SUBSURFACE FLOAT
6.7. DRIFTING PROFILING FLOATS (ARGO FLOATS)
6.7.1. Profiling Observations from Polar Regions
Because of a lesser amount of observational data in the Arctic Ocean interior, changes of the Arctic Ocean circulation, oceanographic parameters, and sea-ice conditions remained
unclear. In addition, there is debate over how changes in the Arctic Ocean are affecting global climate, e.g., as related to the global ocean circulation. Even though the Arctic Ocean plays a critical role in global climate (e.g.,Morison et al., 2000;Hassol, 2004), sea ice had previously prohibited Argo float observations in the Arctic Ocean. Instead of such instruments as Argo floats, ice-drifting buoys have been the main method of year-round observation in the Arctic Ocean.
Compared to hydrographic surveys of the Arctic Ocean by icebreakers or aircrafts, one of the most important advantages of an ice-drifting buoy is that it can obtain data even in darkness and during severe winter conditions. To provide meteorological and oceanographic observation data throughout the year, the International Arctic Buoy Pro- gramme (IABP) maintains a network of ice-drifting buoys (e.g.,Rigor et al., 2000).
To monitor and better understand the thermohaline conditions in the ocean interior of the polar regions and thus to elucidate Polar Ocean change,Kikuchi et al. (2007) developed a new Argo-type ocean profiling system. This Polar Ocean Profiling System (POPS) is an ice-drifting buoy system that tethers an Argo-type CTD profiler and is deployed in multiyear ice. JAMSTEC and METOCEAN began collaboration to develop POPS in 2004 with the aim of obtaining oceanographic profiling data from beneath the Arctic ice. POPS consists of an ice platform and a subsur- face CTD profiler (see Figure 6.18). It also provides
FIGURE 6.18 Schematic view of the Polar Ocean Profiling System (POPS) consisting of an ice platform and a subsurface CTD profiler.
(Source:Kikuchi et al., 2007.)
meteorological data. Iridium and GPS antennae are located at the top of the meteorological mast, which is placed into a drilled hole in the ice.
The ice platform includes a system controller that manages all data acquisition, processing, formatting, and messaging. The profiler is mounted on an oceanographic cable interfaced to the platform. The profiler moves along the cable between depths of 10 and 1,000 m. The inductive modem system provides data transfer between the ice platform and the profiler. The inductive modem (IM) telemetry system, which includes a Surface Inductive Modem (SIM), an Underwater Inductive Modem (UIM), and two Inductive Cable Couplers (ICCs), is used to establish communication between the ice platform and the subsurface CTD profiler. The SIM is located inside the ice platform, whereas the UIM is inside the profiler. The two ICCs, the plastic-jacketed wire, and the water together make the connection between the SIM and the UIM.
The surface unit power supply consists of two 152-Ah lithium battery packs. The expected lifetime of the ice platform is about 2.68 years for meteorological data acquisition with a GPS position every 3 h and oceano- graphic profiling data acquisition transferred and processed every three days.
Iridium satellite communication technology sends the observation data and allows remote commands to be sent from the laboratory to the buoy. Data can also be sent to the global telecommunication system (GTS) in real time. Data can easily be accessed from the Argo data server. The system was successfully tested in the Arctic Ocean near the North Pole.
The major difference between POPS and the standard Argo floats is that the POPS profiler is mounted on a cable and slides between a pair of upper and lower bumpers located at 10 and 1,000 m on the cable. The cable is a plastic-impregnated 7 7 strand galvanized wire rope.
Figure 6.19shows how the profiler is mounted on the cable.
The upper and lower riders attached to the profiler are carefully designed to minimize friction and drag when the profiler is moving along the cable. In contrast to the original Argo float, the POPS controller software is modified to cope with the nonsurfacing properties and to interface with the IM system (UIM and ICC) to commu- nicate with the ice platform. Its functions include main- tenance of the calendar and internal clock, supervision of the depth cycling process, and activation and control of the hydraulic system.
In addition to the 37-kg weight of the 1,000-m cable itself, a 20-kg terminal weight is used to keep the cable as vertical as possible. Although all these calculations allow full data point collection, much faster ice motion will result in missing data due to tilting of the cable. Placing a heavier terminal weight at the end of the cable is possible in order to minimize the risk of missing data points.
