Strait of Gibraltar

The Strait of Gibraltar connects the Atlantic Ocean with the Mediterranean Sea. The water body in the Strait of Gibraltar and its approaches consists of a deep layer of salty Mediterranean water (salinity approximately 38 psu) and an upper layer of less salty Atlantic water (salinity approximately 36 psu). The mean flow is composed of two counter-flowing layers: an upper layer of Atlantic water flowing into the Mediterranean Sea and a lower layer of Mediterranean water flowing into the Atlantic Ocean. The mean depth of the interface between these two layers slopes down from about 80 m at the Mediterranean side of the strait to about 800 m at the Atlantic side. The relative change of density across this interface, which is mainly determined by the salinity difference and is therefore called a halocline, is 0.002. For a comprehensive summary of the oceanography of the Strait of Gibraltar the reader is referred to the paper of Lacombe and Richez (1982). The Strait of Gibraltar has a complex bottom topography containing several ridges as depicted in the topographic map shown in Fig. 1. The shallowest section in the Strait of Gibraltar is at the Camarinal Sill where the maxium water depth is 290 m. The interaction of the predominantly semidiurnal tidal flow with the sills inside the strait, in particular with the Camarinal Sill, gives rise to periodic deformations of the halocline in the sill regions which then give birth to internal solitary waves. The ERS SAR images have revealed that the internal solitary waves only propagate eastwards into the Mediterranean Sea and not westwards into the Atlantic. By model calculations Brandt et al. (1996) have shown that this asymmetry of the internal wave field in the Strait of Gibraltar and its adjacent waters results from the east-west asymmetry of the mean flow in the upper and lower layer of the strait.

Fig. 1: Bottom topography of the Strait of Gibraltar. The shallowest section is at the Camarinal Sill. (Figure reproduced from Brandt, P., Alpers, W. & Backhaus, J. O., Study of the generation and propagation of internal waves in the Strait of Gibraltar using a numerical model and synthetic aperture radar images of the European ERS 1 satellite, J. Geophys. Res., 101, 14237-14252 (1996).)