National Academies Press: OpenBook

An Ocean Climate Research Strategy (1984)

Chapter: THE INTERANNUAL VARIABILITY OF THE TROPICAL OCEAN AND THE GLOBAL ATMOSPHERE

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Suggested Citation:"THE INTERANNUAL VARIABILITY OF THE TROPICAL OCEAN AND THE GLOBAL ATMOSPHERE." National Research Council. 1984. An Ocean Climate Research Strategy. Washington, DC: The National Academies Press. doi: 10.17226/19384.
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Page 11
Suggested Citation:"THE INTERANNUAL VARIABILITY OF THE TROPICAL OCEAN AND THE GLOBAL ATMOSPHERE." National Research Council. 1984. An Ocean Climate Research Strategy. Washington, DC: The National Academies Press. doi: 10.17226/19384.
×
Page 12
Suggested Citation:"THE INTERANNUAL VARIABILITY OF THE TROPICAL OCEAN AND THE GLOBAL ATMOSPHERE." National Research Council. 1984. An Ocean Climate Research Strategy. Washington, DC: The National Academies Press. doi: 10.17226/19384.
×
Page 13
Suggested Citation:"THE INTERANNUAL VARIABILITY OF THE TROPICAL OCEAN AND THE GLOBAL ATMOSPHERE." National Research Council. 1984. An Ocean Climate Research Strategy. Washington, DC: The National Academies Press. doi: 10.17226/19384.
×
Page 14
Suggested Citation:"THE INTERANNUAL VARIABILITY OF THE TROPICAL OCEAN AND THE GLOBAL ATMOSPHERE." National Research Council. 1984. An Ocean Climate Research Strategy. Washington, DC: The National Academies Press. doi: 10.17226/19384.
×
Page 15
Suggested Citation:"THE INTERANNUAL VARIABILITY OF THE TROPICAL OCEAN AND THE GLOBAL ATMOSPHERE." National Research Council. 1984. An Ocean Climate Research Strategy. Washington, DC: The National Academies Press. doi: 10.17226/19384.
×
Page 16
Suggested Citation:"THE INTERANNUAL VARIABILITY OF THE TROPICAL OCEAN AND THE GLOBAL ATMOSPHERE." National Research Council. 1984. An Ocean Climate Research Strategy. Washington, DC: The National Academies Press. doi: 10.17226/19384.
×
Page 17
Suggested Citation:"THE INTERANNUAL VARIABILITY OF THE TROPICAL OCEAN AND THE GLOBAL ATMOSPHERE." National Research Council. 1984. An Ocean Climate Research Strategy. Washington, DC: The National Academies Press. doi: 10.17226/19384.
×
Page 18
Suggested Citation:"THE INTERANNUAL VARIABILITY OF THE TROPICAL OCEAN AND THE GLOBAL ATMOSPHERE." National Research Council. 1984. An Ocean Climate Research Strategy. Washington, DC: The National Academies Press. doi: 10.17226/19384.
×
Page 19

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THE INTERANNUAL VARIABILITY OF THE TROPICAL OCEAN AND THE GLOBAL ATMOSPHERE The Southern Oscillation, a family of naturally occurring, interacting phenomena in the ocean and atmosphere that produces climate anomalies, provides us with the opportunity to carry out experiments in interannual climate forecasting and to develop a climate prediction capability. The phenomena that make up the Southern Oscillation family (e.g., anomalies of sea-surface temperature, atmospheric pressure, precipitation, and temperature) are found in the tropical ocean and global atmosphere. In addition, the Southern Oscillation, centered in the Pacific Ocean, may have analogues in the other tropical oceans. A study of these phenomena, their properties, their linkages, and their climate consequences holds promise of providing us with a predictive capability that far exceeds what we are likely to achieve through atmospheric studies alone. THE SOUTHERN OSCILLATION The Southern Oscillation is a large-scale exchange of atmospheric mass in the atmosphere between the eastern and western hemispheres in the tropics. It can be detected in sea-level atmospheric pressure records as a see-saw of high pressure in the South Pacific Ocean and low pressure in the Indian Ocean alternating with the opposite conditions in the other phase of the cycle. It has a characteristic cycle length of a couple of years and may occur at 2- to 10-year intervals. It is the most obvious instance of interannual climate variability. The Southern Oscillation comprises a global family of climatically varying phenomena. Among these are sea-surface temperature anomalies in the Pacific, Indian, and Atlantic oceans. The changes in the equatorial current system and the heat content of the Pacific Ocean are particularly marked. The largest oceanic oscillation is El Nino, an anomalous warming off the coast of South America. El Nino brings destruction to the fisheries off Peru and Ecuador. Plankton, fish, and birds, depending in a chain for 11

