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3.1 The modified Atlantic water flow (MAW)

The MAW splits into two branches at the entrance to the Sicilian Channel, one flowing
northward to the Tyrrhenian Sea, the other into the Sicilian Channel (Lermusiaux and
Robinson, 2001). The second branch is composed by two streams, the Atlantic Ionian
Stream (AIS) and the Atlantic Tunisian Current (ATC). According to Sorgente et al. (2003)
and Drago et al. (2010) these two main branches of the MAW show a counter phase
behaviour: ATC is stronger in winter and the AIS is stronger during summer .

The AIS starts its path as a meander to the south of Adventure bank. It then precedes south-
eastward and loops back northward around Malta, forming along its path the Adventure bank
vortex (eddy), the Maltese Channel Crest and, as it reaches the sharp Malta escarpment to
the east, it abruptly gains positive vorticity and tends to deflect with an increase looping
northward meander forming the characteristic Ionian Shelf Break Vortex (Fig. 7)

In summer, the AIS is associated with a number of well-known semi-permanent features
including the intermittent northward extension of the AIS (NAIS) at the Ionian shelf break,
which seems to be driven by the surface density contrast between waters of the Sicilian and
the Ionian basins (Beranger et al., 2004). The signature of the Modified Atlantic Water is
seasonal and it is given by a salinity minimum (37.2) that is found at about 50 meters during
summer and near the surface during winter (Manzella, 1988).

The northward flow along the Ionian shelf break is predominant during summer when the AIS
is most intense and follows closely the Sicilian shelf break. The flow subsequently extends as
a relatively strong velocity front into the northwestern Ionian where the summer circulation is
mostly anticyclonic. The contrast in temperature of the MAW exiting the Sicily channel with
the warmer Ionian Sea produces the Maltese front, which constitutes a conspicuous thermal
filament on sea surface temperature AVHRR maps.

During winter, the MAW tends to spread more along the interior of the channel and it is more
steeply sloped towards the African coast (Manzella et al., 1990), consequently the AIS is less
intense. The exit of the MAW is shifted further south and progresses splitting into
southeastward and southward branches. This situation is moreover favoured by an
enhanced cyclonic component in the Ionian circulation especially during winter.

The circulation pattern in spring and fall is more difficult to assess. On the basis of more
updated hydrographic data, (Robinson et al., 1998) it appears that the summer circulation
pattern with a northward veering of the MAW over the Malta Escarpment is also common in
both spring and fall. On the other end, earlier studies (Tziperman & Malanotte-Rizzoli, 1991;
Ovchinnikov, 1996) concluded that on exiting the strait, the MAW will predominantly proceed
to the north during summer and to the south and southeastward during the remainder of the
year, while Zavatarelli & Mellor (1995) does not attribute very pronounced seasonal

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