BACKGROUND  INFORMATION

    ELISA rationale

    1. The general circulation of water masses in the southwestern Mediterranean.
     


    2. The origin, structure and trajectories of Algerian Eddies.

    3. The biological response to mesoscale phenomena.


 
 
ELISA rationale
    The general circulation of the water masses in the Mediterranean Sea has been studied from the beginning of the 20th century. Maps produced in the 1960s did not differ much, and the general circulation schemes were widely accepted.
However satellite imagery becoming available in the early 1980s revealed the existence of upwellings, meanders and eddies in nearly the whole Mediterranean.

    This prompted a re-examination and interpretation of the few available data sets . The main point was to propose a coherent explanation for the episodic departure of water masses from their "normal" path. New hypotheses were first set forth about the Algerian Current and the role of the eddies it generates on the circulation of the water masses in the Western Mediterranean [Millot, 1985]. The first mesoscale-dedicated operation designed to study the algerian eddies in the western part of the Algerian Basin allowed Millot [1987]  to propose revisited schemes of the circulation in the western Mediterranean. The major change was the introduction of the mesoscale activity: eddies in the Algerian Basin were supposed to be able to locally modify the water masses "normal" paths, providing coherent explanation for "unexpected" in situ observations.

    About 10 years later, additional observations enabled to complete and refine the circulation schemes [Millot, 1999]. But they also raised new questions and prompted other hypotheses. It was time for a new large amplitude investigation in the Algerian Basin:  it was time for ELISA.
 
 
 

1. The general circulation of water masses in the southwestern Mediterranean.
    The Mediterranean Sea is called an evaporation basin, because loss of water by evaporation exceeds inputs from precipitations and river runoff. Would the Strait of Gibraltar be closed, the Mediterranean sea level would decrease by ~1m/year. To compensate for this water loss, there is an inflow of Atlantic Water through the Strait of Gibraltar in the upper layer.

    The general circulation of the water masses in the Mediterranean is mainly determined by exchanges of heat and salt (thermohaline circulation) and by earth rotation effect (Coriolis force). As a result, circulation roughly runs counter-clockwise along the continental slopes. However this circulation, as depicted by schemes such as  Nielsen's [1912] or Ovchinnikov's [1966], is more likely to be established if measured over a period of time long enough (several months). Indeed, dynamical activity at mesoscale is superimposed nearly everywhere in the Mediterranean, modifying locally and episodically  this mean path.

Mesoscale is defined by:


    Mesoscale phenomena are typically upwellings, fronts, meanders and eddies. Locally and episodically (up to several months, Taupier-Letage and Millot [1986], Millot [ 1994]), they can strongly perturb the general circulation.

Click here to see the current time series recorded  ~ 25km off the Algerian coast at 100m deep (ELISA mooring 3):
- changes in direction and intensity of the current vector are due to meanders and eddies,
- due to an algerian eddy, which stayed over the mooring for ~5 months inducing strong currents, the current averaged over 1 year is actually northwestward (~10cm/s), when general (mean) circulation is eastward.

A main consequence is that, on new circulation schemes, there are no longer any continuous paths figured in the Algerian Basin.

The main water masses of the western Mediterranean are:


More information can be found e.g. in EUROMODEL [1995], Send et al. [1997] and Millot [1999].
 
 


     Throughout its path in the western Mediterranean, the water of Atlantic origin will be mixed with mediterranean resident water, and therefore will be called Modified Atlantic Water (MAW).
    It first flows through the Alboran Sea and exits usually forming the Almeria-Oran jet, reaching the Algerian coast near ~0°. If only constrained by the Coriolis force, MAW should flow eastward, keeping the coast on its right, in a vein 25 to 50km wide. Thus MAW should flow along the Algerian and Tunisian slopes, and wholly enter the eastern Mediterranean through the Channel of Sardinia. But MAW is also found in the whole northwestern Mediterranean, forming the permanent Northern Current along the Italian, French and Spanish coasts. As well, in situ measurements show that water of Atlantic origin only little modifed (ie newly entered) can also be found offshore in the Algerian basin, especially in the eastern part of the Algerian Basin. Hence previous widely-accepted circulation schemes displayed a progressively widening MAW vein, "splitting" or "branching" as it approached the Channel of Sardinia: one "branch" heading northward into the NW Mediterranean,  the other continuing eastward through the Channel of Sardinia into the eastern Mediterranean. No mechanism was proposed to explain such a "branching".

    Satellite observations becoming more accessible in the 80s brought information about mesoscale dynamics. Satellite images analysis showed that the water of atlantic origin flowing along the Algerian coast does so mainly by way of  meanders and eddies, and as soon as 0°-1°E. This led Millot [1985] to define an Algerian Current, characterized by its instability which generates this intense mesoscale activity, and to link the occurrence of recent MAW in the open basin to Algerian Current- induced eddies, hereafter called Algerian Eddies (AEs). The MEDIPROD-5 (1986-1987) operation [Millot and Bonin, 1990] was the first mesoscale-dedicated experiment in this region. Consequences of AEs on hydrology and circulation were confirmed [Benzora and Millot, 1995 a,b; Millot et al., 1997]. The Algerian Basin thus acts as a reservoir for MAW, disconnecting the flow of Atlantic Water coming in at Gibraltar from those exiting eastward through the Channel of Sardinia and northward between Balearic Islands and Sardinia.
 
