Page 17 - angeo-21-299-2003

R. Sorgente et al.: Seasonal variability in the Central Mediterranean Sea circulation 315
The inflow of MAW into the Tyrrhenian Sea has a high
seasonal variability, especially in the mean and total veloc-
ities associated with the MAW stream, while the salinity is
more stable. The vertical sections of the simulated salinity at
13◦ E (from 38◦ N to 39.5◦ N, in the south Tyrrhenian Sea) in
Fig. 15 show the eastward extension of the MAW flow to the
north of Sicily as a large vein characterized by a minimum of
salinity of around 37.5 psu. Speeds reach values over 20 cm/s
from December to May, while maximum velocities of up to
30 cm/s occur in December. The isohalines are inclined to-
ward the north Sicilian coast and the minimum salinity is
located at the surface close to Sicily (Fig. 15). From May to
August the MAW is progressively eroded from the surface,
developing into a subsurface minimum of salinity at about
50 m depth, and with a decrease in speed. The isohalines
tend to become horizontal, with an increase in the tempera-
ture and salinity values toward the surface, due to a positive
heat exchange and negative water vapour exchange with the
atmosphere.
4.3.2 The Sicily Channel region
The examination of the vertical sections of the simulated
salinity and the total velocity fields, between Cape Bon
(on the Tunisian perimeter) and the western coast of Sicily
(Fig. 16), show that the presence of the MAW is character-
ized by a high seasonal variability in both its temperature and
velocity. From November to May, the isohalines appear in-
clined towards the African side. The MAW flows eastward,
predominantly along the Tunisian side with speeds reach-
ing up to 30 cm/s, in agreement with calculations by Horton
et al. (1997b). The maximum simulated velocity is reached
in November with speeds up to 50 cm/s. Onken and Sell-
schopp (2001) computed speeds of up to 60 cm/s in October
1996 east of Cape Bon. The minimum of salinity is quite
stable, ranging from 37.2 to 37.3 psu, and is located at the
surface toward the African side, while the salinity increases
from 37.3 psu to 37.5 psu toward the Sicilian side (Fig. 16a).
During the summer months, the isohalines tend to progres-
sively become closer to horizontal above 100 m, with two
sub-surface cores characterized by a minimum of salinity of
about 37.3 psu. Both are located at about 40 m depth: the
first is adjacent to the Tunisian side, while the second is more
towards the center of the section (Fig. 16b). In spite of the re- of 20 cm/s. It is attenuated during winter when the circula-
duced summer MAW volume transport, the monthly analyses
of the salinity horizontal distribution in the Sicily Channel
shows a higher lateral spreading during summer. This appar-
ent contradiction appears to be related to the reduced impact
of the wind stress on the MAW/LIW mixing with respect to
winter (Manzella, 1994).
Beyond the Strait of Sicily, the simulated circulation re-
produces the branching of the MAW into two main streams
(Fig. 13). The southern branch follows the Tunisian shelf
edge and eventually spreads over the wide shallow Libyan
continental platform. The northern branch flows along the
Sicilian shelf. Both have a strong seasonal variability, par-
ticularly in their volume transport and in their intrusion ca-
pacity into the Sicilian Channel. The presence of these two
MAW veins is well evidenced by the meridian section along
13◦ E (Fig. 15), from 35◦ N to 37.5◦ N, in the form of sepa-
rate subsurface cores with a salinity of 37.5 psu, situated at a
depth of 50 m. The southern branch, larger and less salty than
the northern, has its maximum in autumn, while the northern
branch is more evident in summer.
4.3.3 The African shelf region
The southern branch spreads over the north African shelf
with a seasonal variability characterized by a decrease in the
minimum of salinity from winter to summer, as a result of
the lower degree of mixing of the MAW with the surround-
ing waters. The monthly distribution of the simulated salin-
ity field increases from Cape Bon to the southeastern side of
the domain, with a gradual modification of surface proper-
ties of the MAW. Strong anomalies of the temperature and
salinity are found in the Gulf of Gabes, where steep horizon-
tal temperature gradients practically persist throughout the
year, and positive salinity gradients towards the coast occur
mainly during the autumn/winter period. The extreme salin-
ity values in the gulf increase in winter, passing from 37.6 psu
in August (Fig. 12b) to 38.4 psu in December–January (not
shown). Such anomalies compare well with reality, as can
be evidenced from AVHRR surface temperature maps (Le
Vourch et al., 1992). During winter, a cold superficial coastal
layer is established in the area, reaching a mean temperature
of about 10.5 ◦C (Fig. 17a). This low winter temperature is
caused by the combined action of the strong negative sur-
face heat fluxes and the shallow bathymetry. In summer the
temperature reaches values over 30 ◦C (Fig. 17c), due to the
reversal of the surface heat fluxes with respect to winter.
The circulation in the rest of the northern African shelf
is characterized mainly by a strong and relatively cooler
eastward flow, which can be called the African MAW cur-
rent, following closely the African shelf edge and making its
way from the Sicily strait, past Lampedusa Island and sub-
sequently steering southwards, coming close to the Libyan
coast from 13◦ E onwards. This flow is particularly evident
in the period May to December (Fig. 17d), developing into
an intense stream during autumn, with flows reaching values
tion patterns are more complex. The dynamics of the area
is also characterized by a permanent barotropic gyre located
southeast of Cape Bon and by a pair of small-scale counter
rotating gyres during summer, residing in the deeper waters
off the Libyan coast between 14◦ E to 16◦ E. This circula-
tion results from the interaction of the African MAW with
the warmer western perimeter of the Sidra gyre, and gives
rise to the establishment of divergence zones and upwelling
events in the proximity of the Libyan coast (Fig. 17d). Over
the Libyan slope area in the southeastern part of the domain,
the influence of the northward Sidra circulation bounded be-
tween longitudes 14◦ E to 16◦ E is replaced in the period De-
cember to April by an opposite general meandering consis-
tent flow of MAW from the north (Fig. 17b). This flow car-