I. Siokou-Frangou et al.: Mediterranean plankton
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Fig. 1. Major seas, connecting straits and bottom topography of Mediterranean Sea.
74
Fig. 1. Major seas, connecting straits and bottom topography of Mediterranean Sea.
The bathymetry (Fig. 1) highlights a key feature of the
MS, i.e. the connection with the neighbouring ocean and be-
tween the deep sub-basins through shallow or very shallow
straits (e.g., Gibraltar, Dardanelles, Sicily, etc.), which pre-
clude any exchange of deep water masses. Nonetheless, the
deep layers are efficiently oxygenated in the present MS, be-
cause deep waters are regularly formed independently in the
western and eastern sub-basins and renewal occurs at yearly
pace (Hopkins, 1978).
The present MS is a concentration basin (freshwater loss
exceeds freshwater inputs), which forces an anti-estuarine
circulation, with saltier and denser water exiting the basin
at Gibraltar and a compensating entrance of the fresher At-
lantic water. As the unbalance between evaporation and pre-
cipitation plus runoff (the E-P-R term) increases towards the
east, the eastern basin is anti-estuarine respect to the west-
ern basin. This creates a single open thermohaline cell, en-
compassing the upper layer of both basins, with a dominant
west-to-east surface transport and a an east-to-west interme-
diate transport (e.g., Zavatarelli and Mellor, 1995; Pinardi
and Masetti, 2000). North-westerly wind stress prevails over
the whole basin in winter, with a rotation towards north-east
in summer, with no significant decrease in the W-to-E wind
driven transport. The wind stress pattern, the morphology
of the basin and the bottom topography produce a somewhat
regular pattern in the distribution of eddies and gyres, which
are mainly anticyclonic in the southern regions and cyclonic
in the northern ones (Pinardi and Masetti, 2000) (Fig. 2).
The Atlantic Water (AW) entering the basin is often re-
ferred to as Modified Atlantic Water (MAW) to account for
the progressive eastward change in its T-S properties. The
MAW adds a haline term to thermal stratification in large ar-
eas of the South West (SW) MS, decreasing the winter mixed
layer depth (D’Ortenzio et al., 2005). From a dynamic view
point, the inflow of MAW into the MS basin favours a sys-
tem of high energy anticyclonic structures in both the Alb-
oran Sea and the Algerian basin, resulting in anticyclonic
eddies along the Algerian current path (Fig. 2), with a du-
ration between several months and three years (Puillat et al.,
2002, and references therein). One of the most striking fea-
tures associated with the MAW is the North Balearic Front
in the North West (NW) MS, which separates two drastically
different sub-regions. The MAW flows across the Straits of
Sicily creating a jet, which is the dominant connecting sur-
face flow among the two MS sub-basins. In the Aegean Sea,
at the north-eastern edge of the MS, the modified Black Sea
Water flows in through the Dardanelles Strait. A strong ther-
mohaline front (the North East Aegean Front) is formed in
the area where colder less saline water (∼30) meets warmer
saltier water (∼38.5) of Levantine origin (Zervakis and Geor-
gopoulos, 2002).
The general circulation of the MS is also character-
ized by the presence of permanent or semi-permanent sub-
basin gyres, which are mostly controlled by the topography
(Robinson and Golnaraghi, 1994). The most important are
the cyclonic Rhodos Gyre (NW Levantine Sea) and the South
Adriatic Gyre, with convective events during winter leading
to the formation of intermediate and deep water masses, re-
spectively. Another quasi permanent gyre, which is mostly
wind driven and displays strong seasonality, is located in
the North Tyrrhenian Sea (Artale et al., 1994), coupled with
anti-cyclonic twin on its southern edge (Rinaldi et al., 2009).
In the southern part of the basin, in addition to the Alge-
rian eddies, quasi permanent anticyclonic structures popu-
late the eastern MS, e.g., Ierapetra (south of Crete Island),
Mersa Matruh (north of the Egyptian coast), and a more vari-
able multipole anticyclonic structure S and SE of Cyprus,
which has recently been analyzed in great detail (Zodiatis
et al., 2005). This structure is often looked at as composed
by the Shikmona gyre (south-east of Cyprus) and the warm
Cyprus Eddy (south or south-west of Cyprus) (Fig. 2). Local
deep convection events occur periodically in the deep troughs
(>1000 m) of the North Aegean Sea and in the deep basin of
the South Aegean Sea (Theocharis and Georgopoulos, 1993;
Theocharis et al., 1999). In the Gulf of Lion (NW MS), a
large scale cyclonic circulation and the extreme atmospheric
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1546
I. Siokou-Frangou et al.: Mediterranean plankton
Fig. 2. Key traits of surface circulation of Mediterranean Sea. Acronyms: AE: Algerian Eddies; AF: Almerian
Front; CE: Cyprus Eddy; WCE: West Cyprus Eddy; CF: Catalan Front; IG: Ierapetra Gyre; MMG: Mersa Ma-
truh Gyre; NBF: North Balearic Front; NEAF: North East Aegean Front; NTC: North Tyrrhenian Anticyclon;
NTA: North Tyrrhenian Cyclon; PG: Pelops Gyre; RG: Rhodos Gyre; SAG: South Adriatic Gyre; SG: Shik-
mona Gyre (sources: Artegiani et al., 1997; Malanotte-Rizzoli et al., 1997; Millot, 1999; Astraldi et al., 2002;
Karageorgis et al., 2008; Rinaldi et al., 2009).
75
Fig. 2. Key traits of surface circulation of Mediterranean Sea. Acronyms: AE: Algerian Eddies; AF: Almerian Front; CE: Cyprus Eddy;
WCE: West Cyprus Eddy; CF: Catalan Front; IG: Ierapetra Gyre; MMG: Mersa Matruh Gyre; NBF: North Balearic Front; NEAF: North
East Aegean Front; NTA: North Tyrrhenian Anticyclon; NTC: North Tyrrhenian Cyclon; PG: Pelops Gyre; RG: Rhodos Gyre; SAG: South
Adriatic Gyre; SG: Shikmona Gyre (sources: Artegiani et al., 1997; Malanotte-Rizzoli et al., 1997; Millot, 1999; Astraldi et al., 2002;
Karageorgis et al., 2008; Rinaldi et al., 2009).
Fig. 3. Climatology of Mixed Layer Depth. Reproduced from D’Ortenzio et al. (2003) by permission of
American Geophysical Union.
76
Fig. 3. Climatology of Mixed Layer Depth. Reproduced from D’Ortenzio et al. (2003) by permission of American Geophysical Union.
forcing, especially in winter, force intense convective events,
which eventually reach the bottom. Based on an analysis
of Ekman wind-driven surface transport, intermittent coastal
upwelling events are also likely to take place in selected
regions of the MS, namely the Alboran Sea, Balearic Sea,
Straits of Sicily, East Adriatic Sea and North-East Aegean
Sea (Agostini and Bakun, 2002). It is worth mentioning
that, between the end of the eighties and the first half of the
nineties, a sequence of events in atmospheric and marine dy-
namics caused the formation of a large volume of very dense
water in the Southern Aegean Sea that spread into the deep
layers of the Levantine and Ionian basins, strongly modifying
their vertical thermohaline structure (Roether et al., 1996).
This event, normally refererred to as the Eastern Mediter-
ranean Transient (EMT), caused traceable changes in biogeo-
chemical processes in the Eastern Mediterranean Sea (EMS)
(e.g., Civitarese and Gacic, 2001; La Ferla et al., 2003).
Biogeosciences, 7, 1543–1586, 2010
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