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320 R. Sorgente et al.: Seasonal variability in the Central Mediterranean Sea circulation
1.9 Sv; this is slightly higher than the 1.6 Sv computed from
the coarse resolution model (Demirov and Pinardi, 2003).
The transport estimates are higher in the period from Novem-
ber to May, with an average maximum of 2.2 Sv, while the
minimum occurs during summer from July to August, reach-
ing below 1.6 Sv. The interannual variability during the “per-
petual” simulation experiment shows a peak value of 2.4 Sv
in December and a minimum of 1.4 Sv in July. The annual
mean of the baroclinic component is 1.4 Sv and carries the
greater share of the seasonal variability. The short-term vari-
ability of the transport values is also significant, with fluctua-
tions of the order of 0.3–0.4 Sv being very common through-
out the year. The seasonality is in good agreement with ob-
servations (Manzella, 1990; Grancini and Michelato, 1987;
Astraldi et al., 1996) and with the OGCM model study of in-
terannual variability (Pinardi et al., 1997); it is, however, in
contrast with Moretti et al. (1993). From hydrographic mea-
surements, Moretti et al. (1993) report a mean volume tran-
port under 1.0 Sv with an opposite seasonal pattern, that is a
mean winter transport smaller than in summer. This appar-
ent discrepancy has been explained by Pinardi et al. (1997)
by observing that Moretti et al. (1993) considered only the
baroclinic contribution to the transport across the Strait, and
that this contribution was only 1/3 of the total transport while
the rest is due to the barotropic component. In our applica-
tion the baroclinic component is found, however, to carry a
higher proportion (about the 74%) of the total volume trans-
port.
The time series of the net heat transport (Fig. 20b) is fairly the Tyrrhenian Sea.
steady during the model integration, although a strong sea-
sonal variability is also evident. The maximum of heat ex-
change is reached in September-October (2.5×1013 Watts),
that is with a phase lag of four months with respect to
the maximum in the total superficial heat flux over the
model domain (Fig. 7b); the minimum is in March–April.
The annual mean of 1.1×1013 Watts, directed towards the
eastern Mediterranean, is in agreement with Pinardi et al.
(1997), who calculated 1.2×1013 Watts, and the coarse reso-
lution model results from Demirov and Pinardi (2003). The
net salt transport (Fig. 20c) has an annual mean of about
1.2×106 psu m3/s toward the western basin, showing, also in
this case, a strong seasonal variability, with a maximum in
December-January and a minimum in July-August.
5 Summary and Conclusions
The Central Mediterranean region is characterized by a num-
ber of significant hydrodynamical processes and phenomena
with strong variability at seasonal and shorter time scales.
The general circulation is dictated mainly by the slow basin
scale (vertical) thermohaline structure of the Mediterranean,
in addition to important components related to meteorologi-
cal forcing and the influence of the complex morphology. An
eddy-resolving version of the free surface Princeton Ocean
Model (POM) nested to the rigid lid MOM-OGCM (Modu-
lar Ocean Model - Ocean General Circulation Model) basin
scale model is shown to correctly reproduce the seasonal cli-
matological cycle of the circulation in the area, as evidenced
by the comparison of the model results with the circula-
tion patterns inferred from the bibliography and from remote
sensed thermal AVHRR data. The five-year “perpetual” sim-
ulation experiment with climatological forcing contributes to
give an improved description of the circulation, especially
in its 3-D phenomenology, as well as to give a firsthand in-
dication of important dynamical features pertaining to the
Tunisian and Libyan near-shelf areas, where oceanographic
knowledge is generally scarce due to lack of data.
The numerically efficient one-way off-line nesting tech-
nique used in this work is that adopted by the Mediterranean
Forecasting System Pilot Project. It is shown to be adequate,
even in the case of a complicated model domain character-
ized by strong exchange flows. The POM-MOM correlations
show that the nested domain is able to generate high resolu-
tion information that is coherent with the nesting large scale.
The simulations have evidenced the presence of a strong,
superficial mesoscale activity in the Sardinia Channel, par-
ticularly from November to March. Such activity is driven
mainly by the inflow of MAW from the Algerian basin, cal-
culated to be twice in winter than in summer. The eddies
are generated by baroclinic instabilities from the meandering
of the MAW stream and the variations in density with the
surrounding superficial waters. The simulations show that
these eddies are important in the transfer of the MAW into
In the Sicilan Channel the model has underlined the pres-
ence of two main superficial MAW streams with a different
seasonality in the mass transport. The southern flux moving
along the north African coast is stronger in autumn, while
the northern flow along the Atlantic Ionian Stream (AIS)
increases in summer, meandering along the Sicilian shelf
break, close to Malta. The model puts in evidence the sea-
sonality of the northward veering of the AIS as it reaches
the Ionian Sea. The pre-conditioning and pathways of the
MAW towards the eastern Mediterranean are well described,
together with the associated evolution of the mesoscale sig-
nals in the form of eddies, meanders and small-scale gyres.
The simulated LIW flow can be followed in its progress along
the Sicilian Channel, and shows seasonal differences in the
mass transport, being greater in winter compared to summer.
The model is also used to quantify the volume transport
across the Strait of Sicily. The flow is found to be predomi-
nantly baroclinic in nature (74%), with a mean total transport
of 1.9 Sv and a strong seasonal signal having a maximum
spread over the winter months and a sharper minimum close
to July.
Acknowledgements. This work has been partially funded by the EU
MAST project MFSPP (Mediterranean Forecasting System Pilot
Project, contract number MAS3-CT98–0171) and by the Italian na-
tional project SIMBIOS, Programma Operativo del Piano Ambiente
Marino, Cluster C10, Progetto n. 13 (D.n. 778.RIC).
The authors thank Marco Zavatarelli from the University of
Bologna for his precious suggestions and Filippo Angotzi for his
essential technical support.