Northward flow of AAIW is blocked by the Chagos-Laccadlve Ridge, the 

 westward flow of BIW, and the preponderance of RSIW to the north. BIW is 

 characterized by a salinity minimum between about 900 and 1200 meters 

 (Figure C-7) and enters the area from the east. Although the southward spread 

 of RSIW and the Chagos-Laccadive Ridge divert the main BIW flow to south of 

 10° S. latitude, substantial concentrations are observed north and south of the 

 Seychelles Islands. 



In the Gulf of Aden, west central Arabian Basin, and western Somali 

 Basin, high RSIW concentrations cause the formation of a strong sound velocity 

 maximum at the approximate depth of the high salinity core (e.g., see Figures 

 B-6, B-7, B-8, and B-18). As previously shown (Fenner and Bucca, Jan 1971), 

 high concentrations of RSIW in the presence of SSW can cause the formation of 

 a "meaningful" upper sound channel in the Gulf of Aden and a limited region 

 east and south of Socotra. In this context, a "meaningful" upper sound channel 

 is defined as having an average velocity at the upper sound velocity minimum 

 that is at least 1 .5 meters/second greater than the comparable velocity at the 

 intermediate sound velocity maximum. Figures B-7, B-8, and B-11 show good 

 examples of a "meaningful" upper sound channel . This sound velocity structure 

 is much less widespread in the northwest Indian Ocean than that observed in the 

 North Atlantic Ocean (Fenner and Bucca, Dec 1969), largely due to the rapid 

 dilution of RSIW in Bab-el-Mandeb and the Gulf of Aden. Less than 200 

 nautical miles east of Bab-el-Mandeb, only 35% RSIW remains unmixed (Figure 

 B-6). In contrast, more than 95% unmixed Mediterranean intermediate Water 

 (MIW; upper North Atlantic Deep Water of Defant, 1961) was found approximately 

 250 nautical miles west of the Straits of Gibraltar (Fenner and Bucca, Dec 1969). 



Throughout much of the area, RSIW depresses the deep sound channel 

 axis and, in concentrations less than 15%, broadens the channel by interacting 

 with various low salinity water masses to form an essentially isovelocity layer 

 between the subsurface negative velocity gradient and deep axial depth. Figures 

 B-21, B-26 and B-28 show such interactions with AAIW and/or SSW in the Arabian, 

 Somali, and Mid-Indian Basins, respectively. Interaction of RSIW with either 

 AAIW or BIW can split RSIW into two lobes, both with a salinity maximum and 

 weak sound velocity maximum (Figures B-25 and B-31 , respectively). The lower 

 RSIW lobe formed by interaction with BIW often lies at depths below 1 100 meters 

 (Figure B-29). In such cases, it is called North Indian Deep Water (NIDW). 



The effects of AAIW on sound velocity structures in the North Indian 

 Ocean are minor due to its sporadic occurrence and relatively small unmixed 

 concentrations, AAIW either can cause the formation of a weak sound velocity 

 minimum below the RSIW induced maximum (Figure B-11) or interact with RSIW 

 and SSW to form an essentially isovelocity gradient above deep axial depth 



