Oceanic Internal Waves - Strait of Messina
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The Strait of Messina is a narrow channel in the Mediterranean Sea which separates the Italian peninsula from the island of Sicily and connects the Tyrrhenian Sea north of the strait with the Ionian Sea south of it. A topographic map of the Strait of Messina is shown in Fig. 1. In the shallowest section of the strait, called the sill region, the maximum water depth is 90 m. While in the southern part of the strait the bottom slopes down very steeply to a depth of more than 800 m approximately 15 km south of the sill, the northern region has a more gentle slope. Here the 400 m isobath is located approximately 15 km north of the sill. Throughout the year, two different water masses are encountered in the Strait of Messina: the Tyrrhenian Surface Water and the colder and saltier Levantine Intermediate Water. In the vicinity of the strait these water masses are separated at a depth of approximately 150 m (Vercelli, 1925). During most of the year, a seasonal thermocline is also present in the strait which overlies this weak stratification. Although tidal displacements are very small in the Mediterranean Sea (of order of 10 cm), large gradients of tidal displacements are encountered in the Strait of Messina, because the predominantly semidiurnal tides north and south of the strait are approximately in phase opposition. Due to this phase opposition and due to topographic constrictions, the current velocities can attain values as high as 3.0 m/s in the sill region (Vercelli, 1925; Defant, 1961).
These hydrological peculiarities of the Strait of Messina may explain why this site has attracted the attention of many ancient writers and philosophers. Homer (800 B.C.) makes two monsters, Scylla and Charybdis, responsible for the violent currents in the strait (Homer, Odyssey, 12th song, line 80-114). Aristotle (384-322 B.C.) argues that hollows in the sea floor and the interaction of two opposing wind-generated currents could produce such intensive currents (Aristotle, Problema Physica, chap. 23) and in the poetry of ancient times, allegories alluding to the danger of sailing in the Strait of Messina can often be found ('Incidis in Scillam cupiens vitare Charybdim', Ovid, Metamorphosis).
The fact that (1) strong tidally induced currents are encountered in the strait, (2) the water body is stratified and (3) there is a shallow sill in the center of the strait which disturbs the tidal flow, suggests that internal waves should be generated in the Strait of Messina. But it was not before 1978 that internal waves were detected in this strait. The first hint came from a synthetic aperture radar image which was acquired by the American Seasat satellite on 15 September 1978. The three rings visible on the Seasat SAR image of the Tyrrhenian Sea north of the strait were interpreted as sea surface manifestations of a train of internal solitary waves propagating northwards (Apers and Salusti, 1983). In the following years internal waves propagating north- as well as southwards have been detected during several oceanographic campaigns by in-situ measurements (Alpers and Salusti, 1983; Griffa et al., 1986; Di Sarra et al., 1987; Sapia and Salusti, 1987; Nicolo and Salusti, 1991). A review of oceanographic investigations carried out in the Strait of Messina up to 1990 can be found in Bignami and Salusti (1990).
Fig. 1: Bottom topography of the Strait of Messina. The numbers on the depth lines (isobaths) denote depth in metres.
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