Salinity stratification in the region of the Somali Current is complex 

 yet much weaker than that found in the North Atlantic. The two intermediate 

 salinity maxima in the northwestern Indian Ocean are weak compared with the 

 intermediate salinity maximum found at a depth of about 1000 m between the 

 27.6 o^ and 27.8 a^ surfaces in the North Atlantic. The small volumes and 

 surface areas of the Persian Gulf (6 x 10-^ km-^ and 239 x 10^ km^) and Red Sea 

 (215 X 10^ km-^ and 438 x 10"^ km^) compared with the Mediterranean excluding 

 the Black Sea (3701 x 10-^ km^ and 2505 x 10"^ km^) are reasonable explanations 

 of the relative weaknesses of the salinity maxima in the Indian Ocean. Weak 

 density stratification between Red Sea and Antarctic Intermediate Waters is 

 also evident in that the two water masses have approximately the same a^ 

 values (27.2 - 27.3) in spite of their strongly differing temperatures and 

 salinities. Between a^ values of 26.4 and 27.0, interfingering of Persian 

 Gulf and Subtropical Subsurface Water is observed (Warren, et^ al_. , 1966). 

 Such interfingering is responsible for temperature inversions and sound 

 channels (Fenner and Bucca, 1972). 



Subsurface temperature analyses in the region of the Somali Current 

 reveal the double-cellular circulation bounded by colder water on the western 

 and eastern edges of the survey area. The ribbon of cold upwelled water 

 between the Great Whirl and the Socotra Eddy is especially evident in the 

 100 m, 200 m, and 300 m temperatures (figures 27, 28, and 29). Cold upwelled 

 water adjacent to the coast appears in all subsurface temperature analyses. 



The analysis of the temperatures at 50 m (figure 26) gives a slight 

 indication of the double-cellular circulation. The 24°C and 26°C isotherms 

 lack the circular pattern found in the other temperature charts, yet the 

 temperature maxima associated with the Great Whirl and the Socotra Eddy are 

 discernible. The 22°C v/ater centered at about 8°N latitude is cold upwelled 

 water entrained between the two anticyclonic gyres. Cold upwelled water 

 (T<120C) is clearly evident adjacent to the coast between 9°N and 10°N lati- 

 tude. Analysis of the temperature field at 50 m was made difficult by the 

 normal variations of the mixed layer about this depth. Since the layer depth 

 was underlain by a strong thermocline, normal variations in mechanical wind 

 mixing resulted in highly variable temperatures at 50 m. Such variations 

 masked to a large extent normal temperature variations caused by the gross 

 circulation of the Somali Current System. 



The analyses of the 100 m, 200 m and 300 m temperatures (figures 27, 28, 

 and 29) each show the Great Whirl and Socotra Eddy to be the salient oceano- 

 graphic features in the area. Low temperatures adjacent to the coast, es- 

 pecially between 9°N and 10°N, signify coastal upwelling. The temperature 

 minimum between the Great Whirl and the Socotra Eddy was a result of the 

 entrainment of cold upwelled water and was found in a zone of strong shear and 

 turbulence. The strong horizontal temperature gradient east of the Great 

 Whirl and the Socotra Eddy was clearly evident at 100 m, yet was not found at 

 200 m or 300 m. The lack of a strong horizontal temperature gradient at 200 m 

 and 300 m in the eastern edge of the survey area was an indication that the 

 zone of shear found there, although strong, was quite shallow. The 300 m 

 temperature analysis, while still showing the double-cellular circulation of 

 the Great Whirl and Socotra Eddy, reveals a considerably weaker horizontal 

 temperature gradient than that found at 100 m or 200 m. The rapidly weakening 

 horizontal temperature gradient with depth is consistent with the dynamics of 

 a strong but shallow wind-driven current. 



12 



