The mixed layer depth (MLD) analysis chart shown in figure 4-2-12 depicts the depth of the mixed layer, in meters. Contours are drawn on the hemispheric chart at 10, 20, 30, 45, 60, 80, 100, 140, 180, 230, and 280 meters. The deeper the mixed layer, the larger the contour interval. The MLD is the bottom of the uppermost layer of the ocean, the Mixed Layer, which, because of mixing by waves and currents, is usually fairly isothermal or shows only a slightly negative temperature gradient with depth. The MLD is also considered the top of the second ocean layer, the Main Thermocline, where the temperature decreases rapidly with depth. Because of the sharp decrease in the temperature gradient with depth at the MLD, the MLD usually, but not always, is the point of maximum sound velocity in the upper 1,500 feet of the sea. In the cases where the MLD is the point of maximum sound velocity, it is also known as the Sonic Layer Depth (SLD).

Prognostic charts of the forecast change in the MLD are available in addition to the analyses. Figure 4-2-13 shows a full hemisphere (Northern) polar stereographic projection of the 24-hour forecast changes in the MLD, which are due to increasing or decreasing wave heights. The contours are in meters, with a 4-meter interval. Negative values indicate an increasing MLD (the MLD becomes deeper); positive values indicate a decreasing MLD.

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