Simulation of sound propagation in the environment of the salt-fingering structure in the east of the Strait of Hormuz

The water environment is considered as a suitable conductor for sound waves propagation and the changes in the horizontal and vertical structures of physical parameters are effective on the speed and propagation of sound. One of the effective vertical processes in the water column is the double diffusion process with two structures of salt-fingering and diffusive convection, which are created due to the vertical gradient of temperature and salinity with different diffusion coefficients. Salt-fingering occurs when a layer of warm and salty water is located above cold and fresh water. In areas such as the Strait of Hormuz, with thermohaline exchange between the salty basin (Persian Gulf) and the open sea (Oman Sea), the conditions for the formation of salt fingers and their growth are significant. To determine double diffusion structures, the Turner angle (Tu) method is used in terms of density ratio (Rρ). Turner angle values (in degree) for the formation of double diffusion structures are defined in the range of -90 < Tu < 90. So that the diffusive convection structure is formed for the values of -90 < Tu < -45 and the salt finger structure is formed for the values of 45 < Tu < 90. With the increase of the warm water inflow entering from the Oman Sea into the Persian Gulf, in spring and summer and the increase of evaporation in late spring, the conditions for the formation of salt fingers are strengthened and salt fall occurs in all the eastern and middle stations of the Strait Hormuz. Also, in the southern stations, salt fingers extend from the surface to a depth of 65 m. The vertical gradients of temperature and salinity in the eastern cross-section of the Strait of Hormuz form a warm and salty surface layer (34 °C and 39 psu) over a cold and fresh water layer (29 °C and 37.5 psu) so that the warm and Salty water mass spreads on the surface and falls from the surface to a depth of 40-70 m as salt fingers. The fall of salt fingers causes the speed of sound waves to be not uniform along the channel so from the surface to the depth of salt finger fall, it has the highest value (1557 m/s). In this study, sound signal propagation at frequencies above 500 Hz (600 Hz and 60 kHz) is simulated using Ray theory and the Bellhop model. The results show that sound rays with a small propagation angle are scattered on the surface after passing through the salt fingers. But by increasing the depth of the sound source, the dissipative effect of the salt structure decreases while the deviation from the location of the strong structure is observed. In general, the propagation and transmission of the sound signal along the channel depend on the stratification leading to the salt finger structure, and the sound rays are propagated with significant deviation and 80-85 dB loss in transmission (10-15 dB increase in loss), while these salt fingers exist in water.