Part Three Techniques of Investigation I4 14 1 Observations and the Impact of New Instruments Ocean Instruments and Experiment Design I was never able to make a fact my own without seeing it and the descriptions of the best works altogether failed to convey to my mind such a knowledge of things as to allow myself to form a judgment upon them It was so with new things So wrote Michael Faraday in 1860 quoted in Williams 1965 p 27 close to the end of his remarkably productive career as an experimental physicist Faraday s words convey to us the immediacy of observation the need to see natural forces at work W H Watson remarked a century later on the philosophy of physics How often do experimental discoveries in science impress us with the large areas of terra incognita in our pictures of nature We imagine nothing going on because we have no clue to suspect it Our representations have a basic physical innocence until imagination coupled with technical ingenuity discloses how dull we were Watson 1963 Faraday recognized the importance of this coupling when he wrote in his laboratory Diary 1859 quoted D ames Baker Jr What wonderful and manifest conditions of natural power have escaped observation M Faraday 1859 We know what gear will catch but we do not know what it will not catch M R Clarke 1977 in Williams 1965 p 467 Let the imagination go guiding it by judgment and principle but holding it in and directing it by experiment In the turbulent multiscale geophysical systems of interest to oceanographers the need for observation and experiment is clear Our aim is to understand the fluid dynamics of these geophysical systems Although geophysical fluid dynamics is a subject that can be described with relatively few equations of motion and conservation as Feynman Leighton and Sands 1964 stated That we have written an equation does not remove from the flow of fluids its charm or mystery or its surprise In fact observations and experiments have been crucial to the untangling of the mysteries of fluid processes in the ocean and in the atmosphere For example on the smaller scales lengths of the order of tens of meters and less the discovery of the sharp discontinuities in density temperature and salinity that was brought to focus by the new profiling instrumentation has given us a whole new picture of mixing in the ocean On the large scale a good example is the explanation of the general circulation of the atmosphere in terms of baroclinic instability The theoretical development was firmly based on the remarkable set of observations of the atmosphere carried out in the 1940s and 1950s As E Lorenz 1967 p 26 noted in his treatise on The Nature and Theory of the General Circulation of the Atmosphere The study of the circulation owes a great deal to the practice of weather forecasting for without these observations our understanding could not have approached its present level Yet certain gaps will continue to exist in our knowledge of the circulation as long as extensive regions 396 D James Baker Jr L l I without regular observations remain The emphasis that Lorenz placed on the need for observations before understanding can occur is equally valid for oceanographic studies of the same scale One must search long and hard for counterexamples where theory has preceded observation in geophysics One of the few such examples in oceanography is the prediction and subsequent confirmation by direct measurement of southward flow under the Gulf Stream by the Stommel Arons theory of abyssal circulation The theory is discussed elsewhere e g chapters 1 and 5 this volume so I shall not pursue it further The point is that observations guide and appear to limit the progress of our science There is no inherent reason that this should be so Why is our imagination so limited that as Watson put it we are so dull Perhaps the historian of science can answer the question If observations guide the science then new instruments are the means for guidance The following two examples show how this has occurred we consider first the North Atlantic circulation Most of our ideas about the ocean circulation have been based on the indirect evidence of the temperature and salinity fields and the assumption of geostrophy With the advent of direct velocity measurements by deep floats and current meters during the 1960s and early 1970s the necessary data for a consistent picture of ocean circulation at least in limited areas began to come in Worthington s 1976 attempt to put together for the first time such a picture of circulation in the North Atlantic was based on the new direct data One of the important pieces of evidence used in the work by Worthington were the data from the neutrally buoyant floats which show a high transport for the Gulf Stream see Worthington 1976 for references Until the direct measurements the distribution of the absolute velocity field was ambiguous With the new data Worthington was encouraged to put together a complete picture that includes a tight recirculation pattern However within the constraints he used Worthington s attempts at a complete mass and dynamic balance for the entire North Atlantic circulation were not successful He decided therefore to choose a circulation pattern that was not consistent with geostrophy This provocative work stimulated a number of attempts to look more closely at the circulation system there Because both scale analysis of the equations of motion and the direct moored measurements of Schmitz 1977 1980 confirm geostrophy to the leading order as do the measurements reported by Swallow 1977 and Byrden 1977 in the MODE region of the Sargasso Sea Worthington s total picture is not correct The moored data are consistent with the recirculation pattern but in addition reveal a flow with an eastward component immediately south of the Gulf Stream and north of the recirculation The latter feature is not clearly contained in any existing picture of the North Atlantic circulation A second approach was taken by Wunsch 1978a who used hydrographic data mass balance and geostrophy to estimate the absolute velocity field in the North Atlantic Since the system is basically underdetermined one requires point measurements to find unique solutions to the relevant fields an auxiliary criterion is required Wunsch found a unique solution by minimizing a measure of the energy This is an example of the geophysical inverse technique that provides estimates in underdetermined systems by optimizing
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