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HEAT TRANSPORT STUDIES Transport and storage of heat by the ocean are central to all theories of the role of the ocean in global climate and thus central to our hope for developing the skill to predict climate variations. In this chapter the problem of estimating heat transport processes in the ocean is considered, and proposals for large-scale experiments that focus on this question are reviewed. Though these experiments are now likely to take place as components of WOCE and TOGA, they are here discussed in a separate chapter because of their importance and the extensive scientific review they have received. OCEAN HEAT TRANSPORT AND STORAGE The ocean dominates the energy storage of the combined ocean-atmosphere system. Heat can be stored in the ocean for periods that are long in comparison with atmospheric residence times. The ocean can transport this heat and can give It up to the atmosphere far from the place where it was received. Oort and Vonder Haar (1976) estimate that the ocean has a heat transport poleward from the tropics to mid- latitudes as large as or larger than the corresponding mid- latitude atmospheric transport. As was discussed in the chapter on TOGA, we have found correlations between tropical sea-surface temperature anomalies and subsequent climate variations in temperate latitudes. Such correlations have been known for some time (e.g., Bjerknes, 1969), but recent work (e.g., Horel and Wallace, 1981) provides a hypothesis of the physical processes that bring this about. The correlations between the tropical Pacific and Northern Hemisphere extratropical latitudes are not strong. But the correlations do suggest that variations in oceanic heat storage and transport may modulate atmospheric fluctuations on time scales that are tied to the oceanic processes. Heat flux is a central variable in all ocean climate models. For model testing, we need to be able to determine experimentally the poleward transport of heat by the oceans 27
28 and its variations with time. Techniques for estimating ocean heat transport are subject to uncertainty. Before we can confidently deal with the question of ocean heat transport, we will have to develop means for measuring it so that we can have assurance in our estimates. Our aim should be to measure the ocean's role in heat flux in order to understand the magnitude of poleward heat transport by the ocean and the atmosphere, the distribution of this transport by region, and the processes that control the magnitude and time scale of the transport. THE CAGE PROPOSAL Ocean heat flux experiments have been proposed nationally and internationally to explore the storage, transport, and transfer of heat by the ocean. The Cage experiment was proposed by a group led by F.W. Dobson (Bretherton et al., 1982) to examine the long-term mean heat flux, the annual cycle, and the interannual variability over the North Atlantic. The Cage objective is to compare the results of three different techniques for estimating the heat flux from the ocean to the atmosphere over an ocean basin. The three methods are as follows: 1. Estimation using the distribution of temperature and velocity within the ocean (the Hall and Bryden (1982) method). 2. Estimation by area integration of the heat transfer across the air-sea interface (the Budyko (1974) and Bunker (1976) method). 3. Estimation from the net radiation at the top of the atmosphere together with estimates of atmospheric flux divergence and oceanic heat storage (the Oort and Yonder Haar (1976) method). All three methods are subject to major uncertainties. We need to know how well we can determine ocean heat flux before we can begin to consider its interannual variability. We want to know the random and systematic errors associated with each. By comparing the three techniques over an ocean basin, it might then be possible to estimate the interannual variability of heat transport and its sensitivity to long- term climate change. These experiments would be useful if each of the three methods could estimate the heat flux within an uncertainty of 10 percent. At least 5 years of
29 measurement have been proposed over an ocean-wide region bounded east and west by continents and north and south by transoceanic sections. The goal of Cage would be to determine the mean heat flux to the atmosphere over the North Atlantic to a desired 20 percent accuracy. The Cage proposal was controversial. Uncertainties in the individual components of the sea-surface flux budget are about 30 to 40 percent. With measurement techniques now available, only by taking global averages can these uncertainties be reduced. Critics say that to obtain an estimate of heat flux that is accurate within 20 percent, we must average extensively (over the entire ocean basin, over all depths, over long periods of time). This much averaging will produce an integral result that will not resolve the time- and space-dependent processes that control the poleward flux of heat. A further problem is that satellite radiation measurements, such as might be made by an Earth Radiation Budget Experiment (ERBE) satellite, may not be available during Cage. Should Cage go ahead without such a satellite? A knowledge of the radiative fluxes at the top of the atmosphere is fundamental to obtaining the heat balance over an ocean basin. Without ERBE, Cage will be seriously weakened. To estimate the heat flux from the net radiation at the top of the atmosphere, we will need estimates of the atmospheric divergence of latent and sensible heat and of potential and kinetic energy over an ocean basin. We do not know if such measurements can be made with the requisite accuracy (less than 10 W/m2). Furthermore, we do not yet have a procedure whereby we can test such estimates for bias. At the Tokyo Study Conference (CCCO, 1983), it was suggested that, at least in the Atlantic, the interannual variability in heat flux is about 10 W/m2. If this is the case, it is contended, all components of an Atlantic Cage experiment need not be done simultaneously. In particular, satellite measurements could be made at some later stage, when a satellite becomes available. It was thus proposed at the Tokyo Study Conference that the North Atlantic Cage experiment be redefined into a step- by-step process. This would begin with a study of the interannual variability of each of the heat budget components. If this study shows that the interannual variability of these components is greater than is now apparent, it might be necessary to return to a simultaneous Cage experiment.
