(Figures B-12 and B-33). In contrast, BIW has marked effects in forming a 

 shoaler and narrower sound channel at lesser velocities (Figures B-34 and B-36). 

 Along the southern boundary of the area, the deep sound channel structure is 

 controlled by BIW. Mixing of BIW and RSIW can form sound velocity perturbations 

 below deep axial depth (Figure B-31). 



Deep Water Masses 



The circulation of NIDW (formed by sinking of RSIW south of the 

 North Indian Equatorial Front, Shcherbinin, 1969a) and North Indian Bottom 

 Water does not change the basic shape of the positive sound velocity gradient 

 below about 2000 meters. Although sound velocity varies somewhat from basin 

 to basin (see Table I), deep sound velocity profiles are quite similar throughout 

 most of the area. However, in the Andaman Basin, sound velocity values below 

 2000 meters are 10- to 17-mfc;iers/second (m/sec) greater than those in the rest 

 of the North Indian Ocean (Table I). In addition, T-S values below 2000 meters 

 in the Andaman Basin are essentially isothermal and isohaline (Figure B-17), 

 Andaman Sea Deep and Bottom Water is considerably warmer and more saline 

 than NIDW, and probably is formed by sinking and local modification of RSIW 

 and lEW trapped inside the Andaman Basin. 



SOUND VELOCITY PERTURBATIONS 



Highly sporadic perturbations in the sound velocity structure above deep 

 axial depth were found throughout most of the northwest Indian Ocean, in a band 

 between about 5° N. and 5° S. latitude across the Mid-Indian and North Wharton 

 Basins, and in the region south of Java. Sound velocity minima associated with 

 these perturbations generally occur at the approximate depth of the SSW or AAIW 

 low salinity core (Figure B-ll). Sound velocity maxima associated with such 

 structures generally occur at the approximate depth of the RSIW high salinity 

 core (Figure B-ll). Sound velocity maxima associated with the PGIW high 

 salinity core were found only in the Gulf of Oman and northern Arabian Sea 

 (Figures B-3 and B-4) . 



Figures 2 and 3 show the annual approximate depth and average areal 

 extent of the upper sound velocity minimum and intermediate sound velocity 

 maximum. The data base for both figures is shown on Figure 4. Throughout 

 most of the area, sound velocity perturbations were highly sporadic, particularly 

 in terms of depth. More than one perturbation often was found on a given sound 

 velocity profile. Therefore, it was impossible to contour the depth of either the 

 upper sound velocity minimum or intermediate sound velocity maximum. Rather, 

 Figures 2 and 3 show regions of similar average depth of the most pronounced 

 minimum and maximum (i.e., minimum with least sound velocity and maximum 



