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THE ROLE OF THE OCEAN IN CLIMATE This report is principally concerned with understanding the role of the ocean in interannual climate variability. That is, what are the mechanisms, if any, by which the ocean influences year-to-year variations in the earth's climate? Does the ocean play a role in producing climate anomalies, such as droughts, floods, heat waves, and abnormal frosts? If it does, can we understand the processes whereby this occurs? Can we develop a capability for predicting climate change? This chapter provides a review of what is known of the role of the ocean in the earth's climate system. The ocean influences the mean climate state of the earth. There is growing evidence that the ocean has, as well, a major influence on climate variations. This review will not be exhaustive, since many existing documents and papers present the scientific background. This chapter simply sets the stage for the rest of the report with a review of the scientific basis for studies of climate variability and the role of the ocean. The list of references will provide guidance to the extensive literature. Terms such as "climate state" and "climate variability" are used throughout this report. These terms have been defined by the U.S. Committee for the Global Atmospheric Research Program (1975) and are reproduced here in Appendix B. MEAN CLIMATE STATE The ocean plays a major role in determining the mean climate state of the world. It covers 70 percent of the surface of the earth and is a source of moisture for the atmosphere. As such, it is critical in controlling global patterns of precipitation and evaporation. The heat capacity of the ocean is huge. The upper 3 m of the ocean can contain as much heat as the entire atmosphere. The ocean absorbs energy from the sun and releases energy to the atmosphere at times and places distant from the point where the energy was received. The
seasonal temperature range is reduced over land areas adjacent to the ocean because of the large heat inertia of the ocean. The poleward flux of heat in the ocean is of the same order of magnitude as that in the atmosphere, but the processes of the oceanic transport are not well understood. Any attempt to understand the mean climate state of the world must take account of the role of the ocean in establishing and maintaining the global heat balance. The mean climate state of the ocean is not now well understood. Unless we can define oceanic variability in terms of its departure from some mean state, we may be unable to explain the role of the ocean in maintaining or modifying global climate. CLIMATE VARIABILITY Both ocean and atmosphere show climate variability on time scales of months to centuries. The annual or seasonal cycle is generally large, but the nonseasonal variability can exceed the seasonal in some regions, particularly in some oceanic areas. Ocean heat storage, transport, and transfer to the atmosphere are variable. It may be that such variations are the principal oceanic factor controlling climate variability. Thus an understanding of the uptake, transport, storage, and release of heat by the ocean may lead to an understanding of global climate variations. This report reviews plans for large-scale ocean heat flux experiments, such as Cage. Models of the atmosphere with and without a moving ocean show that oceanic effects influence the mean atmospheric temperature distribution (Manabe and Bryan, 1969; Spelman and Manabe, 1983). The circulation of the ocean appears to affect climate variability on all scales. Thus there are proposals to study the general circulation of the ocean and the climate state of the ocean. In this report the proposal for a World Ocean Circulation Experiment (WOCE) is reviewed. Ocean heat transport and storage processes have lifetimes that are long in comparison with those of atmospheric processes. Atmospheric predictability may inherently be limited to a week or two. But the chain of events involved with the Southern Oscillation, a global- scale atmospheric and oceanic climatic anomaly, has a duration of about 18 months. Though the ocean and
atmosphere interact, this long time scale seems to be dominated by the high thermal and mechanical inertia of the ocean. Thus long-range climate forecasting probably must take ocean processes into account. The largest nonseasonal variable climate signal is the interannual, which, particularly in some tropical regions, may be larger than the annual or seasonal signals. Year-to- year variations in the earth's climate are of great economic importance. Effects such as unusual rainfall, drought, or heat waves can have significant agricultural impacts. Oceanic thermal variations related to climate variability can affect marine fisheries. Thus, there are economic incentives to seek to develop a predictive ability. Variations in climatic conditions can affect marine fisheries. Changes in temperature, light, and ocean currents can affect such factors as the reproduction, feeding, and location of fish