curve between the points T=10.0°C, S=35.4 o/oo and T=5.0°C, S=34.9 °/oo is 

 evidence of pronounced mixing along isopycnal surfaces between these two water 

 masses. Weak stratification between Subtropical Subsurface Water and North 

 Indian Deep Water is indicated on a T-S diagram in which the curve of the two 

 water masses parallels rather than crosses lines of constant a^. This lack of 

 density stratification leads to strong mixing of two water masses of differing 

 temperatures and salinities, yet similar densities. Oxygen concentrations 

 could be a means to tag the Subtropical Subsurface and North Indian Deep 

 Waters of the northwestern Indian Ocean. Owing to its origin in upper and 

 middle latitudes of the southern Indian Ocean where it is subjected to storm- 

 induced wind mixing, the former should have the higher concentration of dis- 

 solved oxygen. 



At about 2500 m, the water in the Somali Basin is principally Antarctic 

 Circumpolar Water originating in the broad band between the Antarctic and 

 Subtropical Convergence (Warren, e^ al_. , 1966). The formation of this water 

 explains the oxygen maximum which underlies the comparatively oxygen-poor 

 North Indian Deep Water. 



Although Warren, Stommel , and Swallow (1966) do not mention the presence 

 of Antarctic Bottom Water in the Somali Basin, Lowrie (1980) is convinced that 

 there is strong flow of this water into the Somali Basin. Antarctic Bottom 

 Water, having bottom potential temperatures of less than 1°C, is formed along 

 the coast of Antarctica, principally in the Weddell Sea. This water, under- 

 lying Antarctic Circumpolar Water, enters the northern part of the Indian 

 Ocean by flowing along the eastern edge of the Madagascar Plateau into the 

 Madagascar Abyssal Plain off the east coast of Madagascar. From the i^adagascar 

 Abyssal Plain, Antarctic Bottom Water enters the Somali Basin through the 

 Amirante Passage between the Seychelles and the Farquhar Island Group north of 

 Madagascar. The extreme depth of the Amirante Passage, which is in excess of 

 5000 m in some places, and the coarse grain sizes (Lowrie, 1980) found along 

 the bottom, indicate considerable flow of Bottom Water through this opening. 

 From the Amirante Passage, Antarctic Bottom Water is free to fan out onto the 

 floor of the deep Somali Basin. The three deep stations taken on the first 

 leg of the survey, station numbers 15 (8° 27.1 'N, 51° 42.0'E), 20 (10° 16.3'N, 

 53° 29.7'E), and 22 (11° 04.8'N, 55° 53.9'E) revealed bottom potential tempera- 

 tures of 0.970c, 0.97°C and 1.12°C, respectively. Although these three 

 stations taken in the northern part of the Somali Basin can hardly be con- 

 sidered conclusive proof of the existence of Antarctic Bottom Water in this 

 area, the low bottom potential temperatures observed at these locations are in 

 good agreement with previous measurements. 



The first station (figure 25) taken at 0° 03.0'N, 50° 56.1 'E, only three 

 nmi north of the equator, shows a shallow, yet strong, salinity maximum 

 between 50 m and 100 m. This salinity maximum, located as it is in an area of 

 mass convergence, is the result of wind-driven transport of warm, saline water 

 toward the equator. Since a similar salinity maximum occurs along the Atlantic 

 equator and is associated very closely with the Atlantic Equatorial Undercur- 

 rent, this feature might be attributed to an Indian Ocean Equatorial Under- 

 current. However, the Equatorial Undercurrent in the Indian Ocean is associated 

 with the northeast monsoon and is absent during the southwest monsoon (Wyrtki, 

 1973). The lack of an Equatorial Undercurrent in the Indian Ocean during the 

 southwest monsoon is explained by the thermohaline structure of the equatorial 