12 nutrients provided by the upwelling of cold seawater off the coast, die. This has economic effects on the global markets for fish, poultry, and fertilizer. El Nino also brings heavy coastal rains that cause flooding and damage crops along the South American coast. The Southern Oscillation has climate significance because it is a strong signal and because of its time scale. Though the Southern Oscillation does not occur regularly, an occurrence has correlated manifestations that normally persist for nearly two years from first to last appearance. This duration offers the potential for us to develop a predictive capability of perhaps a few months. The stages of the oscillation are phase-locked to the annual cycle. That is, the component phenomena of the Southern Oscillation normally occur at specific seasons of the year. From the viewpoint of the United States, the correlations of the Southern Oscillation with North American climate anomalies present an intriguing challenge. Can we, with a better understanding of the Southern Oscillation, use it to predict wintertime climate anomalies over the United States a season in advance? Let us review the evidence. Correlations between the Southern Oscillation and North American climate anomalies were first described in the 1930s by Sir Gilbert Walker. Since that time, there has been growing evidence of the reality of these correlations. Wintertime climate anomalies in the Northern Hemisphere are correlated with earlier atmospheric pressure anomalies over the South Pacific and with sea-surface temperature anomalies in the equatorial Pacific Ocean. Warm sea-surface temperatures are followed by high atmospheric pressure over Indonesia and Australia. Through a global-scale process of physical links, which has been called "teleconnection," these events are correlated with above-normal wintertime temperatures in the southeastern United States and below-normal wintertime temperatures in northwestern Canada. During a normal event, an El Nino begins in January. During the year, warmer-than-normal sea-surface temperatures spread over vast areas of the eastern and central equatorial Pacific. By the following September, surface atmospheric pressure over Indonesia reaches a maximum. Wintertime temperature anomalies over North America may follow in December through February (Horel and Wallace, 1981). The chain of events in the ocean and atmosphere may be a basis for prediction. However, we must be careful not to overstate the case.

13 During the winter of 1982-83, the strongest El Nino event ever observed took place. It was not forecast, it was not recognized as an El Nino occurrence until it was well developed, and its subsequent evolution and duration were not anticipated. Considerable research has been stimulated by this event, which underlined the incomplete state of our understanding. Correlations of North American temperatures with earlier Pacific sea-surface temperature anomalies are found only during the winter season and only over the northwestern and southeastern parts of the continent. Over most of the United States, the correlations are not significant. Where the correlations are significant, they account for considerably less than half the variance in those regions (Barnett, 1981). Nevertheless, Horel and Wallace (1981) suggest that the patterns of correlation may be blurred images resulting from the superposition of an ensemble of sharper patterns that correspond to the various states of the equatorial ocean and atmosphere. If so, then a sharper specification of the equatorial sea-surface temperature and of tropical rainfall might, with effective modeling of the processes of teleconnection, lead to better advance climate anomaly predictions over the United States than is now possible. This hope motivates the general excitement that today exists among oceanographers and meteorologists for a large-scale ocean-atmosphere experiment to explore the interannual variability of the tropical ocean and the global atmosphere (TOGA). CLIMATE PROCESSES IN THE TROPICAL ATLANTIC AND INDIAN OCEANS The link between the tropical Pacific Ocean and the atmosphere has attracted considerable scientific attention. The Atlantic and Indian oceans also provide interesting but different examples of large-scale interactions between the tropical ocean and the global atmosphere. Atlantic sea-surface temperature anomalies correlate with droughts in Brazil. Those in the Indian Ocean correlate with variations in the Indian monsoon. As in the Pacific, tropical sea-surface temperature anomalies influence and in turn are influenced by the atmosphere. An El Nino-like phenomenon may occur in the tropical Atlantic (Hisard, 1980; Merle, 1980), but the smaller dimension of the Atlantic basin may be the reason for the lack of clear evidence for its existence (Moore et al.,

14 1978). The tropical oceans respond to changes in the atmospheric wind stress. The circulation in the ocean is forced by the winds in a way that is distinctive in each ocean. For example, the propagation time of planetary waves across the ocean basin is a critical factor. The Pacific Ocean, being wider than the Atlantic, responds differently to a similar wind forcing, and a strong El Niflo thereby occurs. The Indian Ocean region appears to play an important role in the Southern Oscillation. In addition, there is a large seasonal change in the Indian Ocean in response to the monsoon. The Somali Current, for instance, reverses its direction seasonally. The Indian Ocean thus provides a unique location for studying some kinds of large-scale interaction between the ocean and the atmosphere. Indeed, the early evolution of the Southern Oscillation appears to occur in the atmospheric circulation over the Southern Indian Ocean. THE TOGA PROPOSAL A large-scale ocean-atmosphere experiment to study the Southern Oscillation family of phenomena has been proposed. It is called the Interannual Variability of the Tropical Ocean and the Global Atmosphere Experiment (TOGA). (Other names for components of the same investigation are the El Nino and the Southern Oscillation Experiment (ENSO) and the Ocean Atmosphere Climate Interaction Studies (OACIS).) The basic aims of the TOGA studies are as follows (TOGA Study Group, 1983): 1. to determine the nature of the interannual variability of the tropical oceans and global atmosphere, and 2. to understand the mechanisms that determine the interannual variability and the predictability of the variations. These aims encompass the principal planetary-scale aims of a related atmospheric research program, the Monsoon Climate Program. TOGA as proposed today would include these elements: * a description of the time table and chain of events in the Southern Oscillation and El Nino, * the relationship of the Southern Oscillation to the