in the sea. Regions of advection, upwelling, and convergence can be correlated with the abundance, or lack thereof, of fish. The subject of climate and its impact on the living resources of the sea is scientifically and economically interesting and deserves further study. It is, however, not directly within the scope of this study. As has already been mentioned, some interannual climate signals are particularly strong in the tropical ocean. The strongest of these signals, known best as the Southern Oscillation and discussed in the next chapter, is an interannual atmospheric and oceanic signal that has global dimensions (Wright, 1978). Sea-surface temperature anomalies, changing wind patterns, excessive precipitation, and continental cold spells are among the manifestations that sometimes are associated with the Southern Oscillation. In this report, plans are reviewed for a study of this phenomenon and the interaction of the tropical ocean and the global atmosphere (the TOGA study). Climate variations having scales of approximately a decade are known to exist but are less well documented than those of annual time scale. Their economic effects are also less well documented, but they can be important. The "dust- bowl years" were possibly a result of decadal climate variations. There is evidence that the ocean plays a role in decadal climate variability, and some proposals for large- scale ocean experiments to understand decadal variability have been made. The consensus seems to be that to develop a predictive capability, research on interannual climatic phenomena should have first priority. At this time, no
plans for ocean studies explicitly directed to decadal climate scales have emerged in the World Climate Research Program (WCRP). Long-period climate variability, having time scales between decades and centuries, is not well documented (Hecht, 1981). The study of such phenomena obviously requires a long-term commitment. The economic impact is uncertain. It is even unclear how we would today make use of the knowledge of long-term climatic variation if it were available. CLIMATE CHANGE The paleoclimatic record shows evidence of climate change over the millennia. In addition to these natural effects, the climate may be changing as a result of man's activities. Such inadvertent modification of climate has received considerable public attention. In particular, the climatic impact of changing carbon dioxide levels has been the subject of extensive discussion. The issue of possible changes in the earth's climate as a result of man-induced changes in the level of carbon dioxide has been and is being reviewed elsewhere (Climate Board, 1982; Carbon Dioxide Assessment Committee, 1983) and is outside the scope of this study. A major scientific problem is that of distinguishing anthropogenic climate changes from those that would have happened naturally. The ocean must be considered in any examination of the carbon dioxide question. The large thermal inertia of the ocean will affect the response of the atmosphere to warming. Thus the actual warming at any time could be less than that calculated on the assumption that thermal equilibrium is quickly reached. The ocean will, as well, absorb carbon dioxide and provides a long-term geochemical buffering of the atmospheric carbon dioxide. Ocean warming could melt polar ice and trigger feedback effects that in turn could modify the atmospheric thermal response. The conclusion of the Climate Board (1982) is that "the role of the ocean in time-dependent climatic response deserves special attention in future modeling studies, stressing the regional nature of oceanic thermal inertia and atmospheric energy transfer mechanisms." Carbon dioxide is not the only radiatively important trace substance that can affect the earth's radiation, and hence heat, balance (Chamberlain et al., 1981). Substances such as ozone, oxides of nitrogen, and water vapor can have
10 an effect similar to that of carbon dioxide. The total influence of all these other substances could be as great as that of carbon dioxide, though the requisite studies to determine the actual effects have yet to be made. Again, the ocean plays a role in global storage and transport. Sediments and remains of living organisms on the sea floor can give us a record of past climates. The study of paleo-oceanography is an important component of the study of climate variations on scales of centuries or longer (Geophysics Study Committee, 1982). These investigations, however, are generally not directly related to the oceanic processes involved in climate variability at the scales we are considering here. (An interesting exception is the use of radiocarbon measurements of corals in the Galapagos Islands to construct a time series of El Nino occurrences (Druffel, 1981).) The knowledge we gain about natural processes at such long time scales (millennia) may be more closely related to solar phenomena. Though important to an overall understanding of the global climate, this subject is not within the scope of this study.
