Isabelle Taupier-Letage

Algerian Eddies lifetime can near 3 years

The Algerian Current (AC) is unstable and generates mesoscale meanders and eddies. Only anticyclonic eddies can develop and reach diameters over 200 km with vertical extents down to the bottom (f3000 m). Algerian Eddies (AEs) first propagate eastward along the Algerian slope at few kilometers per day. In the vicinity of the Channel of Sardinia, a few AEs detach from the Algerian slope and propagate along the Sardinian one. It was hypothesized that AEs then followed a counter-clockwise circuit in the eastern part of the basin. Maximum recorded lifetimes were known to exceed 9 months. Within the framework of the 1-year Eddies and Leddies Interdisciplinary Study off Algeria (ELISA) experiment (1997–1998), we exhaustively tracked two AEs, using mainly an f3-year time series of NOAA/AVHRR satellite images. We show that AEs lifetimes can near 3 years, exceeding 33 months at least. We also confirm the long-lived AEs preferential circuit in the eastern part of the Algerian Basin, and specify that it may include several loops (at least three). D 2002 Elsevier Science B.V. All rights reserved.

XBT monitoring of a meridian section across the western Mediterranean Sea

During the Thetis-2/MAST-2 tomography experiment, T7-XBT calibrated (accuracy ∼0.05°C) probes were launched ∼28 km apart between France and Algeria, twice a month from Feb. to Sep. 1994. Combined with infrared images, altimetric data and ship drifts, they provide definite information on the structure, drift and role of the eddy-like mesoscale phenomena generated by the Algerian Current instability. When embedded in this alongslope current, these phenomena generally propagate downstream at a few km/day and are markedly asymmetrical. Because of the topography in the eastern part of the Algerian Basin, they separate from the current, become more symmetrical and follow an anticlockwise circuit in the open basin. These phenomena are deeper than ∼750 m and entrain seaward pieces of the Levantine Intermediate Water (LIW) vein flowing along the Sardinian slope, thus being responsible of the large spatial and temporal variability of the LIW distribution in the open basin. The non-existence of a LIW vein flowing westward across the Algerian Basin is definitely demonstrated. In the Gulf of Lions, new insights are provided into the formation and spreading of the Winter Intermediate Water (WIW), which is the WesternMediterranean counterpart of LIW. Considering the large amount of WIW formed during this mild winter, it is clear that this water has not received enough attention yet, and is certainly a major component of the Mediterranean outflow at Gibraltar. Finally, the XBT data account for the eastward flow of the WesternMediterranean Deep Water (WMDW) off Algeria.

Circulation off Algeria inferred from the Médiprod-5 current meters

Abstract Eight moorings were deployed off Algeria (1–5°E) during the Mediprod-5 experiment (June 1986–March 1987). The 24 current meter time series recorded at nominal depths of 100, 300, 1000 and 2000 m are analysed together with hydrological data (May–June 1986) and satellite infrared images. As expected, the circulation features are markedly different inside and outside of a ∼ 50 km-wide coastal zone. At ∼ 25 km from the coast, five out of six moorings are well within the Algerian Current and the current profile is strongly sheared, with low correlations at depth. All water masses flow eastward along the Algerian slope, thus completing consitently our circulation diagrams. At ∼ 75 km, the currents are more correlated between 300 and 2000 m and more dependent on the occurrence of mesoscale (100–200 km) anticyclonic eddies called “open sea eddies”. An event was recorded, propagating eastward at ∼ 3 km/day across the mooring array. It is thought to consist of a meander (width 50–150 km) of the Algerian Current, extending to ∼ 100 km from the coast and associated with teo superimposed anticyclonic eddies. One eddy, enclosed within the meander, involved the surface layer and had the infra-red signature of what we previously called a “coastal eddy” (30–120 km apparent diameter). The other eddy seemingly involved the whole deep layer, rapidly became barotropic and large in diameter (up to ∼ 150 km) which made it coastal too. Sooner or later, both coastal eddies are expected to merge together. These measurements have slightly modified our former hypotheses as, instead of assuming that “open sea eddies” are old stages of coastal surface eddies becoming larger and deeper, we now expect them to be old stages of the merged coastal eddies. This new understanding of such a coastal event is more similar to an open sea eddy and seems consistent with both theoretical models and laboratory experiments. Whatever this structure, recent data support our former hypotheses that mesoscale eddies generated by the Algerian Current can have a deep extent, propagate along the Algerian and then Sardinian slopes (where they entrap Levantine Intermediate Water) and strongly influence the circulation of all water masses.

Deep structure of an open sea eddy in the Algerian Basin

The Algerian Basin dynamics is dominated by the presence of very energetic mesoscale structures. Deep open sea eddies are key features that influence the regional circulation of water masses at all depths. In May 1998, an open sea eddy was sampled near 38j N2 jE by means of CTD, Acoustic Doppler Current Profiler (ADCP) and surface Lagrangian drifters. For the first time, it has been possible to characterise one of these mesoscale structures in its full depth, down to 2800 m. The density distribution indicated the presence of lighter water in the centre of the eddy from the surface to the bottom. The direct velocity measurements in the surface layer, plus the determination of the baroclinic shear from CTD data, evidenced that the anticyclonic motion was present in the whole water depth. D 2002 Elsevier Science B.V. All rights reserved.

The Algerian eddies

Abstract 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°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 ≈ 200km; 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≈ 3°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.

The Algerian current: comparisons between in situ and laboratory data sets

Abstract The Algerian current is compared to a laboratory current created in a rotating tank. The experimental current flows along a regular wall and over a motionless denser water, with a constant flow rate and a uniform velocity at the source. Data collected in the laboratory are compared with satellite and in situ data sets. The various characteristic phenomena are shown to be similar: both currents are unstable and generate meanders and cyclonic–anticyclonic eddy-pairs. Eventually, the anticyclone remains and is associated with a marked lowering of the interface. The phase speed and growth of the meanders and the shape and size of the eddies in the laboratory are in quite good agreement with those deduced from in situ data and satellite images. The observations and the velocity and interface height measurements in the laboratory allow a better understanding of the Algerian current dynamics and its instability process.

Large particulate matter in the Western Mediterranean: I. LPM distribution related to mesoscale hydrodynamics

The relationship between mesoscale hydrodynamics and the distribution of large particulate matter (LPM, particles larger than 200 Am) in the first 1000 m of the Western Mediterranean basin was studied with a microprocessor-driven CTD-video package, the Underwater Video Profiler (UVP). Observations made during the last decade showed that, in late spring and summer, LPM concentration was high in the coastal part of the Western Mediterranean basin at the shelf break and near the continental slope (computed maximum: 149 A gCl 1 between 0 and 100 m near the Spanish coast of the Gibraltar Strait). LPM concentration decreased further offshore into the central Mediterranean Sea where, below 100 m, it remained uniformly low, ranging from 2 to 4 A gCl 1 . However, a strong variability was observed in the different mesoscale structures such as the Almeria–Oran jet in the Alboran Sea or the Algerian eddies. LPM concentration was up to one order of magnitude higher in fronts and eddies than in the adjacent oligotrophic Mediterranean waters (i.e. 35 vs. 8 A gCl 1 in the Alboran Sea or 16 vs. 3 A gCl 1 in a small shear cyclonic eddy). Our observations suggest that LPM spatial heterogeneity generated by the upper layer mesoscale hydrodynamics extends into deeper layers. Consequently, the superficial mesoscale dynamics may significantly contribute to the biogeochemical cycling between the upper and meso-pelagic layers. D 2002 Elsevier Science B.V. All rights reserved.