winter the Northern Current is observed to flow closer to the continental slope, showing a narrower and deeper structure. Its major variability occurs in two period bands of 2-5 days and 7-10 days at the entrance of the Gulf (off Marseille), occurring in slightly longer periods (bands of 3-6 days and 10-20 days respectively) upstream (i.e. off Nice). The meandering amplitude of the 3-6 days band is of 30- 60 km wavelength, with propagation speeds of 10 km/day, whereas the 10-20 day variability band (observed to reduce to a 10 day variability band in spring) has been mostly associated with Northern Current pulses. There is offshore advection of mesoscale variability associated with the Northern Current towards the open sea, mainly of cyclonic sign. The origin of the mesoscale variability of the Northern Current has been normally associated with baroclinic instability, although other processes such as barotropic instability may also play a role in the observed variability (Flexas, 2003; Jordi et al., in press).
Once in the Balearic basin, at its southernmost extent, the Northern Current is divided into a flow along the northern coasts of the Balearic Islands and a southward flow intruding the Alborán Sea. The latter flow completes the entire ‘cyclonic’ gyre of the AW.
Western Mediterranean Intermediate Water
During winter, the northwestern Mediterranean Sea is characterised by strong cold northwesterly winds. When these winds are persistent and strong they can cool the surface AW so it becomes denser and sinks. This newly-formed water will be overlaid by AW from the surroundings and will receive the name of Western Mediterranean Intermediate Water (WIW). Therefore, this winter mechanism produces a water with the same characteristics as the AW but with a minimum temperature located just beneath the AW. This mechanism of formation was proposed by Conan and Millot (1995). The formation of WIW is common in the whole northwestern Mediter- ranean sea in winter, but the coldest WIW is found in the north. The WIW follows the AW along the continental slopes of France and the Iberian penin- sula to finally intrude into the Algerian basin, although it can also be introduced directly from the Gulf of Lions if it interacts with eddies associated with the North-Balearic front.
The formation of WIW has special importance in the mixing of AW with the underlying water, the
LIW: since the WIW has intermediate characteris- tics between AW and LIW, the presence of WIW will facilitate their mixing.
Intermediate and deep water masses
Levantine Intermediate Wate , Eastern Mediter- ranean Deep Water and Tyrrhenian Deep Water
The Levantine Intermediate Water (LIW) flows under the AW and the WIW. This intermediate water has its origin in the eastern Mediterranean basin and is introduced in the western Mediterranean basin through the Channel of Sicily. It has a maximum temperature and salinity below 200 m depth.
The usual hypothesis of Tyrrhenian Deep water (TDW) formation was that it originates though a mixing of newly entered LIW from the eastern basin and ‘old’ LIW which has been travelling along the western Mediterranean and is returning from the Algerian basin to the Tyrrhenian Sea. This has been recently disregarded (Fuda et al., 2002) and instead a hypothesis of dense water formation occurring within the Tyrrhenian Sea has been proposed.
Recently, in the Strait of Sicily a cold dense vein of slightly different characteristics flowing beneath LIW directly over the sea bottom and following the LIW path has been observed. This water is present in the Ionian Sea between the LIW and the Eastern Mediterranean Deep Water (EMDW). Recently, this vein has been referred to as transitional Eastern Mediterranean Deep Water (tEMDW) (Vetrano et al., 2004; Sparnocchia et al., 1999), Transitional Ionian Deep Water (Iudicone et al., 2003), and also EMDW (Astraldi et al., 2001). At the entrance to the strait this vein presents mean hydrographic charac- teristics of T = 13.53ºC; S = 38.723 psu and σθ ~ 29.17.
The LIW and EMDW enter the western Mediter- ranean from the eastern basin through the western side of the strait, along the Tunisian coast (Astraldi et al., 2001; Fuda et al., 2002). This occurs at a rel- atively shallow depth, at ~300m, and below a strong LIW flow. The entrainment stress between AW and LIW plays an important role in the along-channel pressure gradients, which finally push the EMDW vain through the Sicily Strait (Astraldi et al., 2001). They then enter the Tyrrhenian Sea; there is no evi- dence of any current path from the eastern Mediter- ranean coming directly to the Sardinia Channel (Onken et al., 2003). When LIW decreases in veloc- ity, the deep vein re-establishes its geostrophic posi-
MEDITERRANEAN DEEP-SEA BIOLOGY 17