30 Finally, at the fourth session of GCCO (OCOO-IV, 1983) the issue of an explicit Cage experiment was addressed. The committee felt that intercomparisons between different techniques for estimating heat flux were required. Only by this means can systematic differences in the estimates produced by the techniques be identified. However, rather than planning for a single comprehensive experiment, opportunities for pairwise intercomparisons should be exploited in WOCE and TOGA, and possibly in special regions such as enclosed ocean basins, particularly in the North Atlantic. The comnittee urged work to improve our techniques for developing atmospheric assimilation models and for obtaining direct estimates of surface fluxes. The heat transport estimates recommended by the Cage Study Group should be undertaken as part of the precursors to and observations for the main WOCE experiment. THE PATHS PROPOSAL The Pacific Transport of Heat and Salt (PATHS) Program has sometimes been referred to as a "Pacific Cage," but as presented by the Pacific Cage Study Group (1983), led by G.A. McBean, it has significant differences in objectives and approach. The principal objective is to understand the horizontal processes of large-scale transport of heat and salt in the mid-latitude North Pacific Ocean. The study should investigate the short-term climatic variability in the distributions of heat and salt in both the subarctic and subtropical gyres of the North Pacific, on time scales of months to years, for a period of approximately 10 years. The measurement should give the net flux of heat and water from the ocean to the atmosphere. The principal focus, however, is internal heat redistribution by horizontal processes. Another objective is to estimate transports and air-sea fluxes from time-dependent measurements of heat and salt storage. The southern boundary could be 30"N. Together with the Atlantic studies, this would close off the globe north of 30"N and permit a zonally averaged estimate of the meridional heat transport. A positive feature of Pacific heat flux studies is the extensive existing data base. The Japanese in particular have collected regular measurements in the western Pacific for several decades and produce analyses based on them. In this country as well, programs such as the North Pacific
31 Experiment (NORPAX) and the North Pacific ship-of- opportunity program known as TRANSPAC have accumulated a significant data base. These programs are ongoing, and coordination of them (by GCCO) is mainly what is needed. However, if the Pacific heat flux is to be estimated to the same accuracy as that proposed for Cage in the North Atlantic (say, within 0.2 x 1015 W), the observational demands may be more stringent. The surface area of the Pacific north of 25"^ is about 35 percent greater than that of the Atlantic. Thus the corresponding average surface flux of heat will be less, perhaps about 5 W/rn^. Measurements accurate to this level will be difficult, and it is not yet clear that they are technically feasible. The PATHS plans include a number of component activities. In the Kuroshio region, the Japanese are planning several experiments. The Ocean Heat Transport Experiment (OHTEX) will observe the Kuroshio with a current meter array for one year. The Ocean Mixed Layer Experiment (OMLET) will monitor the variability of the mixed layer, measuring the processes that are involved in heat fluxes and developing the techniques needed to carry out the measurements. An American program, Heat Advection Investigations in the Kuroshio (HAIKU), will observe the Kuroshio southeast of the Ryukyu Islands, a region that may be analogous in ocean-basin heat transport to the well- studied Florida Straits. PATHS also proposes that a transoceanic section be made to measure the heat transport analogous to the estimate of Hall and Bryden (1982) in the Atlantic. The PATHS Study Group has proposed a set of component programs within a broad framework. The resultant program appears to lack the overall strategic flavor that characterized the Cage proposal. If Pacific heat flux plans are further developed, they may come to resemble more closely those of Cage. North Pacific interannual variability is less well known than that in the Atlantic, but appears to be relatively large compared to the mean. Thus, unlike Cage, a nonsimultaneous PATHS experiment has not been proposed. For PATHS as with Cage, CCCO-IV (1983) recommended that the heat transport work be done as part of WOCE and TOGA. The committee noted that PATHS has a strong resemblance to many aspects of WOCE. Experience gained in PATHS could contribute to WOCE planning. Thus PATHS might be considered a precursor to WOCE.
