J F M A M J Jasondj

Figure 3.17 Temperature contours (°C) for a region of the Texas-Louisiana continental shelf in the Gulf of Mexico (Etter and Cochrane, 1975).

for example, the mixed layer is approximately 100 m deep while in August the MLD is seen to be shallow, about 25 m. These variations are further illustrated in Figures 3.18 and 3.19, which are vertical sections across the shelf. In winter (February), for example, the isotherms tend to be vertically oriented, evidencing strong mixing and vertical (versus the more typical horizontal) stratification. In summer (July), however, the isotherms tend to be more horizontal, the result of weak mixing, with attendant horizontal stratification.

3.7.2 General propagation features

A computer-generated ray diagram for a source in a typical mixed layer is shown in Figure 3.20. Under the conditions for which the diagram was drawn, the ray leaving the source at an angle of 1.76° becomes horizontal at the base of the mixed layer. Rays leaving the source at shallower angles remain in the layer, and rays leaving the source at steeper angles are refracted downward to greater depths. A shadow zone is produced beneath the layer at ranges beyond the immediate sound field. The shadow zone is not completely

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