Analysis of historical sound velocity and temperature-salinity data north of ICS latitude and west of 80*E longitude indicates that mixing of relatively warm, saline Red Sea and Persian Gulf Intermediate Waters with relatively cold, dilute Subtropical Subsurface and Antarctic Intermediate Waters results in an extremely strong negative velocity gradient to about 200 meters, sporadic sound velocity perturbations above primary axial depth, and a wide, depressed primary sound channel. Perturbations in the sound velocity structure above primary axial depth were found more than 80% of the time throughout much of the Somali Basin, with minima associated with low salinity cores and maxima associated with high salinity cores. A meaningful upper sound channel was found west and south of Socotra and in a pocket northeast of Zanzibar associated with intrusions of Red Sea Intermediate and Subtropical Subsurface Waters, respectively. The absence of a widespread upper sound channel in the Northwest Indian Ocean is attributable to the lack of sufficient cold, dilute water north of the equator and to intensive mixing of the four intrusive high and low salinity water masses, particularly in the Somali Basin.

In the region north of about 8°N latitude, primary axial depths are anomalously deep (greater than 1700 meters) and primary axial velocities are anomalously high (greater than 1496 meters/second) in the presence of high concentrations of Red Sea and/or Persian Gulf Intermediate Waters. Areal patterns of both variables show the preferential flow of Red Sea Intermediate Water along the African coast and a ZOM of relatively rapid transition south of the equator that corresponds to the limit of present Antarctic Intermediate Water occurrence. During the northeast (winter) monsoon, deejay critical depths are found throughout the Somali Basin due to southward transport of higher temperature and salinity waters from the Arabian Sea by the Northeast Monsoon Current. During the southwest (summer) monsoon, anomalously shallow critical depths are found in association with two widespread centers of upwelling along the Somali and Muscat- Oman coast. Seasonally reversing monsoons do not appreciably affect the sound velocity structure below 200 meters, except in centers of upwelling. Rather, intensive mixing between the four intrusive water masses is the predominate cause of the complex and irregular sound velocity structure and extremely broad primary sound channel found throughout the Northwest Indian Ocean.