15 regular annual cycle, * the fluxes of heat across the Pacific, * the coupling between the oceanic mixed layer and the deeper waters, * the relationship between equatorial convection and precipitation and sea-surface temperature anomalies, * the conditions that lead to a major versus a minor El Nino, * the relationship between sea-surface temperature anomalies and atmospheric planetary wave patterns, * the relationship between sea-surface temperature anomalies and higher-frequency fluctuations such as storm tracks and atmospheric blocking, and * the observing system needed to predict climate change associated with the Southern Oscillation. The Climate Research Committee (1983) has proposed the following scientific and operational objectives for ENSO: 1. To develop an improved understanding of the in situ local and remote atmospheric forcing of and response to fluctuations in equatorial Pacific sea-air transfers of moisture and sensible heat due to sea-surface temperature anomalies. 2. To identify the processes that control the development and time evolution of the thermal anomalies associated with the Southern Oscillation and the El Nino in the equatorial Pacific Ocean. 3. To understand the large-scale ocean-atmosphere interactions responsible for much of the short-term interannual fluctuations of the coupled climate system and to determine the predictability of the system. Of particular interest are relationships between the Southern Oscillation phenomenon and (1) climate fluctuations in mid- latitudes, particularly North America, and (2) interannual variations in the Asian monsoon, including both its regional characteristics and its relationship to the planetary circulation. 4. To develop improved schemes for prediction of short-term climate variability. 5. To design the optimum operational observing system required to provide the data base for such predictions. TOGA is an exciting opportunity. The Southern Oscillation is a strong climate signal. The economic benefits that could be derived from predicting some of the associated climate anomalies could be great. A number of

16 excellent scientists are enthusiastically working on the problem. Progress is being made in data analysis, field experiments, and theoretical work. On the negative side, there is as yet no comprehensive theoretical framework for TOGA. The first fragments of a theory exist, and some linking physical mechanisms have been hypothesized. However, we do not yet have a strong enough base of theory to be able to design a full TOGA experiment with assurance. Correlation does not always indicate causality. In a system where processes are known to be tied to the annual cycle, it is particularly risky to infer causality simply from correlations and timing. Statistically significant correlations can be found among the Southern Oscillation Index, equatorial Pacific sea-surface temperature anomalies, the El Nino, and wintertime temperature and precipitation anomalies over parts of North America. These correlations do not imply causal links, however. We cannot say that the Southern Oscillation causes the other observed climate variations. Because the events in the Southern Oscillation family are each linked to phases in the cycle of annual variation, we cannot say that event A causes event B, simply because A usually precedes B. Both A and B could be due to other causes. What is needed is a plausible physical mechanism to explain why A is usually followed by B. Then experiments can be designed to test that hypothesized mechanism. We must seek a testable theoretical framework to explain why and how the Southern Oscillation, the El Nino, and the temperate latitude climate anomalies are physically linked. A TOGA STRATEGY FOR NSF NSF should support TOGA planning with an eye to implementing a large-scale oceanographic and atmospheric experiment. Oceanic and atmospheric sciences will have to collaborate, and the scale of the problem will require strong coordination between NSF, NOAA, and NASA. The effort required to design and implement TOGA is justified by the benefits that might be derived. A prediction of regional climate changes over North America, for example, could have great social and economic rewards. Within the United States, the subject of TOGA is receiving considerable attention. Parallel programs that had been developing separately (El Nino and the Southern Oscillation Experiment (ENSO), being prepared under the

