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An introduction to Mediterranean deep-sea biology* - page 10 / 32





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Mediterranean as surface waters, while denser Mediterranean waters (high salinity waters due to low fresh-water supplies and high evaporation) trav- el at mid-depths into the Atlantic Ocean.

The inflow of Atlantic waters spreads along the Algerian coast, resulting in a general anticlockwise flow. This surface water of Atlantic origin is modi- fied during its journey along the Mediterranean coasts, acquiring different thermohaline characteris- tics—it becomes saltier and therefore denser.

The intrusion of Atlantic water into the Alborán Sea draws two anticyclonic gyres, from which only the most westward gyre shows a permanent anticy- clonic motion. The most eastward gyre is more vari- able, and although most of the time it shows up as anticyclonic, it has also been recorded as cyclonic (Viúdez and Tintoré, 1995). When the most east- ward gyre is depicted as anticyclonic, its eastern- most circulation branch that joins the Iberian penin- sula to Algeria is named the Almería-Orán front. Small cyclonic gyres have been observed to be linked to both anticyclonic gyres.

In its eastward flow, the AW flows into the Algerian basin as the Algerian current, a relatively narrow and deep current 30-50 km wide and 200- 400 m deep at the coast, which becomes thinner (in depth) and wider (in horizontal extension) eastwards (Benzohra and Millot, 1995). The Algerian current flows following the Algerian slope to the Channel of Sardinia. The instability of the Algerian current leads to the development of a series of coastal eddies. Although both cyclonic and anticyclonic motions have been observed, only the anticyclonic gyres last for a long time, i.e. for weeks or months. These gyres are associated with a non-wind-induced upwelling. The vertical extent of these mesoscale coastal eddies is what remains more uncertain. The Algerian current and its mesoscale phenomena have been considered as a whole, i.e. as a system in which the Algerian current meanders due to its unstable character and generates a coastal mesoscale eddy and a large eddy in the deeper layer, with both axes initially not centred (Millot, 1994). This motion has been successfully reproduced in the laboratory by Obaton et al. (2000).

The flow of the Algerian current following the Algerian slope can be blocked by the presence of open sea eddies. The interaction of the Algerian cur- rent with an open sea eddy can deviate the Algerian current from the Algerian slope directly towards the Balearic Islands (Taupier-Letage and Millot, 1988). These open sea eddies are considered as late stages

16 F. SARDÀ et al.

of coastal eddies. Their eastward progression is topographically constrained by the Channel of Sar- dinia, where such deep structures are forced to flow northwards, contributing to the unstable flow west of Corsica, before finally turning westwards to return into the Algerian basin. The extension of recent AW from the Algerian basin into the Balearic basin is delimited by the North-Balearic front.

A branch of AW enters from the Algerian basin into the Tyrrhenian Sea, whereas a second branch continues its path to enter the eastern Mediterranean (Font et al., 1998). A part of the former directly returns to the western Mediterranean as part of the Tyrrhenian surface outflow (Astraldi et al., 2002). An occasional cyclonic eddy is placed in the north- ern part of the Tyrrhenian Sea, and is associated with an upwelling induced by westerly winds. The AW flows through the Channel of Corsica with a signif- icant seasonal and annual variability of the flux, which is attributed to the atmospheric conditions over the Liguro-Provençal basin (Astraldi and Gas- parini, 1992).

The northward currents on the east and west coasts of Corsica join to form the Northern Current, which flows mainly following the continental slope in the Liguro-Provençal basin. Part of the flow is directed southwards along the coasts of Sardinia into the Algerian basin, giving the Algerian basin a sup- ply of AW apart from the new AW intruding from the Alborán sea. In the Liguro-Provençal basin, the Northern Current flows following the continental slope along the French coasts and the Iberian penin- sula to the Channel of Ibiza, and flows perpendicu- larly to a thermal front (the Pyrenees front) that sep- arates cold waters from the Liguro-Provençal basin from warmer waters from the Balearic basin. This front shows seasonal variability and has its origin in the cold and strong northwesterlies blowing in the Liguro-Provençal basin, which are blocked by the Pyrenees and therefore damped in the Balearic basin.

The Northern Current flows along the 1000 to 2000 m isobaths at its entrance into the Gulf of Lions. It is a 30-50 km wide current with surface speeds of 30-50 cm s-1. Formally, this current involves AW, LIW, TDW and WMDW, although horizontal velocities are very low below 400-500 m depth. In addition, the presence of a density front that separates continental waters from slope in the Gulf of Lions and all along the eastern coasts of the Iberian peninsula reinforces the surface signature$ of the Northern Current by geostrophy. During the

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