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Modern Observational Physical Oceanography
Understanding the Global Ocean
By Carl Wunsch PRINCETON UNIVERSITY PRESS
Copyright © 2015 Princeton University Press
All rights reserved.
ISBN: 978-0-691-15882-2
CHAPTER 1
Introduction
Study of the ocean circulation is a problem in fluid dynamics. Traditionally, however, descriptions of the oceanic general circulation have begun with pictures of the large-scale temperature, salt, and oxygen and other chemical tracer properties of the deep sea. This approach rests on good historical and logical grounds: until recent times, the only properties measurable on a global basis were these scalar "tracers." Furthermore, their overall distributions have proved remarkably stable in time, and in turn that has made it possible to combine data over many decades to achieve global pictures from shipboard measurements.
In contrast, this book begins with an emphasis on the time-varying flow field as observed from a variety of modern instruments. The more traditional discussion of the time-average properties of velocity, temperature, and salinity is postponed. These latter are to be set into a context more relevant to an observer coping with a changing velocity field. Conventional pictures showing the large-scale temperature, salinity, and related distributions led to the concept of the ocean circulation as a quasi-geological phenomenon, with little or no change occurring either spatially or temporally. In the process, sometimes it was forgotten that the ocean is a fluid, and not a series of slabs sliding over one another unrelated to the equations of physics. As long as the study of the circulation was primarily of interest to academic physical oceanographers, the consequences of this distortion were of little practical consequence. Today, however, the circulation is widely regarded as an essential element in the understanding of the climate system and as a dominant factor in such politically charged phenomena as global change, sea level, and biological variations. But misconceptions concerning the very character of the circulation generate unrealistic programs for climate forecasting, observing the ocean, interpreting the record of past climate, and a host of related practical issues such as the management of fish populations.
The term "oceanography" historically denoted a descriptive science, paralleling "geography"—with its heavy emphasis on terrain, crops, economic assets, regional particulars, etc. That traditional beginning is today recalled in "descriptive oceanography," to distinguish it from the wider subject employing the dynamical equations with much mathematics. Every region, depth, season, and probably year in the ocean is distinct from all others. A very large and growing literature exists depicting the elements and eccentricities of many geographical regions. Most of that subject is omitted here—rather, the focus is on those elements that can be understood in a more global context, because of their generality or exceptionality. But the reader must understand that no clear distinction exists between the regional- and global-scale descriptions, be it verbal or mathematical, and too much should not be made of the division.
Physical oceanography can no longer be encompassed in a single manageable volume, and I make no claim to being expert in more than a fraction of it. References are provided that should permit a reader interested in pursuing a subject in greater depth to do so by starting with the various papers and books cited. No serious attempt has been made to provide a historically correct attribution to the originator of an idea, and when a reference is given, unless explicitly stated otherwise no implication is intended that it refers either to the hrst, or even the most important, discussion. These references might be regarded as the analog of navigational beacons: they are neither the channel nor a shoal, but indicators of where those are to be found. Parts of the field are undergoing rapid development as I write, with new papers appearing weekly. Obsolescence in a book must be expected, with the navigational markers being more like bread crumbs in a world of birds and rainfall. Modern electronic search tools now permit easy access to both the earlier and later literature. Occasionally, a historical sketch is provided where it enables a better understanding of some concept.
My intention has been to make the book self-contained if not comprehensive; specific references to the fluid dynamics literature (e.g., Tritton, 1988; Kundu and Cohen, 2008) and to the more theoretical textbooks noted in the preface are provided so that the reader can locate a fuller derivation, a wider discussion, or illuminating applications. Much useful material can be found in the recent compendium of Siedler et al. (2013); like most multiauthor collections (there more than seventy), it is neither easily digested nor without internal contradictions.
By employing "boxed" discussions and appendices, I have tried to make the basic concepts, borrowed from a wide variety of subfields, at least heuristically sensible and have provided references for anyone who would like to know more. Thus sketches are provided of the singular value decomposition, the Radon transform, Bessel functions, etc. Within the text, in many cases, results are simply stated; in others, where the derivation is particularly easy or interesting or illuminating, it is at least sketched. I do not claim to have been consistent. The ocean and climate are nonlinear systems, a property one must always remember. Nonetheless, this book leans almost completely on linear mathematics on the grounds that most intuition and insight are built that way, and as has been found across the sciences, linear analyses often have skills well beyond their formal domain of validity.