17 auspices of the Climate Research Conmittee of the National Research Council, and Ocean Atmosphere Climate Interaction Studies (OACIS), being prepared by NOAA) are being merged. The merged program will likely be a major U.S. contribution to TOGA. In addition, several ongoing and proposed American ocean research programs are related to TOGA: the Equatorial Pacific Ocean Climate Studies (EPOCS), the Sub-Tropical Atlantic Climate Study (STAGS), the Pacific Equatorial Ocean Dynamics Experiment (PEQUOD), the Seasonal Response of the Equatorial Atlantic (SEQUAL), and the Tropic Heat Experiment. These programs can serve as components of a U.S. contribution. Even more importantly, these programs provide a reservoir of scientists prepared to work on the climate links between the tropical ocean and the atmosphere. A decision will have to be made concerning the extent of U.S. commitment to TOGA. Planning is proceeding actively for a Pacific-based program to study the Southern Oscillation (ENSO, OACIS). However, our position with regard to research in the Atlantic and Indian oceans is not clear. A large-scale U.S. program, the Seasonal Response of the Equatorial Atlantic (SEQUAL), is based in the Atlantic but does not address all the broad issues of atmospheric and oceanic climate linkages. NSF should participate with other agencies and the universities in a discussion of our overall involvement with TOGA. Can the United States afford to be involved in all three oceans? The question of setting priorities becomes more difficult if the U.S. program is to be global. Or should U.S. involvement be principally in the Pacific Ocean, where the Southern Oscillation studies are centered? If the United States does concentrate its efforts and resources in one area, what should be the extent of U.S. collaboration with other countries who may, for their own reasons, need to concentrate their efforts somewhere else? As TOGA develops, NSF and the other agencies should focus on first obtaining answers to a set of basic questions as a prelude to a full-scale program. Some of these questions are, What is the chain of events in the ocean and the atmosphere and what imposes the time scale on the Southern Oscillation? How are the component elements linked to the annual cycle? What differentiates a major El Nino from a minor one? These basic questions will not all be answered in the early stages of TOGA. Keeping them in mind, however, will help to focus the program to provide a physical understanding leading to an improved predictive skill.

18 INTERNATIONAL INVOLVEMENT IN TOGA As this report was being prepared, a statement of TOGA's international objectives was also being prepared. The Conmittee on Climatic Changes and the Oceans (CCCO) and the Joint Scientific Committee for the World Climate Research Program (JSC) have established a TOGA Steering Committee that is developing a scientific framework. Among the questions that need to be answered is, Should TOGA include all tropical oceans? A three-ocean TOGA has some attractive aspects. Each ocean has its special processes, and it may be that large-scale ocean-atmosphere interactions can best be understood by examining the contrasts between the three ocean basins. Some countries may be prepared to participate in TOGA if they can do so in the Atlantic or Indian oceans, whereas a TOGA confined to the Pacific Ocean might not be acceptable to them. This was, in fact, the case for some countries in the First GARP Global Experiment. The United States should seek a balance between scientific and political factors in considering the question of the scope of TOGA. Particularly if the United States should decide to restrict its TOGA program to the Pacific Ocean, it would probably be unwise to take a firm international position before exploring the consequences. An El Nino-like phenomenon sometimes occurs in the tropical Atlantic, but our observations are not adequate to be certain about its existence. In general, the Atlantic provides an opportunity to study large-scale tropical air- sea interactions without the complicating effects of a strong El Nino. If a program were to be developed simply to look at equatorial oceanic processes, the Atlantic might be a better choice than the Pacific, first, because the annual cycle is more regular and, second, because the region to be covered is smaller. Large-scale experiments in the Atlantic Ocean are already under way. The U.S. program SEQUAL and the French program Francais Oc6an Climat Atlantique Equatorial (FOCAL) are working cooperatively to understand the oceanic response to seasonally varying winds. A study of the corresponding atmospheric, and hence climate, response to the changing oceanic conditions is not an explicit part of these programs. The potential importance of SEQUAL and FOCAL to understanding global climate variability is great, but some effort will have to be made if the results of these programs are to be channeled effectively into the climate research program stream.

19 The Southern Oscillation extends into the Indian Ocean, where the monsoon provides a sharp alternating sea-surface forcing. The Southern Oscillation appears to have origins in Southeast Asia (though appearances may be deceiving). There may be significant oceanic exchange of mass and heat between the western Pacific and the eastern Indian ocean. Thus it may be scientifically valuable to include the Indian Ocean as part of TOGA, to complement a major Pacific Ocean TOGA experiment. No U.S. ocean climate program is currently under way in the Indian Ocean. Some American oceanographers are cooperating in a French program, SINODE (Surface Indian Ocean Dynamic Experiment), whose purpose is to study the seasonal and interannual variability of the currents and heat content of the upper layer of the northwest Indian Ocean. French research activities in the Indian Ocean are providing benefits to American research. If the tropical Indian Ocean is included within the U.S. TOGA program, the benefits from the work of other countries could outweigh the additional expenses that might be incurred. To summarize, the Southern Oscillation, centered in the Pacific, presents an exciting natural signal that seems to promise a predictive capability for climate variations in temperate latitudes. The opportunity to study this phenomenon should not be missed, and the United States should support a major TOGA experiment in the Pacific. At the same time, complementary TOGA research activities should be supported in the Atlantic and Indian oceans, though it may be that other nations will play the principal role there.

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