Basin, deep velocities are 10- to 17-m/sec greater than those in the North 

 Indian Ocean proper. This is due to the higher temperatures and salinities of 

 Andaman Sea Deep and Bottom Water (Figure B-1) which is formed by local 

 modification of RSIW and lEW. The deep profile in the Arabian Basin has 

 consistently higher sound velocities than those in the other four major basins 

 due to the higher T-S values found below 2000 meters in both the Arabian Basin 

 and the Arabian Sea (influence of ArSW, PGIW, and RSIW). The deep profile 

 in the North Wharton Basin has consistently lower sound velocities due to lower 

 T-S values found below 2000 meters. The Wharton Basin extends to about 35° S. 

 latitude and is a primary route for deep water flow between the Antarctic and 

 the North Indian Ocean (Zaklinskii, 1963). The deep velocity profiles for the 

 Somali, North Mascarene, and Mid-Indian Basins are intermediate between those 

 for the Arabian and North Wharton Basins. 



SEASONAL CRITICAL DEPTHS 



Figures 9 and 1 1 show critical depths for the northeast and southwest 

 monsoon. The data bases for these figures are given on Figures 10 and 12. 

 Overall, 47% of the existing critical depth data represents northeast monsoon 

 conditions, 53% southwest monsoon conditions. During the southwest monsoon, 

 data are inadequate to evaluate in a large region north and east of Ceylon. 

 Critical depths were derived using the deep sound velocity profiles shown on 

 Table 1. Critical depth contours for both seasons are accurate to ±50 meters 

 throughout the area. 



During the northeast (winter) monsoon, critical depths range from greater 

 than 5200 meters in the southern Somali Basin to less than 4700 meters off 

 Pakistan. In the Andaman Sea, critical depths are approximately 4100 meters 

 due to the anomalous deep sound velocity profile for the Andaman Basin (Table 

 1). The marked differences in deep velocity structures on either side of the 

 Andaman-Nicobar Ridge result in an apparent 900-meter change in critical depth 

 in the various passages into the Andaman Sea. Critical depths less than 4800 

 meters in the northern end of the Arabian Sea correspond well with sea surface 

 temperature minima shown for December through February by La Violette and 

 Frontenac, August 1967. Critical depths less than 4900 meters in the northern 

 Bay of Bengal correspond to similar sea surface temperature minima shown by 

 La Violette, August 1967. Critical depths less than 4800 meters off the Kenya 

 and east India coasts represent local regions of northeast monsoon upwelling 

 (Figure C-1). In the western Somali Basin, the northeast to southwest tendency 

 of the 4900- and 5000-meter isolines is caused by the Northeast Monsoon Current 

 which carries warmer, more saline ArSW into the region north of the Seychelles 

 Islands. This mechanism combined with surface insolation leads to deeper critical 

 depths throughout the southern Somali Basin. Warmer, more saline waters carried 



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