DEEP SOUND CHANNEL 



Figures 5 and 6 show the annual average depth and velocity of the deep 

 sound channel axis. The data base for both figures (number of observations 

 deeper than deep axial depth) is shown on Figure 7. These values of depth and 

 velocity are larger than those at similar latitudes in the Pacific Ocean (Johnson 

 and Norris, Apr 1967). However, the deep axial depths throughout most of the 

 area are similar to those in the North Atlantic Ocean in the presence of unmixed 

 MIW (Fenner and Bucca, Nov 1971). Deep axial depths and velocities greater 

 than 1200 meters and 1490 m/sec, respectively, are controlled by RSIW, that 

 (like MIW) depresses the deep sound channel. Deep axial depths and velocities 

 less than 1200 meters and 1490 m/sec are controlled by BIW that causes a 

 shoaler and narrower sound channel . AAIW does not have a marked effect on 

 deep channel structure in the North Indian Ocean since it is present only 

 sporadically and in slight concentrations. Deep axial depths and velocities 

 less than 1200 meters and greater than 1493 m/sec found in the Andaman Basin 

 are intermediate between those controlled by RSIW and BIW. Here, the deep 

 channel structure probably is controlled by locally modified RSIW and JEW that 

 enter the Andaman Basin north of Sumatra and at about 10° N. latitude. 



Deep axial depths and velocities greater than 1700 meters and 1494 m/sec 

 consistently are found in the Gulf of Aden, Arabian Sea, and north Arabian Basin 

 in the presence of RSIW and ArSW. The marked southward tendency of the 1600- 

 through 1800-meter and 1494- through 1496-m/sec isolines along about 55° E. 

 longitude corresponds well to preferential flows of RSIW (Figure C-5). The 

 relatively tight packing of deep axial depth and velocity isolines south of the 

 Equator corresponds roughly to the northern limit of persistent BIW occurrence 

 (Figure C-7) and the position of the North Indian Equatorial Front (Shcherbinin, 

 1969a). The northward tendency of the 1500- and 1600-meter deep axial depth 

 isolines along about 90° E. longitude corresponds to a flow of RSIW into the Bay 

 of Bengal (Figure C-5), Deep axial depth isolines shown on Figure 5 are accurate 

 to ±100 meters north of the Equator, but are somewhat less accurate between the 

 Equator and 10° S, latitude (±100 to 200 meters). The same is true of the deep 

 axial velocity isolines shown on Figure 6 (accurate to ±0.5 m/sec north of 

 Equator, to ±1 .0 m/sec south of Equator). The greater variability of both para- 

 meters south of the Equator is related to mixing between various intermediate 

 water masses. 



The sound velocity cross-sections illustrate these and other features of the 

 deep axial depth and velocity distribution. Figure A-2 shows a relatively stable 

 deep sound channel at approximately 1800 meters extending across the Gulf of 

 Aden and northern Arabian Basin. Axial velocities less than 1495 m/sec probably 

 represent regions lacking substantial RSIW flows (Figure C-5). Figure A-3 shows 



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