A preliminary survey of thermal microstructure in the Strait of Gibraltar

The 1958 survey provided the data for a preliminary synoptic study of the complicated dynamic phenomena of the Strait of Gibraltar. For this purpose, attention was focused near the boundary surface of the deep layer of pure Mediterranean water east of the sill. West of the sill this dense layer loses its character in a process of turbulent mixing with the lighter Atlantic water.

The survey of the thermal microstructure of the sill area was made possible by the use of the new towed thermistor chain of the Woods Hole Oceanographic Institution (Richardson and Hubbard 1959). In 66 crossings of the Strait (north to south) above the sill and one passage (west to east) along the axis, the following were the most interesting findings:

• 1.

  (1) - The boundary between deep Mediterranean water and Atlantic water oscillates vertically with tidal periods.

• 2.

  (2) - The boundary layer was found to slope down sharply at the centre of the channel at the time of internal high tide.

• 3.

  (3) - Vertical motions of the isotherms of semi-diurnal period and in phase with the moon transits were dominat.

• 4.

  (4) - Fluctuations of high amplitude and non-tidal period were superimposed on the dominant motions.

• 5.

  (5) - Solitary rapid depth variations of some isotherms were recorded more frequently over the southern portion of the sill. The maximum depth change recorded was 80 m.

• 6.

  (6) - The surface temperature is warmer on the Spanish than on the African side of the Strait. A four degree Centrigrade difference was found at one time along the four-mile long transverse above the sill.