The concept of the three-layered ocean-mixed layer, main thermocline, and deep layerwas mentioned earlier in this unit. Of these three layers, the mixed layer is the most variable in its properties (primarily depth), and requires considerable attention.

Another layer of high variability and of great importance in antisubmarine warfare is the sonic layer. The sonic layer is a layer that basically traps sound waves. The sonic-layer depth is the level of maximum sound velocity based primarily on temperature, but also controlled by density and pressure. The sonic-layer depth and mixed-layer depth often coincide.

We determine layer depths from temperature versus depth profiles obtained in bathyther-mograph observations. The mixed-layer depth is determined by finding the first subsurface depth with a temperature at least 1C colder than the surface temperature. From this point, proceed back up the profile to the previous depththis is the mixed-layer depth, or MLD. Refer to the example in figure 1-4-5.

The temperature-versus-depth profile in this example shows an isothermal layer of 24.5C water in the first 15 meters. Between 15 meters and 55 meters the temperature decreases to 22.1C. This temperature is maintained to a depth of 75 meters. Between 75 meters and the last

The first reported depth with a temperature at least 1C colder than the surface temperature is 55 meters. The preceding temperature is 24.5C at 15 meters. This is the depth of the mixed layer. Had the isothermal layer at the surface not been in existence and the temperature had simply decreased from the surface to 55 meters, the MLD would have been at the surface. In other words, there would have been no mixed layer.

The importance of the mixed layer lies in the fact that it is most often synonymous with the sonic layer and plays a vital role in ASW operations. The mixed layer also plays a key role in the food chain. Some of the most productive fishing areas are found where strong mechanical and/or convective mixing occur.

The sonic layer depth (SLD) can also be deter-mined from temperature-versus-depth profiles. The depth where the warmest temperature occurs is normally the SLD, but not always. The speed of sound in water is normally greatest near the surface, where the temperature is warmest, and decreases with depth as the temperature decreases. However, at greater depths, sound speed increases because pressure becomes the dominant controlling factor. Figure 1-4-6 is a BT profile that shows the MLD and SLD at different depths. The SST is 24.5C, and the temperature increases to 26.0C at 10 meters. The temperature then decreases gradually to 25.1C at 28 meters before falling rapidly to 15.6C at 80 meters. The MLD in this example is at 28 meters, while the SLD is at 10 meters. The SLD is at 10 meters because this is the level with the highest temperature.

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