SUMMARY 



Interaction of high salinity Red Sea Intermediate Water (RSIW) and 

 Persian Gulf Intermediate Water (PGIW) with low salinity Subtropical Subsurface 

 Water (SSW) , Antarctic Intermediate Water (AAIW), and Banda Intermediate 

 Water (BIW) results in anomalous sound velocity structures throughout much of 

 the North Indian Ocean. Interaction of PGIW and SSW causes an extremely 

 strong negative velocity gradient between the surface mixed layer and about 

 200 meters. Mixing of RSIW with SSW and/or AAIW results in either sporadic 

 sound velocity perturbations or an essentially isovelocity layer directly above 

 deep axial depth. Relatively unmixed RSIW results in anomalously large deep 

 axial depth values, whereas relatively well mixed RSIW causes a broader and 

 somewhat shoaler deep sound channel. Relatively unmixed BIW results in the 

 formation of a somewhat shoaler and narrower deep sound channel. Intensive 

 mixing of RSIW and BIW can cause sound velocity perturbations below deep 

 axial depth. 



Sound velocity perturbations (Figures 2 and 3) occur greater than 80% 

 of the time in the Gulf of Aden and much of the Somali Basin; 20-80% of the 

 time throughout the Arabian Basin, southern Somali Basin, and in a band between 

 about 5° N. and 5° S. latitude east of India and Ceylon; but are effectively 

 absent in the northeast Arabian Sea, Bay of Bengal and southern Mid- Indian 

 Basins. The lack of high unmixed RSIW concentrations throughout most of the 

 area precludes the formation of a well defined upper sound channel and sub- 

 surface sound velocity maximum such as found in the Northeast Atlantic Ocean 

 in the presence of high salinity Mediterranean Intermediate Water (Fenner and 

 Bucca, Dec 1969). 



RSIW is the major factor affecting deep sound channel structure in the 

 North Indian Ocean (Figures 5 and 6). In the Gulf of Aden, northern Arabian 

 Basin and Arabian Sea, the sound channel is relatively narrow, deep (greater 

 than 1700 meters), and high velocity (greater than 1493 m/sec) in the presence 

 of 15-35% unmixed RSIW. In the southern Arabian and Somali Basins, in the 

 Bay of Bengal, and in most of the Mid-Indian and North Wharton Basins the 

 sound channel is relatively wide, somewhat shoaler (1200 to 1600 meters), and 

 somewhat lower velocity (1490 to 1493 m/sec) in the presence of less than 15% 

 unmixed RSIW. In the southeast corner of the area, the channel is narrow, 

 shallow (less than 1100 meters), and low velocity (less than 1490 m/sec) due to 

 low salinity BIW, In the Andaman Sea, the channel Is very narrow, shallow 

 (less than 1100 meters) and relatively high velocity (about 1493 m/sec) due to 

 RSIW and local modification of Indian Equatorial Water. An analogous situation 

 occurs In the North Atlantic Ocean in the presence of varying concentrations of 

 Mediterranean Intermediate Water (Fenner and Bucca, Nov 1971). 



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