Acoustic monitoring of the transport and temperature variability in the Strait of Gibraltar

Abstract

Three acoustic transceivers transmitted across the Strait of Gibraltar during April–May 1996 to determine the feasibility of using acoustic methods to measure the transport and average temperature in the Strait. The first arrivals, from ray paths confined to the (lower layer) Mediterranean water, give sum travel times that are remarkably stable, with a slow peak-to-peak variation of only about 3 ms over a two-week period, corresponding to an integrated temperature change of about 70 m °C. The difference travel times from a path diagonal to the axis of the Strait show a peak-to-peak tidal variation of about ±5 ms, corresponding to integrated currents parallel to the acoustic path of about ±50 cm/s, which is consistent with estimates derived from a moored current meter array. One of the transceivers also had a two-element horizontal receiving array, to determine the feasibility of using arrival angle to measure the current perpendicular to the acoustic path. The measured phase differences are encouragingly similar to phase differences predicted using the current meter data. Later arrivals, from ray paths that sampled the (upper layer) Atlantic water, are much more complicated, in part due to the effects of the internal wave bores present in the Strait.