Adjacent Seas of the Indian Ocean
Extending between lat. 12°N and 30°N, the Red Sea fills a long and narrow basin which at the northern end is closed except for the communication through the Suez Canal and at the southern end is separated from the Gulf of Aden by a shallow sill. The total length of the Red Sea is nearly 1800 km and the width 270 km. Outside the shallow and reef-bound coastal waters the general depth is about 700 m, but the bottom is very irregular and apparently isolated depressions exceeding 2000 m occur in several places. The sill at the southern entrance of the Red Sea lies 140 km inside of the narrow Strait of Bab el Mandeb off Hanish Island, where the greatest depth is only about 100 m. The fairly complete sonic soundings of the John Murray Expedition have failed to reveal any deeper channel (Thompson, 1939b).
The Red Sea is located in a region which is characterized by such an arid climate that evaporation from the water surface greatly exceeds the small precipitation. There is no runoff because no rivers enter the Red Sea. Along the entire length the prevailing winds blow consistently from the north-northwest during half of the year, from May to September, but during the other half of the year, October to April, the north-northwest winds reach only as far south as lat. 22° or 21°N, and south of 20°N the wind direction is reversed, south-southeast winds dominating.
The climatic conditions and the prevailing winds determine the character of the waters in the Red Sea and the exchange of water between the Red Sea and the Gulf of Aden. Owing to the excessive evaporation the surface salinity of the water in the northern part of the Red Sea reaches values between 40 ‰ and 41 ‰. In summer the temperature of the surface water is very high, mostly exceeding 30°, but in winter the temperature is decreased, particularly in the northern end where the average temperature in February is as low as 18°. The lowering of the temperature in winter, together with the intense evaporation in that season, leads to the formation of deep water that fills the entire basin of the Red Sea below the sill depth and has a salinity between 40.5 ‰ and 41.00 ‰ and a temperature between 21.5° and 22°. The formation of this deep water is further facilitated by the character of the currents, which is related to the character of the prevailing winds. According to Barlow (1934), who has examined 6100 direct observations of currents, averaging them by months for the areas 12° to 20°N and 20° to 28°N, the surface current flows toward the north-northwest (into the Red Sea) from November to March and to the south-southeast (out of the Red Sea) from June to September, with transition stages in April, May, and October. During November to March, when the current flows in, the velocity is less in the northern half of the sea where the flow is directed
The oxygen content of the deep water is very low in spite of sinking of surface water in winter. According to Thompson, the oxygen content shows an annual variation, values higher than 2.0 ml/L being observed at the end of the winter, whereas at the end of the summer most values were lower than 1 ml/L. Thompson attributes the low oxygen values and the annual variation to a very rapid consumption of oxygen, which at depths between 350 and 600 m appears to amount to about 2ml/L/year. This rate of oxygen consumption is by far the highest which has been found at such depths, but appears reasonable in view of the very high water temperature (about 22°C). The observations of the John Murray Expedition in September and in April-May both indicate the existence of a region of minimum oxygen content in the central portion of the Red Sea at depths between 300 and 500 m. Thompson attributes this minimum to a vertical rotational movement, which in winter is related to the sinking of surface water in the northern part of the Red Sea and rising of deep water inside of the sill, but which, in summer, in part reverses owing to the piling up of water inside of the sill.
The exchange of water between the Red Sea and the adjacent parts of the ocean takes place through the Suez Canal and through the Strait of Bab el Mandeb. The exchange through the Suez Canal is of no importance to the water and salt budget of the Red Sea but shows some interesting details (Wüst, 1934). Wüst points out that any flow of water through the Suez Canal is greatly complicated by the fact that the canal passes through the Bitter Lakes, the bottoms of which consist of layers of salt which are gradually being dissolved, thus increasing the salinity of the waters in the canal to a concentration above that of the Red Sea or Mediterranean Sea waters. In October-December the salinity at the surface of the canal above the Great Bitter Lake is as high as 50.00 ‰ and at the bottom it is above 55.00 ‰. The flow through the Suez Canal is mainly determined by three factors: (1) the difference
The exchange of water between the Red Sea and the Gulf of Aden is subject, according to Vercelli (1925) and Thompson (1939b), to a distinct annual variation which is related to the change in the direction of the prevailing winds in winter and summer. In winter, when south-southeast winds blow in through the Strait of Bab el Mandeb, the surface layers are carried from the Gulf of Aden into the Red Sea, and at greater depths highly saline Red Sea water flows out across the sill. In summer, when north-northwest winds prevail, the surface flow is directed out of the Red Sea and at some intermediate depths water from the Gulf of Aden flows in, having a lower salinity and a lower temperature than the outflowing surface water. At still greater depths highly saline Red Sea water appears to flow out over the sill, but it is probable that this outflow is much reduced as compared to the outflow in winter. On the basis of direct measurements of currents at anchor stations, Vercelli found that in winter the average inflow amounts to approximately 0.58 million m3/sec, whereas the outflow of Red Sea water amounts to approximately 0.48 million m3/sec. No measurements are available for summer.
Owing to the complicated character of the water exchange in summer, the average salinity of the in- and outflowing water is not known and, furthermore, the excess evaporation from the Red Sea is not well determined, for which reasons a computation of the exchange of water cannot be based on a consideration of the salt balance. Vercelli even thinks it possible that more salt is carried in than out and that the salinity of the Red Sea is increasing. He also points out that owing to the rapid change in salinity through the Strait of Bab el Mandeb, the outflowing tidal currents carry water of higher salinity than the inflowing, such that tidal currents assist in the transport of salt out of the Red Sea. He concludes that in winter the net inflow of 0.1 million m3/sec is nearly twice as great as the net loss of water by evaporation, for which reason the water level in the Red Sea must rise during winter. According to Vercelli the average annual evaporation excess from the Red Sea amounts to about
The final result of this discussion is not very conclusive. As far as the conditions in the Indian Ocean are concerned, the greatest interest is attached to the amount of Red Sea water that flows out over the sill and spreads at an intermediate depth in a manner similar to the spreading of the Mediterranean Sea water in the Atlantic Ocean. According to Vercelli the amount that flows out in winter is, as already stated, 0.48 million m3/sec, but in summer the outflow must be considerably smaller. The average annual amount is therefore probably between 0.3 and 0.4 million m3/sec, that is, approximately one sixth of the amount which flows out through the Strait of Gibraltar. This conclusion is in agreement with the fact that, as presently will be shown, the Red Sea water is of less importance in the Indian Ocean than is the Mediterranean water in the Atlantic Ocean.
The Persian Gulf is so shallow that any exchange of water between it and the adjacent Gulf of Oman is of small significance. The average depth of the Persian Gulf is only 25 m and the maximum depth is about 90 m. It appears to be filled by water of a nearly uniform salinity of about 38.00‰ and some exchange must take place with the waters of the Gulf of Oman, but the character of this exchange has not been examined.