The total weight of the POPS (ice platform, profiler, cable, and 20-kg terminal weight) is less than 150 kg in air.
Therefore, not only a big icebreaker but also a small airplane or helicopter allows accessing the target sites for the POPS deployment on the Arctic multiyear ice. Because of the presence of sea ice, it is necessary to drill a hole through the ice for the POPS deployment. A 10-in.
(25.4 cm) diameter hole is enough to put the terminal weight, profiler, and cable into the seawater.
When the profiler has reached and is stabilized at its parking depth, it will establish communication with the platform. The profiler travels at approximately 8e10 cm/s in ascent while traveling at approximately 5 cm/s in descent.
The parking and maximum profiling depths are set to 300 and 1,000 m, respectively. This bidirectional communica- tion system provides the users with flexibility and control of the observation after the deployment. For example, it is possible to fine-tune these parameters for optimum perfor- mance of the system when ice condition warrants closer attention.
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198 Measuring Ocean Currents
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Chapter 7
Horizontally Integrated Remote Measurements of Ocean Currents
Using Acoustic Tomography Techniques
Chapter Outline
7.1. One-Way Tomography 203
7.2. Two-Way Tomography (Reciprocal Tomography) 210 7.3. Acoustic Tomographic Measurements from Straits 212
7.4. Coastal Acoustic Tomography 215
7.5. River Acoustic Tomography 226
7.6. Acoustic Tomographic Measurements of Vorticity 230 7.7. Horizontally Integrated Current Measurements Using
Space-Time Acoustic Scintillation Analysis Technique 232
References 235
Bibliography 237
Satellite remote-sensing techniques, employing active and passive optical, thermal, and microwave signals, and coastally operated remote-sensing techniques employing active electromagnetic signals in the HF, VHF, UHF and microwave bands are used on an operational scale for remote detection and quantitative mapping of ocean surface current vectors and circulation patterns. However, the inability of electromagnetic signals to penetrate below the surface layer of the ocean has rendered these techniques unusable for remote measurements of subsurface currents and their circulation features. Physical oceanographers have an interest in subsurface current measurements to gain insight into the water circulation in the ocean layers at various depths and its dependence and possible effects on climatological conditions. Apart from this, subsurface currents are of considerable importance in marine geology because of their influence on the transportation and depo- sition of sediment. Knowledge of deep currents is also of interest to biologists because of the currents’ influence on the dispersal of organisms and the maintenance of supplies of nutrients. Regions of convergence or divergence in the horizontal movements of water mass are of particular interest because of their association with vertical move- ments in the form of sinking or upwelling.
The traditional means of making observations of subsurface currents was an indirect one, the so-called dynamical method, based on highly precise measurements
of water temperature, salinity, and depth; the hydrographic tables for computing density; the geostrophic equation; and an assumption regarding the “depth of no motion.” The valuable review of Bowden (1954) focuses attention on the assumptions made and uncertainties involved in the dynamical computations regarding the depth of no motion and the mean subsurface current charts. In fact,Stommel’s (1955) letter to the editor provides an indication of an almost total lack of knowledge on subsurface currents in the early 1950s.
For lack of proper tools, direct measurements of subsurface currents were limited to those made from current meters tethered from anchored ships, moored current meters, freely sinking/rising vertical profilers, and so on. An example whereby much effort has been expended with a large variety of techniques is the Straits of Florida.
Current meter moorings (Lee et al., 1985), Pegasus sections (Leaman et al., 1995), sea-level differences (Maul et al., 1985), and undersea cables (Larsen and Sanford, 1985) have all been employed for measuring the water-current flow there. For a variety of reasons, such limited measurements were not adequate to resolve the spatial structure of deep-water motions. Tracking of subsurface drifters (see Chapter 6) yielded Lagrangian descriptions of subsurface currents. However, these tools were inadequate to provide basin-scale horizontally integrated current measurements.
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201
Since the infancy of oceanographic research, oceanog- raphers have been strapped for observations over large regions of the ocean in anything like rapid enough time to get synoptic views or snapshots. If one goes out and surveys a large region of the ocean with a ship, the ocean might have already changed by the time the survey is completed.
