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An introduction to Mediterranean deep-sea biology* - page 12 / 32

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tion along the Sicilian coast, which is, due to its high density, at ~1500-1800 m depth. This sinking caus- es strong mixing and increases the volume of TDW (Sparnocchia et al., 1999), which together with LIW forms the Tyrrhenian basin outflow at intermediate and deep levels.

The LIW and TDW follow the same path, flow- ing along the western coasts of Sardinia. On the southwest coast of this island, the LIW + TDW shows its maximum variability: from being a rela- tively narrow vein to the south of Sardinia, it becomes wider (in horizontal direction), thinner (in depth) and cooler to the west of the island. These changes have been attributed to the interaction with open sea Algerian eddies that migrate from the Algerian slope to open waters, and also to the inher- ent instabilities of the intermediate water vein, which is presumably able to generate anticyclonic structures. Millot (1999) proposed calling these anti- cyclones ‘Leddies’, similar to the naming of the Atlantic Meddies.

The LIW + TDW continue flowing along the continental slope of Corsica on an anticlockwise path similar to the AW, along the slopes of the Lig- uro-Provençal basin (T = 13.4-13.5ºC and 38.50- 38.55 psu), only disturbed by occasional winter sea- ward spreading. Its general path continues along the continental slopes of the Iberian peninsula towards the Alborán Sea and part of it flows towards the Algerian basin following the Almería-Orán front or the westernmost Alborán anticyclonic gyre. Once there, the LIW + TDW is no longer present as a vein, but it still follows the Algerian slope until it interacts with an Algerian eddy and is exported northwards.

Western Mediterranean Deep Water

The deep water beneath the TDW is the Western Mediterranean Deep Water (WMDW). The WMDW is formed in the Gulf of Lions during severe winter conditions, due to cold Mistral and Tramontane wind events (MEDOC Group, 1970), by small con- vective cells or “chimneys” with high downward vertical speeds.

Its origin lies in vertical mixing produced by small plumes of the order of a few hundreds of metres in diameter with vertical velocities of the order of 10 cm s-1 that develop into convective cells with downward vertical speeds of 1 mm s-1 (Gas- card, 1978). Eddies of a few km in diameter devel- op (due to baroclinic instability) and contribute to

18 F. SARDÀ et al.

re-stratification. The WMDW does not form every winter: during mild winters this mixing process forms intermediate waters resting above 1500 m depth. The mean hydrographic characteristics of WMDW are 12.7-12.80ºC and 38.44-38.46 psu, although they have been seen to change with a 10- year period, showing an increasing tendency in both salinity and temperature of 0.03ºC/10 years and 0.02 psu/10 years (Millot, 1999).

Deep water formation has historically been divided into three phases: preconditioning, vigorous convection and relaxation (MEDOC Group, 1970). All three phases are spatially highly non-uniform and show mesoscale activity at short time scales, together with advection processes like intrusions of LIW and surface capping (Gaillard et al., 1997). Specifically, the preconditioning phase involves cooling of the water surface due to the effect of the cold N-NW winds and leads to a rising and tilting of the isopycnals, affecting the horizontal and vertical density distribution of the shelf-slope front.

Although deep water formation is mostly found to occur in the open sea, strong wind events (i.e. Mistral and Tramontane) occurring over the Gulf of Lions continental shelf can also lead to the formation of deep water. During severe winters, the wide conti- nental shelf in the Gulf of Lions enhances deep water formation on the continental shelf. When this occurs, the newly-formed deep water spreads into the open sea perpendicularly to the continental slope, as a cas- cade, down to 1500 m depth. This cascading is impor- tant not only from a dynamic point of view but also in terms of biogeochemical cycles: during most of the year the Gulf of Lions continental shelf is a sink for nitrate, but during winter dense water cascading is responsible for nitrate exportation toward the open sea (Tusseau-Vuillemin et al., 1998).

The WMDW formed in the Gulf of Lions follows basically the continental slope at depths greater than 1500 m. Once it reaches the Alborán Sea, part of the flow is released up to 300 m depth by mixing processes and part is introduced into the Algerian basin, where it continues flowing along the conti- nental slope. In this region it acquires a general anti- clockwise circulation. It is worth noting that close to the bottom, at about 2500 m depth, along-slope mean speeds are of the order of 3-4 cm s-1, and they are greater than 1500 m above. It has been hypothe- sised (Millot et al., 1997) that this is due to the effect of the anticyclonic eddies, which are believed to induce intense currents in the whole deep layer and near the bottom.

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