Search

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2005.12.017 Byline: Claude Millot (a), Julio Candela (b), Jean-Luc Fuda (c), Youssef Tber (d) Keywords: Long-term changes; Mediterranean outflow Abstract: The Mediterranean Sea transforms surface Atlantic Water (AW) into a set of cooler and saltier typical Mediterranean Waters (tMWs) that are formed in different subbasins within the sea and thus have distinct hydrological characteristics. Depending on the mixing conditions along their route and on their relative amounts, the tMWs are more or less differentiated at any given place, and some mix together up to forming new water masses. We emphasise the fact that any of these Mediterranean Waters (MWs) must outflow from the sea, even if more or less identifiable and/or in a more or less continuous way. Historical data from the 1960s-1980s showed that the densest MW outflowing through the Strait of Gibraltar at Camarinal Sill South (CSS) was a relatively cool and fresh tMW formed in the western basin, namely the Western Mediterranean Deep Water (WMDW). At these times, the sole other tMW identified in the strait was the Levantine Intermediate Water (LIW); no mention was made there of, in particular, the two densest tMWs formed in the eastern basin (in the Aegean and the Adriatic) that are now named Eastern Overflow Water (EOW) when they reach the Channel of Sicily (where they cannot be differentiated). A fortiori, no mention was made of the Tyrrhenian Dense Water (TDW) that results from the mixing of EOW with waters resident in the western basin (in particular WMDW) when it cascades down to [approximately equal to]2000m from the channel of Sicily. New measurements (essentially temperature and salinity time series) collected at CSS since the mid-1990s indicate that the densest MWs outflowing through the strait have been continuously changing; temperature and salinity there have been increasing, being actually (early 2000s) much warmer ([approximately equal to]0.3[degrees]C) and saltier (0.06) than [approximately equal to]20 years ago. These changes are one order of magnitude larger than the decadal trends shown for WMDW in particular. We thus demonstrate that, in the early 2000s, (i) the densest MW outflowing at Gibraltar is TDW and (ii) TDW is mainly composed of EOW (the percentage of MWs from the western basin, in particular WMDW, is lower): the densest part of the outflow is thus 'more eastern than western'. This Mediterranean Sea Transient (a shift from the western basin to the eastern one) could be linked to the Eastern Mediterranean Transient (a shift from the Adriatic subbasin to the Aegean one). Whatever the case, we demonstrate that the proper functioning of the Mediterranean Sea leads to a variability in its outflow's composition that can have consequences for the mid-depth water characteristics in the North-Atlantic much more dramatic than previously thought. Author Affiliation: (a) Laboratoire d'Oceanographie et de Biogeochimie, Antenne LOB-COM-CNRS, BP 330, F 83507 La Seyne/mer, France (b) Centro de Investigacion Cientifica y de Educacion Superior de Ensenada (CICESE), Mexico (c) Centre d'Oceanologie de Marseille (COM), Marseille, France (d) Service Hydrographique et Oceanographique de la Marine Royale du Maroc (SHOMAR), Casablanca, Maroc Article History: Received 28 June 2005; Revised 21 December 2005; Accepted 27 December 2005

Infrared satellite imagery and drifting buoy trajectories have provided new data on mesoscale phenomena produced by the Mediterranean Sea's Algerian Current. These include meander of current linked to surface anticyclonic eddies within its crest, deep anticyclonic eddies below the meander, and transitory surface cyclonic eddies occurring upstream from the crest.

The general circulation features of the oceans off Algeria have long been the subject of debate and have been difficult to evaluate. A new study, based on the Western Mediterranean Circulation Experiment, centres its findings on infra-red images, plus temperature and current data collected at 0-5 deg East and at depths of 100, 300, 1000 and 2000 mt, and shows that the surface and deep eddies around the eastern Algerian Basin remain linked and describe a cyclonic circuit. The findings support the hypothesis that mesoscale eddies formed by the Algerian Current can have a strong influence on the circulation of all water masses. Details of the experiment are presented.

Tests are conducted to determine the comparative performance and accuracy of Sippican Inc.'s eXpendable Conductivity Temperature Depth (XCTD) probes as opposed to CTD sondes. As previously seen in the THETIS-I experiment, errors that occurred due to experimental conditions are segregated from errors caused by the equipment. Results show the relative accuracy of XCTD sensors based on the manufacturer's specifications.

An anticyclonic open sea eddy was found moving near the Algerian coast that deflects the flow of the Algerian Current away from the coast. The eddy is characterized as cylindrical with a depth of 800 meters and a diameter of 120-140 kilometers. Water from the Algerian current mixes with the eddy in small streams, while some modified Atlantic water and Levantine intermediate water may also be mixed with the eddy.

A strong flow of Atlantic water moving against the Algerian coast is known as the Algerian current and it moves near the coastline until it encounters and open sea eddy that makes it flow seaward. The Algerian current produces two anticyclonic eddies, the winter intermediate water and the levantine intermediate water. These two eddies flow eastward from the Algerian coast and encounters the Algerian Current again at the Alboran sea to flow together towards the Mediterranean sea.

Millot, C., Taupier-Letage, I. and Benzohra, M., 1990. The Algerian eddies. Earth-Sci. Rev., 27: 203-219. This paper synthetizes various previous analyses of the Algerian eddies. These mesoscale anticyclonic eddies are of primary importance to the circulation of all the water masses in the Western Mediterranean. They are generated by instability processes disturbing the flow of Modified Atlantic Water (MAW) shortly after it reaches the African coast near 0[degrees]E, namely the Algerian Current. Young eddies, which have diameters of 50-100 km, and the upwelling generally attached to their southwestern edges, drift eastward along the coast at a few km per day, sometimes during several months; as they grow older and larger, the most vigorous ones leave the coastal zone and drift seaward. Some of them may reach the Sardinian continental slope where a well-defined vein of Levantine Intermediate Water (LIW) flows; we suspect them to be capable to pull fragments of LIW seaward, and in fact, the least modified LIW we encountered in the middle of the Algerian Basin had the form of rings or filaments trapped by one eddy. Eddies several months old may have diameters as large as [is nearly equal] 200 km; they also entrain the Mediterranean Deep Water (MDW), and they probably extend down to the bottom. In 1984, two such eddies occupied most of the Algerian Basin; the MAW was deflected seaward by the westernmost one, from the Algerian coast at [is nearly equal] 3[degrees]E as far as the Balearic Islands, and was then dispatched by other eddies through the whole basin as though by a set of paddle-wheels. Such interactions are perhaps not as exceptional as was previously expected. These mesoscale phenomena have strong biological implications. The Western Mediterranean Sea thus appears to be a very suitable place for the observation and modelling of such mesoscale coherent structures.