The situation faced by oceanographic researchers is quite different from the situation of meteorologists, who can get virtually instantaneous pictures of the atmosphere from either a global measuring network or satellite images of clouds and so forth. They really do get images that are, effectively, snapshots. In contrast, oceanographers are not so fortunate, with the exception of satellite maps of surface temperature, color, and roughness and possibly, satellite altimetry measurements used as sea surface tomography. But none of these measurements extends beneath the surface. It was hoped that acoustic trans- missions and receptions could well turn out to be one of the few ways to acquire measurements of what is happening in the ocean over a large spatial scale fast enough that one is not hopelessly mixed up between time variations and space variations.
Ocean acoustic tomography (OAT) is a method employed in measuring the ocean by utilizing the favorable properties of sound propagation through water. The outlines of such a system (i.e., the feasibility of monitoring and ultimately studying the oceans by measuring acoustic transmissions between moorings over large distances) were originally proposed by Walter Munk (Scripps Institution of Oceanography) and Carl Wunsch (Massachusetts Institute of Technology), as provided inMunk and Wunsch (1979).
In subsequent years there has been a substantial effort to demonstrate both the practicality of the idea (Spiesberger et al., 1980) and to further analyze the theoretical aspects of sound propagation in this context (Munk and Wunsch, 1982a,b, 1983; Spofford and Stokes, 1984). The theory was further developed and tested to a large part with active contribution from Robert Spindel (Woods Hole Oceano- graphic Institution).
Ocean acoustic tomography techniques have been developed for remote measurements of large-scale subsurface currents and the associated large-scale circula- tion features. Similar to the use of X-rays to produce medical computer-assisted tomography (CAT) scans in hospitals to examine the interior of the human body and the use of seismic waves by geophysicists to determine the Earth’s internal structure, OAT employs low-frequency sound waves to probe large sections of the oceans. Ocean tomography describes, layer by layer, the interior features of the ocean by transmitting sound waves along many transmitter-to-receiver paths between distantly spaced instrument moorings. The details of the ocean’s interior are revealed by interpreting the arrival times of the sounds, since their speed is either accelerated or decelerated by the
temperature and current of the interior ocean. By trans- mitting sound waves through hundreds of miles of ocean, it becomes possible to make measurements over an area that would otherwise require a fleet of ships working for many weeks. With OAT it becomes possible to take a look at large-scale ocean circulation in a synchronous manner, which is a requirement to better understand a fundamental problem in oceanography. No other technique has the potential to make these ocean-interior measurements. The appeal of acoustical techniques is several-fold (Cornuelle et al., 1985):
1. Tomography techniques are by nature integrating, automatically filtering out undesirable small-scale features that “contaminate” normal point measure- ments. The unwanted features can range from micro- structure and internal waves, if one’s interest is in the mesoscale (order 100 kilometers), to the mesoscale itself, if one’s interest is in the gyre-scale circulation.
2. In principle, the information content of tomographic arrays grows quadratically with the number N of moorings deployed rather than approximately linearly, as with conventional point moorings. It may be noted that over long distances, a source may not be heard by all receivers, and the addition of new instruments thus may not provide information quite as fast asN2. 3. As a consequence of the waveguide nature of sound
propagation over much of the ocean, a single source- receiver pair can provide information about the hori- zontal average of the vertical structure of oceanic disturbances that would otherwise require large numbers of vertically distributed instruments.
The theory on paper advanced to sea in two tests conducted in the Atlantic Ocean during 1981 and 1983 and subsequent tests funded by the U.S. National Science Foundation (NSF) and the Office of Naval Research (ONR). Each at-sea test became more complicated as the tomography itself grew more sophisticated. There are essentially two kinds of tomography: (1) one-way tomography (called the zero-order kind of tomography) and (2) two-way tomog- raphy (called reciprocal tomography). The former is essentially measuring the temperature of the ocean by measuring the time it takes for sound to traverse from a source to a receiver; the latter measures the difference in time it takes to send a signal from a source to a receiver and back again.
Although the principle of OAT was well founded, its practical realization on an operational scale took a long time because the issues had to do more with engineering developments. The principles had been demonstrated, but it was necessary to develop better, cheaper, very reliable, long-lived sources and receivers. The science that could be developed with the application of acoustic tomography did not advance rapidly enough, because oceanographers had
to wait for the more complicated engineering to catch up to the need. The elaborate electronic equipment for the acoustic tomography experiment was not yet commer- cially available and had to be designed and built by the technologists at various oceanographic research institutions.