•Background

•References

•Data Access

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 in the vertical). 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. As part of the corrections for the aforementioned effects, in situ estimates of the ocean transport are obtained using either velocity profile (Lowered Acoustic Doppler Current Profiler) or GPS-equipped floats (Dropsondes) and these transport estimates are compared to the cable estimates and are used to calibrate the cable-derived transports and also determine voltage offsets. Since 1982 cables have been used to measure the transport of the Florida Current between Florida and the Bahamas near 27°N. The time series of transport estimates from the first 16 years of cable measurements is shown in Figure 1. 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].

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 are being recorded by a system at Eight Mile Rock. 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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