Intimate ’96: Shallow water tomography in the Sea of the Condemned

Abstract

As is well-known, the tidal force of the moon and the sun can cause notable changes in the sea level. Besides this so-called barotropic effect, the tidal force also drives internal waves in a daily rhythm. Thus, the internal wave spectrum is often dominated by a single component with perhaps 10 km from crest to crest. This ‘‘internal tide’’ tends to propagate toward shore and has its greatest height near the shelfbreak. As this tide propagates it modulates the surface duct and its acoustic signature is often seen in data. The Intimate ’96 experiment (conducted off the coast of Portugal) was specifically designed to acoustically image the internal tide with an eye toward a more precise understanding of its structure and acoustic impact. A towed source emitted chirps every 8 s for several days and the chirps were received on the SACLANTCEN portable array. The data show a textbook multipath structure with early refracted paths followed by some 30 distinct bottom and surface echoes which shift with the internal tide. The acoustic and oceanographic interpretation of this data will be discussed.