Currents in a Strait 



533 



This also gives a good fit between observed values and the theoretical current 

 profile (Fig. 246); the low value at 130 m depth is apparently due to the uncertain 

 elimination of the tides. From the Gulf of Aden to the Red Sea the sea level falls 

 M cm in 100 km and the internal current boundary surface rises about 40 m. Since 

 the sea bottom from the sill out into the Gulf of Aden falls almost steadily from 150 to 



cm/sec -4 

 To the South 



4 8 12 



To the North 



Fig. 247. Vertical stratification of the basic current in the Strait of Messina (according to the 

 observations of the anchor station of the R.N. "Marsigli", 16-30 August 1922). 



about 350 m, the internal boundary surface will also decline in the same direction, so 

 that the value given above is merely the deviation from the bottom slope. Because of 

 changes in the direction of the wind from the winter monsoon (east to south-east) to 

 the summer monsoon (north-west to west) the currents in the Strait of Bab el Mandeb 

 are subjected to oscillations with a semi-annual period. In the winter the inflow into 

 the Red Sea is a wind-drift current of strong permanence in speed and direction but 

 larger variations are to be expected during the summer monsoon. 



{d) Strait of Messina 



The smallest cross-section in this strait between the Ionian and the Tyrrhenian Sea 

 is at the northern end of the strait. Here it has a cross-sectional area of only \ km'* 

 with a mean depth of about 80 m and a maximum one of about 120 m. From this sill 

 the sea bottom slopes downward, uniformly and rather rapidly in valley form on either 

 side. At the northern outlet the mean depth is 140 m and towards the south it is already 

 about 900 m at about 30 km south of the sill. Since the water of the Ionian Sea is 

 heavier, the current flows from the Tyrrhenian Sea into the Ionian Sea in the upper 

 layer and in the opposite direction in the lower layer. Current measurements over a 

 15-day interval by the research vessel "Marsigli", at different depths down to 90 m 

 at a section in the narrowest part of the strait, have been analysed harmonically by 

 Vercelli (1926). Figure 247 shows the vertial current profile. Down to a depth of 30 m 

 the current flows to the south, below this, down to the sea bottom, to the north. 

 The velocities are small, in accordance with the low density differences, with on the 

 average about 4-3 in the upper current and about 9-3 cm/sec in the lower current. 

 Strong tidal currents are superimposed on the basic current and there are also strong 