Only elementary statistical methods are employed: sample means and variances, spectral estimates, etc.—just enough to get by on, given the existence of useful handbooks dealing with a variety of powerful techniques. Historically, oceanography and climate have almost never raised issues in which very fussy statistical tests were required—if apparent signals were so weak as to require powerful tests, they usually proved unimportant compared to much more conspicuous, and still unexplained, signals. Many statistical methods exist for extracting weak signals from noise. In practice, however, oceanographic and climate measurements are usually subject to such basic problems as calibration drifts, sampling distribution changes, unknown external contributors, small sample size, and poorly understood statistical characteristics (e.g., they are never truly Gaussian, never truly statistically stationary) such that results dependent upon the use of elaborate methodologies should continue to be regarded as very tentative unless subjected to careful study of their sensitivity to the underlying assumptions. Common sense is useful. For example, if a process is obviously non-Gaussian, don't use ordinary statistical tests that assume it is normally distributed. The future belongs to the Bayesians, but as these methods have not yet broadly been used in the ocean literature, no explicit use of them is included.
Organization of a book such as this presents a conundrum. Discussion of instruments and measurements is almost impossible without some understanding of how the data are used—and that requires some theoretical background. But much of the theory is not very compelling without an understanding of what is measurable. Ocean variability is not interpretable without knowledge of the time-mean circulation—suitably defined—and that in turn is determined in part by the variability. A linear narrative is thus not possible—leading to a need for parallel and iterative discussions; the reader can expect to jump around among chapters. The book opens with a description of measurement methods, followed by a qualitative description of both the time mean and variability. A chapter sketching the variability theories leads to one discussing observations. Later chapters then turn attention to the more traditional ideas about the time-mean circulation.
TERMINOLOGY
Many scientists are impatient with discussions of terminology ("It's just semantics"). But precise language is an essential shorthand. Furthermore, unnecessary jargon is a serious obstacle to the exchange of ideas within the field, and much more so with neighboring disciplines. Muddled thinking is often most immediately apparent in the choice of language. Anyone who has worked in oceanography for awhile will have had the experience of reading through a paper or sitting through a talk before recognizing that one's growing bewilderment and confusion resulted from some careless or unorthodox use of the label for a concept. Examples abound. For example the term "barotropic velocity" has at least six different incompatible definitions, and a very large amount of unnecessary confusion has ensued by their sometimes unthinking, undefined use. A number of examples are discussed in Appendix C.
AN OPEN MIND
Most textbooks are directed at explaining to their readers the facts of the subject. One interpretation of the central role of science is in its overthrowing what "everyone knows." In the grand scheme of things, everyone once knew that the Sun orbitted around the Earth, that the geological record was the result of the Noachian Flood, and that species were immutable. Many scientists share the human need for near-religious faith in what they "know" about the world, to the point that dogmatism becomes a major obstacle to understanding. In physical oceanography, as with all fields, many examples exist of somewhat plausible ideas being converted into a kind of faith-based science. The advice by Chamberlin (1890) to always maintain multiple scientific hypotheses remains most sensible.
CHAPTER 2
Observing the Ocean
Compared to the atmospheric sciences, oceanography has a special flavor because measurements are so difficult. Radio waves do not propagate through the sea at useful frequencies. Among other problems, (1) no analog exists of the capability in meteorology of measuring cloud velocities and temperature profiles from space; (2) information cannot be sent back to shore electronically in the way weather balloons transmit data to observers at the surface; (3) from a ship, ocean surface properties alone can be measured, or an instrument must be placed physically at the depth where the observation is required; and (4) if long-duration measurements are needed, the instrument must be kept in place, internally recording the changing values being sensed.
The problem of observation is further complicated by the high pressures at depth: each 10 m of ocean depth increases the ambient pressure by about one atmosphere of pressure; seawater is corrosive everywhere; biological fouling is a major problem in the upper regions; and surface instruments are mechanically stressed by thousands of wave cycles over weeks and months. A somewhat brief description of many (far from all) of the major measurement technologies is given here because so much of what is believed understood of the ocean is, and has always been, filtered and distorted through the prism of the available observational tools.
Because of the present intense interest in climate trends, questions of whether the ocean has been getting warmer or fresher or moving faster or slower over the last decades and centuries, or even whether its volume is changing, are very conspicuous. Some understanding must be gained of how the measurement technology, its coverage, and its accuracy have evolved through time. Deacon et al. (1971) and Peterson et al. (1996) provided useful and interesting summaries of observations dating from antiquity to the late nineteenth century. More extended accounts of measurement methods of their era can be found in von Arx 1962, Baker 1981, Heinmiller 1983, Laughton et al. 2010, Robinson 2004, and Martin 2004, the latter two for satellites. Here only a sketch is provided to give of the flavor of the subject and as background for understanding of some of the difficulties observational oceanographers have had, and continue, to face. Many instrument types and concerns are revisited later in discussions of the meaning of the observations for the physics.
