Barotropic flows observed by the towed transport meter

The structure of the meridional transport east of Abaco, the Bahamas, was simultaneously observed with moored current meters and a towed electric field sensor. Important currents in this region include the northward flowing Antilles Current near the surface and the Deep Western Boundary Current flowing southward at depth. The towed measurements of vertically averaged velocity were obtained with the Towed Transport Meter version 2 (TTM2) based on the observation of motionally induced electric fields. The electric field component parallel to the track of a ship is equal to the vertical component of the Earth´s magnetic field times the difference in the cross-track velocity component of the ship and the cross-track velocity component of the electrical conductivity-weighted, vertically averaged horizontal velocity (denoted as v¯*) of the ocean. The velocity of the ship normal to its track was determined from radio navigation methods (e.g. LORAN-C) and the ship´s gyrocompass. The difference between the ship´s velocity normal to its track and the TTM2 observed velocity was v¯*. The differences between v¯* and the conventional v¯ indicated that the conductivity weighting and the effect of bottom electrical conductances were small, of the order of 5%. The comparison with the moored current meter array revealed gyrocompass errors of up to 2.5°. Such errors will be reduced with the application of GPS heading sensors in the near future. A change in the vertically averaged north velocity component of more than -0.25 m s^-1 over less than 20 km was observed in both methods. The cumulative northward transport reached about 10 Sv 70 km east of Abaco, and by 130 km offshore it had decreased to -8 Sv. The velocity and transport structure measured by the two techniques was very similar, supporting the accuracy of the towed measurements as well as the transport estimates from the moored array