408 Tides in the Mediterranean and Adjacent Seas 



attempt an explanation of the tides of the Red Sea; he regarded them as the 

 superposition of a standing wave produced in this sea by the tide-generating 

 forces and a progressive wave penetrating through the Strait of Bab el Mandeb. 

 Blondel (1912), following a suggestion of Poincare (1910), has used a special 

 method of the calculus of variations for the computation of the tides of this 

 sea; however, due to an error, he did not obtain a good agreement between 

 the theory and the observations. This mistake was corrected by Chandon 

 (1930), with the result that the agreement was improved. Defant (1919, p. 1 10) 

 has given a theoretical discussion of the spring tides by applying the step-wise 

 integration of the equations of motion. The tides were considered as the 

 superposition of a tide co-oscillating with the Gulf of Aden and an inde- 

 pendent tide. The observations available at that time consisted only of the 

 establishments and mean heights of the spring tides 2{M. 2 J r S^ given by the 

 Admiralty Tides Tables which were not very accurate. The agreement was 

 quite good and it showed that the tidal motion is due in equal parts to the 

 independent tide and to the co-oscillating tide. 



One result of the cruise of the "Ammiraglio Magnaghi" was the harmonic 

 constants of eleven coastal localities in the Gulf of Suez and in the Red Sea 

 computed by Vercelli (1925). He also analysed a 15 days' anchor station 

 in the Strait of Bab el Mandeb and got the harmonic constants of the tidal 

 current at the open end in front of the Gulf of Aden. Table 54 gives a com- 

 pilation of these values, on which are based all further theoretical investi- 

 gations of the tides of this sea. 



From the distribution of the amplitudes and phases along the longitudinal 

 axis of the sea it can be concluded that the semi-diurnal tides have three 

 nodal lines: one south of Assab, a second one near Port Sudan and a third 

 one in the Gulf of Suez between Tor and Ashrafi Island. Generally the tide 

 has the nature of a standing wave with a rapid transition of the phase in 

 the vicinity of the nodal lines. The Gulf of Agaba oscillates, according to 

 the rough values of the tide tables, with the tides of the main basin without 

 a nodal line (v < 0-5). The diurnal tides, on the contrary, show only one 

 nodal line, namely, for K x between Kamaran Island and Massaua, for O x 

 somewhat more south of the former locality; here also it is essentially a stand- 

 ing wave. 



The character of the tides is given by the ratio (K 1 J r0 1 )/(A4 2 J rS 2 ) (see 

 Table 54). In the areas outside the nodal lines it is definitely semi-diurnal; 

 however, the nodal lines of the semi-diurnal tide stand out as areas with 

 an extreme diurnal type; this is especially the case in the Red Sea. It is also 

 remarkable that the elliptic tide N 2 is remarkably great in the entire sea, 

 which also applies to the Gulf of Aden. 



The tidal currents in the Strait of Bab el Mandeb are very strongly de- 

 veloped and, essentially, of the same nature over the entire cross-section of 

 the strait. The type of the current can be designated (see p. 307), as "mixed, 



