabilities, have revolutionized and continue to revolutionize our ability to observe the physical state of the ocean. One possible outcome is that there may well be a shift toward the paradigm of meteorology, if society comes to believe that operational ocean observation systems are justified in terms of products useful to society, as operational atmospheric observational systems are now. The looming specter of anthropogenically-induced climate change may well lead society to this conclusion, although it is not the only aspect of the ocean in which society has a vital interest. The atmosphere and the ocean form a coupled system that determines the climate of the planet. We as a society have a vital interest in measuring how the earth’s climate is chang- ing, regardless of the cause. We also have a responsibility to measure what we are doing to the planet and to assess the effectiveness of any measures that we take to mitigate these impacts. In meteorology, those running operational monitoring and prediction systems are distinct from those performing basic research, with different reward systems. There is already some movement in this direction in oceanography, as is evident in the implementation of the equatorial Tropi- cal Atmosphere Ocean (TAO) project and IOOS. Those running the obser- vational meteorological systems are rewarded for providing data prod- ucts in near-real time, rather than for publishing papers. If oceanography moves in this direction, academic oceanographic institutions will have to decide whether or not to participate in the operation and maintenance of operational, as opposed to research, ocean observing systems. Those choosing to participate in the operation of OOSs, as some already are (at least in part to make up for the inadequacy of other funding sources), will likely find it necessary to modify the academic reward systems currently in place in order to suitably reward those who maintain and operate the observing systems. Given the importance of the ocean to society, it seems that there is a reasonable likelihood that the current movement toward operational ocean observation systems will continue, although perhaps with different

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0 OCEANOGRAPHY IN 2025 degrees of emphasis on the coastal and open oceans. These systems will evolve over time as the effectiveness and costs of various possible ocean measurement technologies become clearer. Of the triad of observational tools available to physical oceanographers, active and passive acoustic systems have not yet been widely applied in ocean sensing systems, however, in spite of the fact that they have capabilities that are difficult or impossible to obtain using other technologies. OCEAN ACOUSTIC THERMOMETRY/TOMOGRAPHY Long-range acoustic transmissions have the ability to measure gyre- scale changes in ocean temperature with unparalleled precision and tem- poral resolution (Figure 1). The acoustic travel times are inherently spa- tially integrating, which suppresses mesoscale variability and provides a precise measure of range- and depth-averaged temperature. The measure- ments are nearly instantaneous and can be made at any desired sampling rate at essentially no additional cost. Unlike other sensors, the acoustic methods are not subject to calibration errors, because the fundamental measurement is one of travel time. Climate change is an important, but not the only, issue. It is neces- sary to differentiate between the ability to measure large scale changes in ocean properties and the ability to measure the smaller-scale variability that impacts ocean ecosystems. Profiling floats, as implemented in the Argo program, form (by design) an incoherent array that is not well suited to providing information on smaller-scale ocean variability. Acous- tic systems may well be able to help fill this gap. The ability of acoustic methods to resolve mesoscale variability with high temporal resolution may well make important contributions to the study of the interactions between the physical and biological environments. After a century of measuring what turned out to be ocean climate, physical oceanographers are finally developing the tools, including acoustic methods and gliders, needed to measure ocean variability on the time and space scales impor- tant to ocean ecosystems. TRACKING OR UNDERWATER GPS Relatively crude acoustic tracking of neutrally buoyant floats (Sound Fixing And Ranging [SOFAR] and RAFOS) is being and has been used extensively to make Lagrangian measurements of ocean circulation. Mod- ern tracking systems using broadband signals can provide much higher accuracy positioning not just for neutrally buoyant floats, but for gliders, AUVs, and profiling floats. The potential benefits might well rival those provided by GPS.

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1 PETER WORCESTER AND WALTER MUNK –0.10 WOA05 600 Travel Time (ms) 300 0 0 T(°C) –300 –600 0.10 OA JPL–ECCO POP 1998 1999 2000 2001 2002 2003 2004 2005 2006 FIGURE 1 Measured travel times for transmissions from an acoustic source near Worcester_Fig1.eps Kauai to a bottom-mounted receiver approximately 3500 km distant to the north- west of Kauai (bold) are compared with travel times derived from four inde- pendent estimates of the North Pacific (light): (i) climatology, as represented by the World Ocean Atlas 2005 (WOA05), (ii) objective analyses of the upper ocean temperature field derived from satellite altimetry and in situ profiles (OA), (iii) an analysis provided by the Estimating the Circulation and Climate of the Ocean project as implemented at the Jet Propulsion Laboratory (JPL-ECCO), and (iv) simulation results from a high-resolution configuration of the Parallel Ocean Program (POP) model. The time means have been removed from all of the time series. The nominal travel-time trend corresponding to a warming of 5 m°C per year on the sound-channel axis is also shown (straight line). Approximate ray- average temperature perturbations corresponding to the measured travel-time perturbations are shown on the right-hand axis. MONITORING OF UNDERWATER SOUND Monitoring ocean sound is valuable for a variety of reasons, rang- ing from detection of underwater earthquakes, to detecting clandestine explosive tests (e.g., Comprehensive Nuclear-Test-Ban Treaty Organiza- tion [CTBTO] hydrophone arrays), to measuring wind and rain, to long-

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2 OCEANOGRAPHY IN 2025 term monitoring of biological processes (cetaceans, fish, invertebrates), to determining trends in the “climate” of ocean sound levels. OCEAN MODELS, DATA ASSIMILATION, AND OPERATIONAL OCEANOGRAPHY Ocean acoustic tomography has been successfully used to measure ocean temperatures and currents on a wide variety of scales for thirty years, but acoustic data still seem unfamiliar to many oceanographers. As the normal use of ocean data becomes assimilated into ocean models to estimate the present (and future) state of the ocean, acoustic data will be placed on an equal footing with other data. The issue will become the abil- ity of the various measurements to effectively constrain ocean models. There also seems to be a perception that acoustic methods are inor- dinately expensive. Oceanography has not in the past been sensitive to the issue of life-cycle versus capital costs, as this issue does not arise in short-term, process-oriented basic research. Life cycle costs are critically important in gauging operational observation systems, however. Con- sideration of true life-cycle costs has the potential to make systems with higher capital costs, but lower operational and maintenance costs, such as acoustics, more attractive. My conclusion is that given the unique capa- bilities of acoustic sensing systems and their relatively low operational costs, it would seem that the wider application of acoustic methods to measuring the ocean will ultimately be inevitable.