Transport and heat flux of the Florida Current at 27°N derived from cross-stream voltages and profiling data: theory and observations

Abstract The variability in the volume transport of the Florida Current at 27°N has been observed from the voltage measured between a point 16 km east of Jupiter Inlet, Florida and Settlement Point, Grand Bahama Island by the use of an abandoned communications cable. The voltage is caused by the separation of electric charges in the ocean that are caused by the Florida Current crossing the Earth’s magnetic field (motional induction), by the electric currents induced in the ocean caused by time varying ionospheric and magnetospheric electric currents (geomagnetic induction), and by the voltage bias between the cable-ocean contacts caused by differences in the temperature, salinity and electrochemical state of the contacts. The geomagnetic-induced voltage was removed by the use of shore-based magnetic data and transfer functions. Changes in the voltage bias of the cable-ocean contact at Settlement Point were minimized by the use of a silver-silver-chloride electrode. The comparison of the daily mean motion-induced voltages with 137 daily profiling-derived transports, yields a voltage calibration factor of 24.42 + 0.56 Sv V-1, a correlation squared of 0.94, and a root mean square misfit of 0.77 Sv (1 Sv = 106 m3 s-1). The misfit is consistent with an error of 0.9 Sv in the voltage-derived transport estimated from the velocity and temperature profiling data, the electric current effects, and the profiling derived transport error. Hence the voltages accurately monitor the transport variations. The mean voltage-derived transports for the Florida Current at 27°N is 32.3 + 3.2 Sv based on 4862 daily mean values from 1969 to 1990. The annual variation has a range of 4.4 Sv. There is no evidence since 1969 for any long-term trend in the transport but there are numerous shorter term variations as large as 15 Sv lasting from days to months. The mean northward heat flux is 1.30 + 0.13 PW derived from 130 days of profiling the velocity and temperature of the Florida Current and assuming the North Atlantic has a southward, velocity-weighted temperature of 9.4 °C. The variations in heat flux are, essentially, due to the variations in transport because the velocity weighted temperature estimated by the profiling data has a nearly constant value of 19.1 ± 0 .6°C.