Experimental Evaluation of the Influence of the Seawater Characteristics on Spatial and Temporal Variations of the Sound Speed in the Southern Abyssal, Intermediate and Shelf Zones of the Caspian Sea

The results of the evaluation on the spatial and temporal changes of the underwater sound speed in the southern abyssal zone of the Caspian Sea are investigated. Experimental measurements were carried out along three survey line in the west, middle and east parts of the south deep basin during years of 2014–2016. The findings indicated that the variability range of velocity across the deep water layer was 1450–1454 ms –1 . The maximum amount of the speed in abyssal water was about 1454 ms –1 . Temporal variations of observed data demonstrated the influence of thermal structure of the water column in warm months on the sound speed is more than salinity fields. According to the high correlation of the sound speed and temperature in the Caspian Sea water, their vertical structures have uniformity and similarity from the sea surface to the bottom. Spatial analysis on the underwater sound speeds showed that the vertical gradient of the values across the surface and intermediate waters are more than monthly and seasonal changes in the abyssal area. Vertical difference of the speed across the thermocline layer in August was about 40 ms –1 . The range of the measured values of speed in February (winter) was between 1463–1451 ms –1 , while it was recorded between 1501–1451 ms –1 in June. Results indicated the formation of the deep sound channel across the intermediate and abyssal water zones. The analyses demonstrated that the axis of the sound channel occurs at 150 m depth approximately (intermediate layer) in warm months, while it appears between 270–300 m depth in February (midwinter) .