Subsurface Water Masses 



At subsurface depths, sound velocity structures throughout most of 

 the area are altered by Intrusions of PGIW (Rochford, 1964) and SSW (Ivanenkov 

 and Gubin, 1960). PGIW is characterized by a salinity maximum between 250 

 and 400 meters (Figure C-3), and enters the Gulf of Oman across the Straits of 

 Hormuz. SSW Is formed at the Subtropical Convergence (approximately 40° 

 S. latitude) and initially is characterized by an oxygen minimum in a layer of 

 decreasing salinity. However, as SSW flows north into regions with increasing 

 concentrations of RSIW, a salinity minimum is formed at a depth somewhat 

 deeper than the characteristic oxygen minimum (Warren, et al., 1966). In 

 the North Indian Ocean, this salinity minimum generally is found between 400 

 and 700 meters (Figure C-4). South of 5° S, latitude, SSW often cannot be 

 distinguished by T-S analysis alone. 



In the Gulf of Oman and the northern Arabian Sea, high concentra- 

 tions of PGIW cause the formation of a sound velocity minimum at the interface 

 with ArSW and a sound velocity maximum at the approximate depth of the high 

 salinity core (Figures B-3 and B-4), However, throughout most of the area, 

 lesser concentrations of PGIW retard the formation of negative velocity gradients 

 below the surface mixed layer (e.g., see Figures B-5, B-8, B-16, B-22, and 

 B-36). Intrusion of SSW tends to block the southward spread of PGIW, particularly 

 in the Somali Basin. In the Somali Basin, SSW can cause the formation of a strong 

 sound velocity minimum (Figures B-8, B-18, and B-30). However, throughout 

 much of the area, SSW interacts with PGIW above and RSIW below to form an 

 essentially isovelocity layer between the strong subsurface negative velocity 

 gradient and the deep sound channel axis (Figures B-19, B-22, B-26, B-28, and 

 B-33). 



Intermediate Water Masses 



At intermediate depths, sound velocity structures throughout most of 

 the area are modified by intrusions of RSIW (Rochford, 1964), AAIW (Sverdrup, 

 et al., 1942), and BIW (Rochford, 1966a). RSIW is characterized by a salinity 

 maximum between about 500 and 1000 meters (Figure C-5) and enters the Gulf of 

 Aden through Bab-el -Mandeb. Two preferential RSIW flows are apparent in the 

 North Indian Ocean, one south along the east coast of Africa and another east 

 into the Mid-Indian Basin between the Maldlve Islands and the Chagos Archipelago. 

 Larger concentrations of RSIW apparently enter the Gulf of Aden during the 

 northeast monsoon due to the lack of a northward flowing current in Bab-el- 

 Mandeb (Serly, 1968). AAIW is characterized by a salinity minimum between 

 about 700 and 900 meters (Figure C-6) and enters the area from the south. 



