• An extremely strong negative velocity gradient is present between the 

 surface and about 200 meters. 



• Sound velocity perturbations are present just above the deep sound 

 channel with the following temporal and spatial distributions: 



- present greater than 80% of the time in the Gulf of Aden 

 and much of the Somali Basin 



- present 20-80% of the time in the Arabian Basin, southern 

 Somali Basin, and between 5° N, and 5° S, latitude east of 

 India 



- present less than 20% of the time in the northeast Arabian Sea, 

 Bay of Bengal, and southern Mid- Indian Basin. 



• No well defined upper sound channel or subsurface sound velocity 

 maximum is present except in the Gulf of Aden and along the coast of Africa. 



• The deep sound channel has the following characteristics: 



- relatively deep, narrow, and high velocity north of about 

 10° N. latitude 



- relatively wide, shoaler, and lower velocity south of about 

 10° N. latitude 



- narrow, shoal, and low velocity in the southeast comer of 

 the area 



- very narrow, shoal, and relatively high velocity in the 

 Andaman Sea. 



• High salinity intrusive water masses cause a relatively narrow, 

 anomalously deep, and high velocity sound channel in large unmixed concen- 

 trations; but result in a relatively broad, shoaler, and lower velocity channel 

 after substantial mixing. 



• Critical depths are similar during both monsoons with the following 

 exceptions: 



- deeper during the southwest monsoon north of about 10° 

 N. latitude 



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