over the Carlsberg Ridge there are perturbations below the deep sound channel 

 axis (mixing of RSIW and BIW) as well as above. Perturbations are absent over 

 the Mid-Indian Basin, but recur sporadically east of the Ninetyeost Ridge above 

 and below deep axial depth. 



In the north-south direction. Figure A-8 shows a single perturbation 

 structure that shoals and becomes better defined north of 5° S. latitude (caused 

 by interaction of SSiW and RSIW). Farther east. Figure A-9 shows a well defined 

 minimum/maximum structure north of the Equator, However, between the Equator 

 and about 5° N. latitude, 3 separate sets of perturbations are found at velocities 

 from greater than 1500 to less than 1495 m/sec (intermixing of RSIW, SSW, AAIW, 

 and BIW). South of Cape Cormorin, Figure A-ll shows transitory perturbations 

 north of the Equator at velocities less than 1495 m/sec, including one perturbation 

 below deep axial depth. In the eastern part of the area. Figure A-13 shows an 

 extremely complex pattern of perturbations between 5° S. and the Andaman- 

 Nicobar Ridge at velocities greater and less than 1495 m/sec. Perturbations at 

 greater than 1495 m/sec probably are caused by mixing of RSIW with SSW; those 

 at less than 1495 m/sec by mixing of RSIW and/or NIDW with BIW. At approxi- 

 mately 2° S, latitude, perturbations above and below deep axial depth merge with 

 the deep sound channel axis. The complicated perturbations north of the Equator 

 also are related to bathymetric interference by the Andaman-Nicobar l^idge to 

 the eastward flow of RSIW and the northward flow of SSW and BIW. 



The highly significant role of RSIW in forming sound velocity perturbations 

 in the North Indian Ocean is analogous to that of MIW in forming an upper sound 

 channel and subsurface sound velocity maximum in the North Atlantic Ocean 

 (Fenner and Bucca,Nov 1971). In the North Atlantic, relatively high to intermediate 

 concentrations of MIW (greater than about 60%) are necessary for the formation of 

 an upper sound channel and subsurface sound velocity maximum. However, sound 

 velocity perturbations frequently are found at greater than 30% MIW concentrations. 

 Using the curve of Defant, 1961, 30% unmixed MIW lies at approximately the 

 same salinity as 10% unmixed RSIW (Figure B-1). RSIW concentrations greater 

 than 10% generally are found in the Gulf of Aden, east and south of Socotra, 

 and along the east coast of Africa (Figures B-7, B-8 and B-11, and B-25, 

 respectively). Therefore, the relative absence of RSIW (due to intensive mixing 

 with low salinity water masses in the Somali and northern Arabian Basins) is 

 largely responsible for the absence of a "meaningful" upper sound channel and 

 well defined intermediate (subsurface) sound velocity maximum in the North 

 Indian Ocean. 



12 