Oceanographers now often download "data" from the Web, sometimes failing to recognize how remote the numbers are from the original measurements. Tabulated values commonly are derived from very complicated measurement systems and are the result of complex manipulation and processing of the observations. An outcome can be theories or models of nonexistent phenomena—or the opposite, the failure to detect significant physical processes.
2.1 SHIPS
Ships have been the major platforms for observations of the ocean since antiquity. Without them, no deep-sea measurements would have existed. Published histories of the evolution of boats and ships abound; platform evolution from human to wind to engine power is well known. What is relevant for understanding oceanographic observations are that they have been, and remain, expensive and slow. Depending upon the region, time of year, and adequacy of the vessel, they can produce extreme discomfort in scientific observers of varying intestinal fortitude. The first recognizable oceanographic expeditions are sometimes traced to Edmond Halley (1698), who measured tides and the geomagnetic field at sea (Cook, 1998), or to the Challenger expedition of 1872. Even at the outset, the financial requirements for ship use were so onerous that only a government agency could sponsor the work—in both these cases, it was the UK Royal Navy. For awhile in the early twentieth century, wealthy amateurs with their own seagoing yachts (the Prince of Monaco, Fridtjof Nansen, Henry Bryant Bigelow) made significant contributions to the subject, but that era came to an abrupt halt with the professionalization that occurred following World War II.
In the modern world (2014), costs remain a major factor. Although inflation renders all such numbers ultimately nugatory, a modern vessel capable of crossing an ocean basin would cost about $30,000/d to operate, not including the cost of the scientists. The best modern vessels steam at about 12 knots (about 22 km/h) and thus require many days to cross an ocean basin—even without stopping to make measurements. With the ocean now known to be changing significantly day by day, and with few scientists willing to spend months and years at sea as they did hundreds of years ago, the era of the ship as the fundamental sampling platform in physical oceanography is over, although a need for both oceanographic ships and seagoing scientists will always exist. (The situation still remains different in biological and chemical oceanography.)
2.2 NAVIGATION
Observations at sea are useful only if their location is known. The accuracy required depends directly on the type of measurement and its purpose. For Benjamin Franklin, the knowledge that water temperature within the Gulf Stream was warmer than outside it, provided a very useful insight, and large uncertainties in latitude and longitude were tolerable. For a more contemporary scientist attempting to calculate the horizontal derivatives of the density field, [partial derivative]ρ/[partial derivative]x, so as to determine the velocity between measurements separated by 10 km, a 1 km error in the distance can totally confound the calculation. Historically, and practically, the problem of horizontal position has been treated separately from that of determining the depth of an instrument, and thus "navigation" here refers to the problem of determining latitude and longitude at sea.
Because of its implications for commerce, the military, exploration, and science, elaborate discussions exist of the history of navigation. Sobel (1996) provided a well-known popularization of the romantic story of Harrison's marine chronometer. But oceanographers have always been intensely interested in obtaining and using the most precise available navigation systems, and a summary will have to suffice. See Bowditch, 2002, for a history and a guide to many of the available methods.
Before the Second World War, navigational techniques had hardly changed in their fundamentals since the invention of the marine chronometer. Determining position at sea depended upon the compass, the ship's log (a speed-measuring device), the determination of the positions of astronomical bodies with sextants and related instruments, elaborate numerical tables, and the very real skill of human navigators. World War II brought systems related to the invention of radar, including LORAN, Decca, Omega, and the like, which permitted all-weather navigation, at least over large areas. These worked a revolution for oceanography in regions such as the North Atlantic where coverage was good. But LORAN was never global, and putatively global systems like Omega were not very accurate. Radar itself provided accurate line-of-sight navigation relative to detectable objects such as the shore, other ships, and drifting buoys.
The advent of Earth-orbiting satellites in the late 1950s eventually brought about the Transit satellite system, which was operated by the US Navy (see Bowditch, 2002) and provided accurate but highly sporadic temporal coverage, and later the revolutionary Global Positioning System (GPS), and soon perhaps, rival non-US networks. Beyond GPS, improved horizontal position accuracy is unlikely to have a major further impact on shipboard oceanography, although measurements will always exist where even greater accuracy and precision can be used. Quotidian navigational skills have eroded to the ability to push a button—one of the ways in which electronics has distanced end users from familiarity with observations.
(Continues...)
Excerpted from Modern Observational Physical Oceanography by Carl Wunsch. Copyright © 2015 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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