National Academies Press: OpenBook

An Ocean Climate Research Strategy (1984)

Chapter: THE WORLD OCEAN CIRCULATION EXPERIMENT

« Previous: THE INTERANNUAL VARIABILITY OF THE TROPICAL OCEAN AND THE GLOBAL ATMOSPHERE
Suggested Citation:"THE WORLD OCEAN CIRCULATION EXPERIMENT." National Research Council. 1984. An Ocean Climate Research Strategy. Washington, DC: The National Academies Press. doi: 10.17226/19384.
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Suggested Citation:"THE WORLD OCEAN CIRCULATION EXPERIMENT." National Research Council. 1984. An Ocean Climate Research Strategy. Washington, DC: The National Academies Press. doi: 10.17226/19384.
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Suggested Citation:"THE WORLD OCEAN CIRCULATION EXPERIMENT." National Research Council. 1984. An Ocean Climate Research Strategy. Washington, DC: The National Academies Press. doi: 10.17226/19384.
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Suggested Citation:"THE WORLD OCEAN CIRCULATION EXPERIMENT." National Research Council. 1984. An Ocean Climate Research Strategy. Washington, DC: The National Academies Press. doi: 10.17226/19384.
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Suggested Citation:"THE WORLD OCEAN CIRCULATION EXPERIMENT." National Research Council. 1984. An Ocean Climate Research Strategy. Washington, DC: The National Academies Press. doi: 10.17226/19384.
×
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Suggested Citation:"THE WORLD OCEAN CIRCULATION EXPERIMENT." National Research Council. 1984. An Ocean Climate Research Strategy. Washington, DC: The National Academies Press. doi: 10.17226/19384.
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Suggested Citation:"THE WORLD OCEAN CIRCULATION EXPERIMENT." National Research Council. 1984. An Ocean Climate Research Strategy. Washington, DC: The National Academies Press. doi: 10.17226/19384.
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THE WORLD OCEAN CIRCULATION EXPERIMENT We must understand the global oceanic circulation to understand the role of the ocean in maintaining the present climate state and in influencing climate variability. Without this knowledge we are unlikely to be able to predict future climate variations. A large-scale oceanographic experiment to examine global ocean circulation and ocean climate processes is being proposed. The World Ocean Circulation Experiment (WOCE) will be directed at describing the circulation of the ocean, defining the linking physical processes in the ocean- atmosphere climate system, and understanding the sensitivity of that system to forcing by changes in the atmosphere. THE GENERAL CIRCULATION OF THE OCEAN The role of the ocean in climate change can be well understood only if we understand the present climate state of the ocean itself, including its circulation and the transient processes with which it interacts. Thus we need reliable global observations of the state of the ocean and of the changes of that state with time. Reliable observations of a natural system can only be made if the variability is first understood. This variability will govern the sampling procedures needed to obtain a representative description. If the sampling procedures are improperly designed, the processes that we are trying to observe could be overlooked or misidentified. Our current description of the ocean, based on decades of observations, may be adequate in some places and for some oceanic processes. In many places, however, our description is not adequate, and the classic set of oceanographic measurements does not define all of the processes occurring in the ocean that are important in the climate system. Recent oceanographic studies have exposed the existence of a number of processes that could be important to the ocean's role in climate variability: mesoscale eddies, tropical waves, isopycnal mixing, the seasonal variation of the mixed layer, microscale mixing in the interior of the 20

21 ocean. We have also developed computer models of the large- scale ocean circulation. The models underline the importance of some of the processes just listed. Thus, to observe and understand the climate of the ocean, we need to describe the small-scale processes in the ocean in enough detail to model them realistically. A major obstacle to obtaining observations of the ocean is the difficulty of obtaining measurements over long time scales and over great distances. But recent technical developments and new means of making measurements have made it feasible to consider carrying out a global experiment to understand the role of ocean circulation in climate. Orbiting satellites give promise of regular global measurements of sea-surface temperature, surface currents, and the wind stress on the sea surface. If these observations are combined with subsurface remote sensing, it may be possible to develop a description of the ocean that, for the first time, would begin to be as complete as our description of the atmosphere. The idea of a world ocean circulation experiment has encountered some resistance. Physical oceanographers have a tradition of research programs that are regional in scope. Some of them are suspicious of any program with "world" or "global" in its title. The personal cost of working with others in a large program can be great. Some oceanographers who have tried it prefer to avoid the hassles and stick to programs that are small enough that they can retain control of their lives. Some meteorologists are suspicious of a world ocean circulation experiment because they view it as an attempt to use the World Climate Research Program (WCRP) as a justification for large-scale oceanographic research that may not have direct links to climate. In spite of these hesitations, the consensus is that understanding global ocean circulation is essential to understanding climate. A global program will complement regional oceanographic programs and will provide observations that will benefit them. The climate state of the ocean is as much a part of global climate as the climate state of the atmosphere. Certainly, the atmosphere has more direct impact on humans and their activities than does the ocean. However, if we want to understand how the climate system works, we cannot continue to treat the climate of the ocean as secondary, as something that can simply be parameterized or treated with bulk formulas. The challenge, then, is to define the global ocean experiment that is justified by the needs of the climate

