Seasonal critical depths (Figures 10 and 12 for northeast and southwest 

 monsoons, respectively) are quite similar (5000 to 5200 meters) except in the 

 Arabian Sea, Bay of Bengal, western Somali Basin, and south of Java, In the 

 Arabian Sea and Bay of Bengal, critical depths are 100-300 meters deeper during 

 the southwest monsoon due to increased surface insolation. However, in the 

 western Somali Basin, off Muscat and Oman, and south of Java, critical depths 

 are substantially less during the southwest monsoon due to upwelling induced by 

 reversing monsoon circulation. During the southwest monsoon, critical depths 

 less than 3500 meters are found in the Somali Basin, less than 4400 meters off 

 Muscat and Oman, and less than 4800 meters south of Java, These values are 

 approximately 1400, 500 and 300 meters less than northeast monsoon critical 

 depths, respectively. In the Andaman Sea, critical depths during both monsoons 

 are about 900 meters less than those found in the Mid-Indian Basin due to the 

 anomalous deep velocity profile for the Andaman Basin, Below 2000 meters, 

 variances of less than 3,0 m/sec were found between the five major basins 

 (Table I), However, deep velocities were 10 to 17 m/sec higher in the semi- 

 enclosed Andaman Basin due to local modification of higher temperature and 

 salinity RSIW and Indian Equatorial Water. 



The 12 sound velocity cross-sections shown in Appendix A and the 36 

 sound velocity/T-S comparisons given in Appendix B (including 13 seasonal 

 comparisons) illustrate the anomalous characteristics of North Indian Ocean 

 sound velocity structure. They also indicate that reversing monsoon circulation 

 does not markedly effect sound velocities below 200 to 300 meters (strong 

 negative velocity gradient) except in areas of upwelling. This finding is in 

 agreement with that of Duing, 1970 concerning vertical extent of monsoonal 

 effects. However, in the major upwelling center off the Somali Republic, 

 seasonal variability was found to depths greater than 1000 meters (see Figures 

 A-4and B-ll). 



In addition. Appendix C contains figures showing generalized surface 

 circulation during the northeast and southwest monsoons; generalized subsurface 

 circulation of PGIW and SSW; and generalized intermediate circulation of 

 RSIW, AAIW, and BIW. Water mass flow diagrams were derived from core salinity 

 depth data in order to determine the causes of various anomalous sound velocity 

 features, but include original work of Rochford, 1964 and 1966a, 



CONCLUSIONS 



• Interaction of subsurface and Intermediate water masses and variations 

 resulting from the reversing northeast and southwest monsoons cause anomalous 

 sound velocity conditions in the North Indian Ocean. 



28 