32 THE GINS CAGE PROPOSAL The Greenland, Iceland, and Norwegian Seas Cage Experiment (GINS Cage) is a proposed program to study climate processes in the northern polar seas. To begin, a study of sea-air-ice processes in the Greenland Sea is proposed. The region of the Greenland Sea is favorable for studies of the interactions among air, sea, and ice. There is vigorous exchange of energy between the ice-covered and the ice-free ocean. The seasonal and interannual variations of ice cover are large and provide a strong signal for study. The data base is large because of extensive shipping in the region. The Greenland Sea area is well configured for observations: there are narrow passages to north and south, there is a coast to the west, and there is an unconfined ice boundary to the east. Since the dominant surface flow is nearly unidirectional, drifters can be used efficiently. Finally, the region is accessible and is already surrounded by a relatively dense network of meteorological observing stations. Plans are still being developed, but the essential research elements can be summarized: ice dynamics, the role of the ocean mixed layer in the heat balance of the bottom surface of the ice, and large-scale convection and its connection to lower latitudes through deep flows into the North and South Atlantic. After 5 years of monitoring, the work could be extended from its Greenland Sea base to include the Norwegian Sea. The proposal is tentative, pending further studies of ocean transport data across the passages of the GINS Cage area. Reaction to the GINS Cage idea has been mixed. On the positive side, the region is nicely defined, the signal is large, and the role of ice in global climate is worthy of study. On the negative side, a Greenland Sea study followed by a GINS Cage experiment would take considerable effort. Many of the Cage enthusiasts would rather spend that much effort on a full-blown North Atlantic Cage experiment. Furthermore, because uncertainties in measurement are reduced by area integration, a Cage-type experiment becomes more difficult the smaller the area. Though the heat flux signal is large in the GINS area, we do not yet have a careful estimate of the feasibility of a Cage experiment in that region. Oceanographers and meteorologists with an interest in the polar regions are not numerous, and thus the enthusiasm for GINS Cage at scientific meetings such as the Tokyo Study
33 Conference is not great. Furthermore, a critical question is whether there is a sufficient number of conmitted scientists. If the problems are worthy of study, we must be assured that enough qualified researchers are available and willing to take them up. The issue is further complicated by organizational factors. In the funding agencies and in the NRC, polar matters sometimes are separately considered. Common development of research plans in polar regions thereby requires an additional effort to assure coordination. A HEAT FLUX STRATEGY FOR NSF The question of strategy to be followed here is a sticky one. Ocean heat flux may be a key factor in controlling global climate variability. However, many oceanographers question our ability to design and carry out an adequate large-scale heat flux experiment in this decade. Before making a commitment to such experiments, NSF should be assured that all three methods for estimating the heat flux are technically feasible within the desired accuracy, that information on processes and not simply integral results will be obtained, and that any proposal for a nonsimultaneous Cage-type experiment will be documented with quantitative estimates of the interannual variability of the component processes. If technically feasible, should we carry out both North Atlantic and North Pacific experiments? Can the United States or the international climate research community afford to do both? If we can only afford to do one, which one? There are arguments in favor of each ocean basin. The poleward oceanic heat flux appears to be greatest in the North Atlantic. As a consequence, the radiation levels per unit area are greater than over the North Pacific, and a precise estimate will be easier to obtain, an argument in favor of the Atlantic. The North Pacific, on the other hand, may be a better choice because it is not complicated by strong interchanges with the Arctic basin, as is the North Atlantic. It is, in effect, closed off to the north, thus simplifying the geography and the heat flux. Deep and bottom water formation is absent in the North Pacific. On the negative side, the poleward heat flux may be less in the Pacific. Because the area of the North Pacific is considerably greater than that of the North Atlantic, with a smaller total heat flux, the surface radiation levels in the North Pacific must be considerably less and thus the air-sea