22 research program and that will provide the understanding of the ocean that we so critically need. An unresolved question is that of the depth to which the ocean interacts with the atmosphere on time scales relevant to climate variability. On one hand, it is often assumed (mostly by meteorologists) that the upper 100 to 300 m of the ocean is sufficient to define the sea-air interactions, at least on time scales shorter than decades. On the other hand, some (oceanographers) contend that this is not sufficient. They believe that all of the ocean needs to be taken into account in order to understand climate variability on scales of a year or longer. Quantitative estimates of the depth of the ocean interacting with the atmosphere are hard to come by. One measure uses the time it would take to warm the ocean in the case, for example, of the global warming that might be produced by rising levels of carbon dioxide in the atmosphere. Thermal time constants were calculated (Climate Board, 1982) using downward mixing modified by the changing sea-surface equilibrium as the ocean warms. The results show that for a new equilibrium temperature 3*C higher, the ocean would warm to a depth of 50 m in 3 years, to 500 m in 30 years, and to 5000 m in 300 years. This type of calculation has been cited as evidence that climate models need only look at the upper 200 m of the ocean for time scales shorter than decades. Those that counter this type of calculation point out that mixing downward is not uniform. Tritium measurements, for example (Ostlund et al., 1976, Figure 2), show mixing down to 5000 m at high latitudes in the North Atlantic within a few years. THE WOCE PROPOSAL A stated objective of WOCE is "to describe and understand quantitatively the general circulation of the ocean, in order to assess within the WCRP the sensitivity of the climate system to changes in external forcing, whether natural or anthropogenic, on time scales of decades to centuries" (CCCO-IV, 1983). The proposal for WOCE has three types of scientific objectives: 1. To describe the general circulation of the ocean. 2. To understand the rates and processes of water-mass transformation. 3. To describe the spectrum of seasonal and broad-band ocean variability.

23 The WOCE Scientific Steering Group has been charged (CCCQ-IV, 1983) with developing a clear description of the WOCE observational and modeling program. WOCE objectives have yet to be specifically defined, and the existing documents give only a general indication of what WOCE will consist of. Curiously, the Tokyo Study Conference in its definition of WOCE observational needs does not highlight the need for determining the general circulation of the ocean. Nor does it emphasize the need for such techniques as global satellite altimetric measurements of ocean surface currents. This oversight was corrected at CCCO-IV (1983, Appendix III), and the primary observational work defined for WOCE includes studies of the following: 1. The global circulation of the ocean. 2. The fields of surface forcing: wind stress, net surface heat flux, and net surface moisture flux. 3. Oceanic temperature and salinity distributions. 4. The statistics of mesoscale eddies in order to characterize lateral mixing. 5. Determination of ventilation times and water-mass conversion processes. 6. Large-scale surveys of signatures of water-mass conversion. 7. Large-scale aspects of the seasonal cycle. 8. Broad-band inherent variability. The observations needed for this list of studies include measurements of wind and wind stress, sea level, solar radiation, diffuse attenuation coefficients, and total precipitable water. Also needed are deep density sections, tracer samples, and extensive in situ temperature, salinity, and velocity measurements. To provide the basis of knowledge to understand the state of the ocean, we must describe the mean circulation of the ocean over several years as well as the space-time variability on time scales of months to years. This might in part be done as a global experiment lasting 5 to 10 years. In addition, special studies could focus on processes that would elude an experiment of this duration. It is evident from the above sunmary that we are not yet fully beyond the shopping-list stage. There is broad agreement that the circulation of the ocean has a strong influence on climate and therefore must be understood. The next step must be to define the circulation measurements that are needed in quantitative terms. A WOCE workshop, to

