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Project Background: Cable physics

Basics of Cable physics

When electrically charged particles move through a magnetic field an electrical field is developed that is perpendicular to the movement of the particles. This has been known since the pioneering experiments of James Maxwell in the mid-1800s. The same physics dictate that when ions in seawater are advected by ocean currents through the magnetic field of the Earth, an electric field is produced perpendicular to the direction of the water motion. Because seawater is a conductive media, these electric fields "short-out" in the vertical, yielding a single electric field corresponding to the vertically averaged horizontal flow (with a minor vertical weighting effect due to small conductivity changes at different depths). Submarine cables provide a means for measuring these "motionally-induced" voltages in the ocean. Using the voltages induced on the cables, the full-water-column transports across the cable can be estimated. The initial demonstration of the ability of a submarine cable to measure transport was made by Stommel in a study of the transport between Key West and Cuba [Stommel, 1948]. More recently, the conversion of the voltage measurements to transport and the errors associated with this conversion, such as geomagnetic fluctuations, temperature, and meandering effects, have been discussed in Larsen, 1992.

The voltage signal on a submarine cable will be a blend of the motionally induced signal due to oceanic transport variations with other fluctuations of the magnetic field of the Earth, which can be forced by both internal changes in the planetary core as well as by external fluctuations due to processes such as solar storms. In order to estimate the oceanic transport fluctuations using the cable it is necessary to remove, as best as possible, the voltage variations that are due to fluctuations of the Earths magnetic field. This is a complex procedure, as the externally-forced magnetic field variations (primarily solar) dominate the observed voltage signals at periods of less than a few days [Luther et al., 1991], while at very long periods, centuries and longer, magnetic field fluctuations due to internal changes within the Earth dominate the voltage signals.  In order to remove the voltage fluctuations that are not oceanic in nature, magnetic field measurements from the nearest magnetic observatories are used to estimate the magnetic field at the site of the cable, and transfer functions are estimated to determine the voltage signal associated with the observed magnetic field variations.  The United States Geological Survey maintains a number of magnetic field observatories at a wide range of sites throughout the US and its territories through their geomagnetism program.  To the extent that the signals at the observatories are represented in the voltage time series from the cable, this should allow for the removal of the non-ocean related voltage changes.  In practice, the high frequency variations are not generally well enough correlated between the cables and the observatories to remove the high frequency signals from the cable with a high level of precision. As a result, the voltage values are commonly low-pass filtered to remove these non-ocean signals and then the voltage is averaged to provide daily averages. (The tide signal must also be removed prior to averaging to avoid aliasing any of the tide signal into the daily mean values.)

An additional part of the conversion from voltage to transport involves the use of in situ estimates of the ocean transport which are obtained using either velocity profile (Lowered Acoustic Doppler Current Profiler) or vertical mean velocity estimates (from GPS-equipped floats called Dropsondes). Historically the Pegasus velocity profiling system was also used for calibration of submarine cables, but due to economic reasons that system is rarely utilized these days. The in situ transport estimates are compared to the cable estimates and are used to calibrate the cable-derived transports, taking into account factors such as the length of the cable and the conductivity of the sediments surrounding the cable.


Application to the Florida Current

Since 1982 cables have been used to measure the transport of the Florida Current between Florida and the Bahamas near 27°N. Geomagnetic data from the San Juan, Puerto Rico, Fredricksburg, Virginia and Stennis, Mississippi magnetic observatories have been used during different segments of this time period to remove the magnetic field fluctuations from the voltage data (the Bay Saint Louis data is being used currently).  The time series of estimated transport from the first 16 years of cable observations is shown in the upper panel of the Figure at right. Also shown are the in situ transport estimates shown as black dots; these in situ transports are estimated to be accurate to better than 0.2 Sv [Hacker et al, 1996, given that the Pegasus values are accurate to 1 cm/s]. The lower two panels show low-pass filtered versions of the time series to illustrate both the large intra-seasonal and interannual signals observed by the cable.

Details of the history of cable measurements in the Florida Current can be found in Larsen, 1992. Most recently, a cable between West Palm Beach, FL and Eight Mile Rock, Grand Bahamas Island has been used. These measurements continued until October 1998, when this cable was retired from telephone service and replaced by a cable between Vero Beach, FL and Eight Mile Rock, Grand Bahamas Island. When the first cable was retired, it was grounded at West Palm Beach with the expectation that voltages could be recorded only at the other end. Technical difficulties in the retirement of the cable and funding problems led to the delay in instrumentation of this cable until March of 2000, resulting in a 17 month gap in the time series. A range of instrumental problems and cable difficulties have further delayed the reintroduction of high-quality calibrated cable measurements in the Florida Current, however most of these problems have now been solved and good quality voltages have been recorded by a system at Eight Mile Rock since October 2003 (there is good reason to hope some of the time period from 2000 to 2003 will be recoverable with additional work in the near future). An additional two month gap in the time series during September-October 2004; this resulted due to the destruction of our recording equipment by the combined forces of Hurricanes Frances and Jeanne during August and September 2004.  New recording equipment was put in place in late October 2004, and voltage recording has been consistent since that time. 

In the future it should also be possible to record voltages on the new active fiber optic cable using the insulated copper shield that is grounded at Eight Mile Rock, providing a second measurement both as a quality control and as a backup for future cable problems. Therefore, low cost voltage measurements of the Florida Current should be able to continue for many years. Figure 2 shows the locations of the various cables crossing the Straits of Florida.

The Florida Current project, part of the Western Boundary Timeseries Project funded by the NOAA Office of Climate Observations, will provide a calibrated time series of Florida Current transports to the community via this web site. Questions about the project, or about this web site, should be directed to Christopher Meinen or Molly Baringer.

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