Observations of Transport Variability in the Baltic Sea by Parasitic Use of a Fiber-Optic Cable

Abstract

Transports between the Swedish mainland and the island of Gotland were studied by means of motionally induced voltages. The copper mantle of an existent fiber-optic telecommunications cable was grounded on Gotland, and the data acquisition system was installed on the mainland, whereby the bulk flow over the section could be observed. The magnetic field was simultaneously measured so as to permit a comparison with the induced voltage. It was found that geomagnetic fluctuations only weakly affected the observed voltage differences at subinertial frequencies, whereas in the superinertial range a considerable degree of geomagnetic influence was detected. The induced-voltage results were also compared to the geostrophic transports determined using tidal gauge data, analysis of which showed that the stratification of the western Gotland basin affected both types of calculations. Transport adjustments, based on taking into account the effects of stratification as well as the bedrock geological characteristics, were thus carried out. It was found that the best correspondence between the two independent estimates of the transport was obtained by locating the main halocline at a depth of 78 m, in good agreement with hydrographic results from the region.