and 0.04 psu , respectively [Béthoux and Gentili, 1997]
(Fig. 1).
Figure 1:
Temperature and salinity trends observed in the deep water of the Algero-Provençal basin
over the 1959-1997 period. Each point represents a mean value for
data acquired at a depth of between 2000 to a maximum of 2700 m ; however, the trends
concern the entire deep water from a depth of about 800 m depth. Hydrological stations
from 1959 to 1994 are referenced in [Béthoux and Gentili, 1996]; 1995-1997 data were
acquired aboard CNRS/Insu ships.
According to [Rohling and Bryden, 1992] these changes were assumed to be imported from the eastern basin and man-induced. Temperature and salinity increases effectively occurred in the eastern Mediterranean where they are the main causes of hydrographic events observed in 1993 and 1995 [Della Vedova et al., 1995, Roether et al., 1996]. The characteristics of deep waters are acquired at the surface; resulting from air-sea exchanges and winter formation of dense water in different northern areas of eastern and western basins [Lacombe et al., 1981, MEDOC Group, 1970, Pollak, 1951, e.g.,]. Due to evaporation and density increase, resulting from cooling and saltening, surface waters sink and mix with intermediate water to form the dense waters that fill the deep inner basins before flowing out to the Atlantic over the sill of Gibraltar. In the Atlantic, a "Mediterranean" layer participates in the dense water formation in the Norwegian Sea [Reid, 1979], a key point for the world ocean circulation, and it was recently assumed that its salinity increase may shift the ocean circulation and cause a glacial age in Canada within the next century [Johnson, 1997]. As a consequence of a residence time of about 15 years for the deep waters of the Algero-Provençal basin (western Mediterranean), the deep trends must have been initiated at least as far back as the early 1940s. However, if a residence time of about 100 years for the whole Mediterranean is considered, then the trends currently being measured may be related to surface changes that took place during the previous century.