S16. Instruments and Methods

CHAPTER S16 Instruments and Methods This chapter on methods for measuring the S16.1. THE IMPACT OF SPACE AND large-scale circulation and water properties TIMESCALES ON SAMPLING AND of the ocean, emphasizing instrumentation, is INSTRUMENTATION published solely online at http://booksite. academicpress.com/DPO/; “S” denotes online The time and space scales of physical ocean- supplemental material. Many of the methods for ographic phenomena were summarized in measuring basic properties such as temperature, Chapter 1 (Figure 1.2). Data collection require- salinity, and pressure were described briefly in ments to study motions with so many time Chapter 3. Some of the satellite observations and space variations are demanding, calling were described in Chapters 3e5. Many of these for a wide variety of sampling methods. As techniques are also used for smaller scale described in Chapter 6, studies at almost every phenomena such as waves. Every decade brings scale require averaging or filtering to remove new advances and thus the descriptions presented space and timescales that are not of interest. It in succeeding editions of this text have been is not possible to measure every space and time- quickly outdated. Nevertheless, it is useful to scale, however, to form perfect averages and understand what types of instruments have been statistics. Therefore observational oceanogra- available at different points in oceanographic phers must understand the sources of error development and their resolution, precision, and and uncertainty, which can be due to instru- accuracy.The information here primarily supports mental or sampling limitations, or to signals at Chapter6,DataAnalysisConceptsandObserva- different frequencies and wavelengths. tional Methods, in the printed textbook. For example, traditional deep oceanographic In Section S16.1 some of the sampling issues for profiles (Section S16.4) were and continue to be physical oceanography are discussed, augment- made from research ships to study the very ing the discussion in Chapter 1. In Section S16.2 largest spatial and temporal scales of the ocean platforms for observations are described. In circulation and property distributions. These Sections S16.3 through S16.8 instruments for in remain the only way to measure the deep situ observations (within the water column) are ocean with high accuracy, and the only way reviewed. Section S16.9 is an overview of satellite to make most chemical measurements. A deep remote sensing, and Section S16.10 briefly oceanographic station can take up to three hours describes oceanographic archives. A recent review and a cross-section across an ocean can take up of oceanographic instrumentation by Howe and to two months, posing limitations to interpreta- Chereskin (2007) is also recommended. tion. The individual, widely separated profiles 1 2 S16. INSTRUMENTS AND METHODS cannot be used to study tides, internal waves, or them as much as possible from the longer eddies, for instance, but these and other smaller timescales. scale motions affect the individual station Returning to observing the largest scale circu- measurements. There are, however, useful lation from the top to the bottom of the ocean, ways to process and analyze the data so that which is the primary focus of this text, it might they can be used to study the large space and appear that employing numerous instruments timescales of interest. that measure the currents directly would be As a second example, satellite altimeters the best approach. Indeed, at the onset of the (Section S16.9.9) measure the ocean’s surface twenty-first century a global program (Argo, height, passing over each point on the ocean’s described in Section S16.5.2) to continuously surface every week or two. Surface height monitor velocity within the water column was depends on several things: the ocean circulation, initiated using relatively inexpensive subsurface surface waves and tides, expansion and contrac- floats that follow the subsurface currents (mostly tion due to more or less heat or salt in the water, at a single depth) and report back to satellites at and the uneven distribution of mass in the solid regular intervals. This program has already earth (variations in the geoid). The geoid, which revolutionized observing of the ocean interior, does not vary in time, dominates the altimetric primarily because of the temperature and signal. Therefore the time-dependent altimetry salinity profiles collected on every trip to the measurements have been most useful, providing surface, which has been standardized at ten- significant information about the time-depen- day intervals; the velocity data have been less dent “mesoscale” (tens to hundreds of kilome- utilized. A global deployment of surface drifters ters) and large-scale time dependence in sea- accomplishes the same objective at the sea surface height, which is associated with changes surface (Section S16.5.1). These ocean-wide in large scale circulation, climate variability such Lagrangian sampling methods were not as El Ninõ, and global sea level rise. possible prior to the beginning of global satellite Interpretation