24 be held in the summer of 1983 under the auspices of the National Research Council, should be an important step in clarifying plans for the U.S. component of WOCE. Today's sketchy WOCE plans should mature into a critical component of the climate research program. A WOCE STRATEGY FOR NSF WOCE planning has not yet begun in detail, and the documents so far prepared (CCCO-III, 1982; WOCE Design Options Study Group, 1982; CCCO-IV, 1983) do not define what WOCE will be. There is, in fact, still some ambiguity about the primary objective: is it to examine global oceanic and atmospheric processes or is it to describe global ocean circulation? This state of affairs is probably normal at this stage of planning for a large undertaking. The uncertainty in the definition of WOCE points to the work that remains to be done in developing a large-scale ocean experiment. NSF may wish to support some research to aid in planning for WOCE. A large-scale commitment to WOCE should await a definition of what the experiment comprises and an international agreement on the scope of the program. WOCE is intended to be a global experiment, and hence international agreements are essential to its planning. NSF should note that a large-scale WOCE is not necessary for some of the observational studies listed in the previous section. These might be carried out as separate projects whether or not WOCE is developed. The stated WOCE objectives have some similarities to those that have been presented for Cage (see the next chapter). The proposed mode of implementation is not the same, however: WOCE proponents talk of generic studies, while proponents of heat flux experiments (sometimes the same individuals) have advocated specific estimates of heat budgets. Nevertheless, both involve estimating surface fluxes of heat and moisture, determining water-mass conversion processes, and measuring solar radiation and atmospheric flux divergences. Thus the recent decision (CCCO-IV, 1983) to undertake heat flux estimates as part of the precursors to and observations for the main WOCE experiment is a logical step in planning. NSF should anticipate further sorting-out of WOCE (and heat flux) objectives. The component generic studies ought to be more fully defined and defended. Are all equally necessary? If we were to support a sequence of studies as

25 part of WOCE or as a precursor to WOCE, how should that sequence be ordered? That is, where should we begin? NSF should seek to have these questions resolved before making a full-scale commitment to WOCE, not because WOCE is of uncertain value, but simply because many of the basic steps in program definition have yet to be taken. WOCE is still young. The ideas and objectives need to mature. A common thread in many WOCE component studies is an earth-orbiting satellite that measures sea-surface elevation by altimetry and surface wind stress by scatterometry. Such a satellite (TOPEX Science Working Group, 1981) would provide a framework for a broad range of climate studies. Sea-surface elevation can define the field of surface geostrophic currents. With complementary measurements, such as of the density field in the interior of the ocean, the circulation of the ocean might be determined. Drifting and fixed buoys could also provide complementary measurements. An intriguing possibility is to combine satellite observations of altimetry and wind stress with ocean acoustic tomography (Munk and Wunsch, 1982) as a means to provide an ocean- observing system. This might be a major step in providing the kind of synoptic information in the ocean that we have long taken for granted in the atmosphere. Proposals within NASA for an altimetric satellite have not been accepted so far by the administrator. This may in part be due to lack of a perceived consensus need for such a satellite. It may be that WOCE can go ahead without a U.S. oceanic topographic satellite. The European Space Agency is planning a SEASAT-like satellite that will measure altimetry, to be launched in 1987. Japan may launch a satellite with an altimeter in 1990. The precision of these satellites may not be as great as that proposed for a U.S. altimetric satellite (TOPEX), but they could allow WOCE to proceed. If WOCE goes ahead without a U.S. satellite for sea- surface elevation and wind stress, U.S. oceanographers could be at a disadvantage. The policies of the foreign space agencies with regard to data availability are not clear, but it is possible that U.S. researchers might have to wait to obtain access to the data until after foreign scientists have had the right to first use. Satellite altimetry and scatterometry are essential for WOCE, for ocean climate monitoring, possibly for heat flux studies, and possibly for TOGA. NSF thus should note the vital need for an earth-orbiting satellite to provide information on surface ocean currents and the wind stress on the sea surface for future ocean and climate research. NSF

26 senior management should discuss this issue with NASA and NOAA. If we are to proceed with large-scale ocean experiments in the next decade, we will soon need to make commitments for satellites to support those experiments. NSF should be aware of what NASA is likely to do in order to make its own plans. INTERNATIONAL INVOLVEMENT IN WOCE WOCE is in need of technical definition and planning. Its present state of development is insufficient to define the activities or the level of support that will be needed. We need to establish an international program office that can plan and that can deal with the issues on a technical level. Adding to existing secretarial activities or creating an office that is part of a secretariat (like that of the Intergovernmental Oceanographic Commission) will likely not result in a group that is technically capable of scientific and technical planning.

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