The boiling-water phenomena at Camarinal Sill, the strait of Gibraltar

A detailed description of high-amplitude steady topographic internal waves recorded at Camarinal Sill during a survey on the R.V. “Investigador” is presented. These internal waves are generated during the maximum outflow (westward) stage of the tidal current and remain over the sill for more than 4 h until the outflow slackens, then being released towards the Mediterranean. Their amplitudes are comparable to the well-known internal bore of Camarinal Sill, occurring during maximum outflow during spring tides. However, they respond to a different physical origin and their spatial features are also quite different. In fact, the favourable hydraulic conditions for the generation of steady waves over the sill inhibit the internal bore generation, and vice versa. Analysis of the observations suggests that steady internal waves are the result of a resonant response of the stratified fluid over the sill to the forcing of the flow on the across-sill topography. An important consequence of the steady internal waves with clear biological implications is the significant mixing phenomena that are induced. Mixing is enabled by an enhancement of the shear at the trough together with a significant induced vertical advection.