of the altimetry measurements communications, and it is still prohibitively in the presence of thermal expansion requires expensive to instrument the ocean at all depths. information on the temperature and salinity Current meters, both mechanical and acoustic, structure beneath the surface, which a satellite directly measure flow at a given point for cannot see. Therefore in situ measurements are several years; they were developed and combined with altimetry. Since the different deployed widely after the 1950s. Current meters data sets are mismatched in sampling frequency give information on the velocity (speed and and location, the combination poses significant direction) of the water only close to the location data analysis challenges, dealt with most (in time and space) of the instrument itself; recently through use of data assimilation experience indicates that large variations in (Section 6.3.4). And as a third example drawn velocity can occur over small distances as well from altimetry, the many days between satellite as over small time intervals. Because of these passes over a given location means that shorter spatial scales and because of the high expense timescales, due for instance to tides, are of current meter deployments, it has not proven measured at different times in their cycles on possible to widely instrument the ocean. Current each satellite pass. This “aliasing” produces meters are now used primarily in well-defined a false long timescale (Section 6.5.3). Great care currents of no more than several hundred kilo- is taken in choosing satellite orbital frequency meters width, or in specific target areas to sample and in interpretation of the data to properly all of the temporal scales (the full time spectrum) deal with these shorter timescales, to remove in that area, sometimes for many years. All of the PLATFORMS 3 direct current measurements of subsurface formalize the use of constraints based on mass currents have provided just a small proportion conservation and on property distributions, of our observed knowledge of the ocean circula- which are affected by mixing. tion. On the other hand, where they have been Some water properties also are inherent used they provide invaluable information; for tracers of time (Sections 3.6 and 4.7). These instance, quantifying the total transport and vari- include tracers that are biologically active and ations of strong, relatively narrow currents like are reset at specific locations. For example, the Gulf Stream or Kuroshio. oxygen content is saturated through contact In the absence of sufficient direct measure- with the atmosphere in the surface layer, and is ments of ocean currents, oceanographers then consumed by bacteria within the water studying the circulation use indirect methods. column, yielding a rough age for a given water One of the oldest, remaining in very common parcel. The built-in clock of radioactive decay use, is the geostrophic or dynamic method, which in transient tracers offers more promise, as it is relates the horizontal pressure distribution to independent of the physical and biological char- horizontal currents (Section 7.6). Most currents acter of the environment. Anthropogenic tracers with timescales greater than a few days (except such as chlorofluorocarbons (CFCs) have been at the equator) are in geostrophic balance, which injected into the earth system by mankind. If is a balance between the horizontal change the history of their release into the environment (gradient) in pressure and the Coriolis force. is known, as is the case for CFCs, then they are The geostrophic velocity is perpendicular to the useful tracers of the paths taken by surface ocean pressure gradient direction due to Earth’s rota- waters as they move into the interior ocean. tion. The pressure distribution depends on sea- surface height and also on the vertical profile of seawater density at a given latitude and longi- S16.2. PLATFORMS tude. Thus the chief method for mapping ocean circulation has been to measure the temperature Manned measurement platforms are described and salinity distribution of the ocean. The density here. Autonomous (unmanned) platforms such as distribution is then calculated, from which the floating or moored instruments, or satellites, are horizontal pressure gradient is calculated at described in later sections. every depth, given an assumption of the pressure gradient at one depth (which could be at the S16.2.1. Ocean Research Vessels surface, due to surface height). The geostrophic currents are then calculated. The majority of oceanographic measurements The step of estimating the pressure gradient have been made from research ships with auxil- at one depth is nontrivial, given the general iary measurements from merchant ships (ocean lack of distributed velocity observations. (The temperature and salinity, weather) and from subsurface float deployments starting in the coastal stations (tide gauges, wave staffs, light- 1990s were first motivated by providing such house temperature and salinity observations, a velocity field at one depth.) The traditional etc.).

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