ABSTRACT 



All available sound velocity data in the North Indian Ocean (north of 

 10° S. latitude) have been analyzed in terms of annual areal extent and depth 

 of perturbations above deep axial depth, annual average depth and velocity of 

 the deep sound channel, and critical depths for the northeast and southwest 

 monsoons. The vertical extent of these and other sound velocity features is 

 shown on six north-south and six east-west cross-sections that extend to a 

 maximum depth of 5000 meters. These analyses indicate that highly saline Red 

 Sea Intermediate Water (RSIW) is the major factor controlling sound velocity 

 structures in the North Indian Ocean. Mixing of RSIW with low salinity water 

 masses causes either sporadic sound velocity perturbations or an essentially 

 isovelocity layer above deep axial depth. In relatively high concentrations, 

 RSIW causes an anomalously deep (greater than 1700 meters) and narrow sound 

 channel with velocities greater than 1493 meters/second (Gulf of Aden, Arabian 

 Sea, and Arabian Basin), In relatively low concentrations, RSIW causes a some- 

 what shoaler (1200-1600 meters) and broader sound channel with velocities 

 between 1490 and 1493 meters/second. The deep sound channel along the 

 southern boundaty of the area (less than 1100 meters at velocities less than 1490 

 meters/second) is controlled by low salinity Banda Intermediate Water. Critical 

 depths show the effects of reversing monsoon circulation north of about 10° N. 

 latitude, in the western Somali Basin, and south of Java. In the remainder of 

 the area, critical depths are similar during both monsoons. In addition, sound 

 velocity/temperature-salinity comparisons for 36 locations and circulation 

 diagrams for the surface (both monsoons) and for subsurface and intermediate 

 water masses are presented. 



DON F. FENNER 

 PAUL J. BUCCA 



Ocean Science Department 

 Science and Engineering Center 



