is isothermal, and in the deep layer it changes slowly. In deep-layer 

 regions, the pressure effect on sound speed prevails. Direct salinity 

 effects on sound speed are usually small. 



Figure 5-12 shows that when an isothermal mixed layer exists as 

 in winter, sound speed increases with depth from the surface at the 

 rate 0.016 m/s/m until the temperature effect of the thermocline takes 

 over. Beneath the thermocline, decrease of temperature with depth 

 becomes small, the pressure effect again takes over and sound speed 

 increases again close to the isothermal rate of 0.016 m/s/m. In 

 figure 5-12, another velocity minimum occurs followed by an increase 

 to the highest sound speeds in the ocean near the bottom. Total 

 variation in sound speed is of the order of about 4 percent depending 

 on water depth. This small variation along with the sound speed minima 

 has a decided effect on propagation of sound over long ranges. 



Sound speed minima are axes of sound channels where rays are 

 trapped or concentrate, whereas the maxima cause a divergence among 

 rays. The sound-trapping region near the surface is called the surface 

 duct, whereas the sound-trapping region at depth is called the deep 

 sound channel or SOFAR (Sound Fixing and Ranging) channel. Figure 5-13 

 shows the latitudinal behavior of the deep sound channel axis depth in 

 the North Atlantic. The deep sound channels are deepest in midlatitudes . 

 An examination of the temperature vs. depth plots of figure 5-6 indicates 

 that this is caused by: (1) very steep thermoclines and shallow mixed 

 layers of tropical regions, and (2) near uniformity with depth of 

 temperature and salinity caused by convective mixing in high latitudes. 



e. Modes of Sound Propagation in the Ocean 



The typical sound speed profile of deep oceans results in four 

 major modes of sound propagation, depending on the depths of the sound 

 source and receiver. In figure 5-14A, the surface duct traps rays by 

 successive bounces off the surface and upward bending by the increasing 

 sound speed with depth. In figure 5-14B, rays from a source at the 

 deep sound channel axis are trapped by continued refraction due to the 

 positive gradients on either side of the channel axis. In figure 5-14C, 

 the convergence zone mode returns a ray bundle by total refraction to 

 the surface from great depths. Figure 5-14D shows the bottom and 

 surface bounce mode that prevails for steep rays, relatively shallow 

 water depths, or high surface temperatures. 



6. Waves in the Ocean 



a. The Sea Surface and Acoustic Effects 



The state of the sea surface has important consequences for 

 ocean acoustics. When wind blows over the water, there is a transfer 

 of energy from wind to water that sets the sea surface in motion. 

 Surface waves are generated and a surface drift current is created. 



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