34 interchanges per unit area must also be less. Hence a program of measurements with 20 percent accuracy could be more difficult to achieve in the Pacific. Doing Atlantic and Pacific heat flux experiments at the same time would have some advantages. The complete Northern Hemisphere ocean and atmosphere heat budget could be determined. This would, in effect, put a cap on the earth north of 25°N. Since the Atlantic and Pacific differ in the dynamics of their heat transport, concurrent observations might shed light on their contrasts. Finally, a heat flux experiment becomes more attainable the larger the area over which one averages. Thus, covering the Atlantic and Pacific together could make the desired accuracy of 20 percent more achievable. One factor that works against the idea of doing a heat flux experiment over both the Atlantic and the Pacific is the scale of operations that would be needed. Could we meet the expense? Are there enough interested scientists to carry out the work? Do we have enough ships and scientific equipment? An earth-orbiting radiation satellite is needed, here, at least, a hemispheric experiment would impose only a small additional expense. If we do carry out heat flux studies over both ocean basins, we may do so within a different context than that proposed for Cage and PATHS at the Tokyo Study Conference (Bretherton et al., 1982, Pacific Cage Study Group, 1983). For example, a WOCE that incorporates heat flux process studies could, because of its generic nature, deal with all ocean basins. If this comes to pass, however, the heat flux experiment may evolve considerably beyond the Cage and PATHS proposals. NSF should anticipate that large-scale heat flux experiments will be proposed in some form. NSF should be prepared to support such work because of its importance in understanding climate change. Nonsimultaneous heat flux studies might provide the opportunity to incorporate heat flux experiments as part of WOCE and TOGA. If this should happen, NSF may be able to support heat flux studies as part of or precursors to the other programs. Many of our ideas about North Atlantic heat flux are stimulated by the direct estimate of Hall and Bryden (1982) of the poleward heat flux across 25TM latitude in the Atlantic. A corresponding estimate for the Pacific does not exist. In fact, we do not even yet know the order of magnitude of the Pacific poleward heat transport across 25°N latitude. Collecting such a measurement will be much easier to do than the large-scale experiments. Though not a quick
35 and easy task, it is a lot easier to justify as a start than an ocean-basin-scale experiment. NSF should consider supporting a trans-Pacific-Ocean poleward heat flux measurement when a suitable proposal appears. We are being held back in developing our ideas about the North Pacific because of the lack of such an estimate. NSF might support other heat flux research activities whether or not we go ahead with full-fledged ocean-basin- scale programs. Existing expendable bathythermograph (XBT) data could be analyzed to estimate heat storage over time scales important to climate. A number of techniques need to be developed to provide a capability for measuring heat fluxes. We should collect radiation measurements in situ to compare with estimates from satellites. We should carry out design studies for heat flux and water-mass conversion experiments. We should continue broad-based satellite radiation measurements. We might undertake limited simulation experiments of an observing system. The aim should be to estimate the extent to which we are able to estimate heat and energy fluxes in the ocean and atmosphere, and to define what improvements are needed to reduce errors to acceptable levels. The GINS Cage experiment could be a valuable contribution to understanding the global heat balance. However, a study of the scientific feasibility is first needed, and we ought to clarify the issue of the scientific resources and manpower that could be called upon to support GINS Cage. NSF should insist that the issue of the role of polar heat fluxes be addressed as part of the justification for either an Atlantic Cage or the GINS Cage experiment. INTERNATIONAL INVOLVEMENT IN CAGE Cage and PATHS have been the subject of international studies sponsored by the CCCO. The Cage Study Group (Bretherton et al., 1982) and the Pacific Cage Study Group submitted reports to the Tokyo Study Conference that were the basis for recommendations (CCCO, 1983) that were submitted to the CCCO. The Pacific Cage Study Group (1983) was able to revise its draft following the Tokyo meeting to reflect the changing ideas on the feasibility of large-scale heat flux experiments. The recommendations to the CCCO have been eclipsed by later evolution in the thinking of those scientists who are active in planning heat flux experiments. The Tokyo Study
36 Conference recommended that a heat flux steering committee be established for "organizing and guiding the development of Cage and PATHS studies. ..." A series of precursor studies were listed, though the issue of a full commitment to the programs was sidestepped.
