Tidal induced variability of mixing processes on Camarinal Sill (Strait of Gibraltar): A pulsating event

The tidal cycle and its amplitude appear as an a priori forcing factor causing periodic mixing events on the Gibraltar Strait sill. These mixing events can supply nutrient-enriched deep waters to the surface atlantic layer flowing to the Mediterranean. Therefore these mixing phenomena have both important biological and biogeochemical implications at the regional and even basin level. Fortnightly variations of the effects caused by changes in tidal amplitude have been studied by analysing the characteristics of surface water masses flowing through the Strait of Gibraltar during several cruises carried out in 2002 and 2003. These variations were studied through the analysis of results from 7 diel cycles performed at a fixed station at the eastern side of the Strait monitoring the atlantic jet during several tidal cycles of different amplitude. When strong tidal currents occur in higher amplitude cycles, the occurrence of mixed water is more frequent than during neap tides causing a distinctive TS diagram which lacks a clear NACW signature. It is also a common pattern that the highest level of fluorescence in each cycle coincides with the presence of these mixed waters, appearing as an intermittent flow of phytoplankton biomass to the Mediterranean sector and suggesting a strong influence of the physical forcing on the biological patterns.