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MIT 12 000 - Study Notes

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Figure 14.2 Sea Data tape-recorder body (right) and COSMOScircuit cards (left). The cards are rounded in order to fit intocommercial cassette tape recorders are also availablenow for oceanographic use.The second thread of engineering design mentionedabove is "platform engineering." Since a review hasrecently been given by Henri Berteaux in his book BuoyEngineering (1975), we need not discuss this aspect indetail here. Some aspects of platform engineering areincluded in the subsections on mooring technology insection 14.3. The reader interested in following currentdevelopments (circa 1980) in ocean technology is urgedto read Exposure, a newsletter produced by R. Mesecarof Oregon State University for discussion of new con-tributions and problems in instrument design and usein the ocean.14.3 Examples of Modern Ocean InstrumentsLooking over the period since The Oceans was written,we can identify four major areas of instrument designthat have had an -important impact on the developmentof our ideas of ocean circulation and mixing. Theseareas are the moored-buoy-current-meter technology,the deep-drifting neutrally buoyant floats, the temper-ature-salinity profilers (commonly known as the STDand CTD), and the velocity profilers. The first three ofa 6"-diameter pressure case. (Courtesy of W. Hill.)these developed essentially simultaneously. The veloc-ity profilers are newer, and, to some extent having builton the technology developed earlier, have not yet hadthe total impact of the other three groups.A second set of instruments is in a state of devel-opment and has not yet had the extended use or theimpact of the four listed above. These include, for ex-ample, bottom pressure gauges, surface drifters, andthe "inverted echo sounder." We also include in thisset the whole suite of remote measurements, e.g., sat-ellite altimetry and laser profiling of the upper layers,and the various acoustic techniques that have beenproposed.Our final discussion in this section covers a set ofproblems that can not yet be attacked with presenttechnology. New instruments are crucial for under-standing and quantitative measurement. Examples areair-sea fluxes in stormy conditions or measurementsin strong currents.Space does not allow me to be comprehensive in anyway here, but only selective. The reader is referred totwo excellent recent review volumes, Instruments andMethods in Air-Sea Interaction (NATO Science Com-mittee, 1978) and the Proceedings of a Working Con-ference on Current Measurements, (Woodward,402D. James Baker, Jr.II _ Mooers, and Jensen, 1978), which cover aspects ofmany of the instruments to be discussed below. Thepaper by Rossby (1979) on "The Impact of Technologyon Oceanography" contains a number of instructiveexamples.Another area of very great impact on ocean meas-urements is navigation. Advances in both shore-based(LORAN) and satellite-based navigation techniques areresponsible for the success of many of the instrumentaltechniques discussed below from mooring location tovelocity determination. The discussion below is lim-ited for reasons of space to instruments themselves.In thinking about instruments and what they meas-ure, we consider the full equations of motion. Theequations include the terms to be measured; ideally,direct measurement of the terms is best, but some-times it turns out to be more feasible to measure theterm indirectly. The terms that appear in the equationsinvolve the velocity, products of velocity, density, pres-sure, turbulent stresses, and viscosity.The instruments that we discuss for velocity includethose that make direct measurements of currentseither at a point or in profile. We have been less suc-cessful in measuring turbulent stresses-products ofvelocity fluctuations-than the meteorologists, primar-ily because of the lack of a stable platform. However,some useful data have been taken from stations on seaice and are discussed below.Density is generally inferred from temperature andsalinity; technical difficulties have precluded any use-ful instrument for measuring density directly. Themain problem is finding an instrument that will workin situ-in the water column or on board ship. Thesmall variations of density and the large accelerationsat sea have prevented much success with direct densitymeasurement. A number of techniques have been de-veloped, however, and some of these will be discussed.Pressure is generally inferred from the density usingthe hydrostatic relation. Without some level of pres-sure reference, however, it is not possible to establishan absolute pressure field in the ocean. Bottom pressuremeasurements (to be discussed below in section 14.3.5)can monitor pressure fluctuations; sea-surface topog-raphy by satellite is a technique currently being devel-oped for measurements of both fluctuations and meansurface field.14.3.1 Current-Meter and Mooring TechnologyThere are two parts to the measurement of currents ata point in the ocean. The current meter must be ac-curate, reliable, and, for most purposes, internally re-cording. The platform, or mooring, must be robust,deployable, and affordable. Major advances in both ofthese areas have been made since the 1960s. It is nowpossible to make long-term (greater than 1 year) meas-urements of currents at levels below the surface layerwith better than 90% data return (e.g., Pillsbury, Bot-tero, and Still, 1977; Tarbell, Spencer, and Payne, 1978).The paper of Richardson, Stimson, and Wilkins(1963) is a good starting point because it marks thebeginning of the modern age of current-meter andmooring technology. This remarkable paper covers thewhole field of mooring and current-meter technologyas it was known at that time, and demonstrates theingenuity of W. S. Richardson and coworkers then atthe Woods Hole Oceanographic Institution. The paperdocuments the early attempts to maintain deep-seamoorings and current meters along a section fromWoods Hole to Bermuda across the Gulf Stream. Forthis purpose, they needed a new current meter thatwould record for a long time, and a sturdy, reliablemooring for a platform.Richardson et al. were influenced by Swallow'smeasurements


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