INTRODUCTION 



Events of the past few years have shown increased Naval interest in the 

 Indian Ocean. However, despite extensive data collected by the International 

 Indian Ocean Expedition (IIOE), there has been no systematic description of 

 the sound velocity structure of the Indian Ocean. In an effort to correct this 

 discrepancy, the Naval Oceanographic Office (NAVOCEANO) prepared a 

 paper (Fenner and Bucca, Jan 1971) in which areal contour charts of various 

 sound velocity features and a limited number of sound velocity/temperature- 

 salinity (T-S) comparisons were used to define the complex sound velocity 

 structure of the northwest Indian Ocean (north of 10° S. latitude, west of 80° 

 E. longitude). This report supplements the above paper in terms of sound velocity 

 cross-sections, additional sound velocity/T-S comparisons, and analysis and 

 extends east to the Malay Peninsula and Indonesia. This report also explains 

 the temporal and spatial distribution of various sound velocity features in terms 

 of water mass analysis and circulation patterns. For purposes of this report, 

 deep axial depth is defined as the deepest sound velocity minimum (usually 

 absolute minimum). Sound velocity perturbations are defined as changes from a 

 negative to a positive sound velocity gradient (or vice versa) that are less 

 effective channels for sound transmission than the deep sound channel. Critical 

 (limiting) depth Is that depth where the sound velocity Is equal to the maximum 

 sound velocity at the surface or In the surface mixed layer. The names of selected 

 physical features used in this report are given on Figure 1 . Locations of sound 

 velocity cross-sections are shown on Figure A-1 . Locations of sound velocity/ 

 T-S comparisons are given on Figure B-2. 



SOURCES AND TREATMENT OF DATA 



All available data from the National Oceanographic Data Center 

 (NODC) were analyzed during the preparation of this report. Additional 

 Nansen cast data collected by the HMAS DIAMANTINA in 1964 and 1966 also 

 are Included (Commonwealth Scientific and Industrial Research Organization, 

 Australia, 1967 and Scully-Power, Apr 1969, respectively). All data were 

 converted Into sound velocity using the equation of Wilson, 1960. 



These data were analyzed for the depth and axial velocity of the deep 

 sound channel, the depth of various sound velocity minima and maxima (pertur- 

 bations) lying above deep axial depth, and for critical depth. The data were 

 compiled by one-degree square and season. One-degree square compilations 

 were averaged by two-degree square (i.e., four one-degree squares) on an 

 annual basis for deep axial depth, deep axial velocity, and the depth of the 

 maxima and minima associated with sound velocity perturbations. Critical 

 depths were averaged similarly on a seasonal basis. The two-degree square 