 


Click on image to see full resolution
MAW Circulation Scheme
from Millot [1999]


    The warm and salty water formed in the eastern Mediterranean (Levantine Basin) enters the Algerian Basin through the Channel of Sardinia and flows approximately between 200-600m deep. As with the MAW, previous schemes [Ovchinnikov, 1966] showed the LIW "branching" as it leaves the Sardinian Channel. One "branch" was to move directly westward along the Algerian slope, the other "branch" flowing counter-clockwise into the interior of the sea. Millot [1985] also dismissed the idea of "branching". He suggested that the LIW main path was northward along the Sardinian continental slope, this circulation being episodically perturbed by algerian eddies passing by, capturing and dispatching recent LIW pieces into the interior of the basin. This hypothesis was proven correct, as lenses of recent LIW were found trapped in an algerian eddy during the MEDIPROD-5 experiment [Millot, 1987b], and later as well with repeated transects from France to Algeria and Tunisia with XBT's (THETIS EU programme, Fuda et al. [2000]). Although the LIW path is still debated by some modellers, in our opinion the present observations from ELISA confirm definitively  this circulation scheme and the non-existence of a LIW vein flowing westward along the Algerian slope.
 
 


Click on image to see full resolution
LIW Circulation Scheme
from Millot [1999]


    WMDW is formed in the Gulf of Lions when particularly intense winter conditions (cold and dry Mistral wind) cause the MAW and LIW to mix and sink (deep convection). The water mass flows below 600m and plays an important but relatively unknown role in the deep circulation of the western Mediterranean. The main path is counter-clockwise along the continental slopes of the western Mediterranean, and can be episodically perturbed by algerian eddies too.
 
 


Click on image to see full resolution
WMDW Circulation Scheme
from Millot [1999]



 
 
 
 
 
 
 
 
 
 

2. The origin, structure and trajectories of Algerian Eddies.


    The Algerian Current (~200m thick) is unstable, and generates meanders and eddies

At that stage, eddies (AE) have an associated upwelling cell (U) on their southwestern edge. Meanders and eddies (with attached upwelling cell) propagate eastward along the coast at few km/day.

Only anticyclonic eddies develop. Algerian Eddies (AEs) diameters range from ~50 to 100km, up to ~250km for some. Their vertical extent can be as deep as the sea floor (~2800m). Lifetimes range from months up to ~3 years, as observed for 2 of them at least.

    AEs can detach from the coast. This is most likely to happen in the vicinity of the Channel of Sardinia, where the narrowing and shallowing bathymetry will constrain AEs to a northward path along the Sardinian slope. AEs will eventually drift westward in the open basin for months. They often come back in the Algerian coastal zone, where they interact with their parent current, potentially leading to dramatic pertubation of the general circulation [Taupier-Letage and Millot, 1986; Benzohra and Millot, 1995]. Algerian Eddies generally describe a counter-clockwise loop in the Algerian Basin [Fuda et al., 2000].

    Before the MEDIPROD-5 experiment in 1986-1987 ( 24,  9-month current and temperature time series) AEs were believed to progressively grow in diameter and depth as they propagated eastward. It appeared that AEs actually get their full vertical extent very rapidly, ie shortly after their creation. However their vertical extent has been observed to vary rapidly (from ~2800m to ~200m), their shape and diameter as well.

The LIW vein might become unstable as it passes the open SW corner of the Sardinian slope. Millot [1999] hypothezised that such instability could produce baroclinic LIW eddies which he named "Leddies", by analogy with Meddies. Leddies would then propagate into the interior of the Algerian Basin, and contribute to the occurrence of recent LIW in the basin.

More info can be found in Millot et al. [1997], and Obaton et al. [2000].

Finally, AEs induce secondary phenomena (limited space and time scales):
- small (diameter a few 10s of km) shear cyclonic eddies (C)
- and filaments (up to 100-200km in length, but very limited in depth)
mainly between AEs and coastline breaks.
 
 
 

3. The biological response to mesoscale phenomena.


    The dynamical signature of mesoscale phenomena can be determined by the Sea Surface Temperature (SST) distribution using satellite thermal infrared radiometer such as NOAA/AVHRR. The corresponding biological signature can be determined using ocean color, as it is linked to algal biomass (roughly the more phytoplankton the greener), using satellites like past CZCS (Coastal Zone Colour Scanner)  and present SeaWiFS (Sea-viewing Wide Field-of-view Sensor) . The close correlation between both dynamical and biological signatures illustrates the fact that, at mesoscale, dynamics is driving biology.
 
 


    The instability of the Algerian Current leads to a relatively richer coastal zone, with richer upwelling cells and entrainments around coastal eddies (however enrichment by upwelling cells is not to be compared with large wind-induced upwelling systems). However, large-sized and long-lived algerian eddies being anticyclonic*, they represent unfavorable situations for biological development. Hence they are signed by very oligotrophic (poor) areas, which can occupy the greatest part of the basin.

    Sometimes small-scale shear cyclonic eddies are generated, that represent the richest spots**. However, as they do not last for long (days to weeks) and are very limited in size (10s of km), their overall contribution is not expected to be large.
 
 

* anticyclonic rotation induces a depression of the isolines within the eddy. After the thermal stratification is established, the euphotic zone (upper layer where light allows algae life) will become nutrient-depleted.

** cyclonic rotation induces a doming of the isolines within the eddy, and enables input of nutrients from the lower layer into the euphotic zone.
 

    Since there is no continental shelf off Algeria (fish is especially abundant on shelves), and no big river (likely to bring nutrients and organic matter into sea) the Algerian Basin  is in a situation not very favorable for natural ressources. It is all the more important to study the Algerian Current.

    As variability is very high in both dynamical and biological fields, correct description, and hence, understanding, require simultaneous sampling at short space and time intervals, over a long period of time. Due to instrumental limitations (especially in biology) and to the large effort needed for such experiments, only a few mesoscale-coherent data sets were available.
The MAST3/MTP2 european programme provided the framework for the large interdisciplinary effort of the ELISA experiment designed to study the Algerian Eddies.