For a target to be visible, it must contrast with its background.

Deep (clear) water is very transparent to the blue portion of the light spectrum and less transparent to the green, yellow, red, and violet portions. In the more turbid coastal waters, green and yellow light penetrates to greater depths than does blue.

Irregular sea surfaces affect visibility in several ways. Variable refraction results in a reduction of the contrast of a target. Winds that barely ruffle the surface reduce contrast of a target by as much as 40 percent.

The amount of incident illumination, as determined by cloud coverage and the sun above the horizon, is a definite consideration in underwater visibility.

The optical image of a target can be due to its own light, to reflected light, or to its being silhouetted against an illuminated background.

GEOGRAPHIC VARIATION OF TRANSPARENCY

Figure 9-14 depicts the Seawater transparency of the North Atlantic. Figure 9-14 also shows that deep North Atlantic waters range in transparency from approximate y 50 feet off the continental slope to over 115 feet in the Sargasso Sea.

EFFECTS OF OCEAN FRONTS, EDDIES, AND UPWELLING

LEARNING OBJECTIVES Define oceanic fronts, eddies, and upwelling. Recognize typical locations of oceanic fronts, eddies, and upwelling in the Pacific and Atlantic oceans. Be familiar with the effects of oceanic fronts, eddies, and upwelling on acoustics. Recognize oceanic front, eddy, and upwelling locations using satellite data.

Figure 9-14.-Seawater transparency of the North Atlantic.

First of all, lets consider the definitions of fronts, eddies, and upwelling.

An ocean front is the interface between two water masses of different physical characteristics. Usually, fronts show strong horizontal gradients of temperature and salinity, with resulting density variation and current shear. Some fronts which have weak horizontal gradients at the surface have strong gradients below the surface. In some cases, gradients are weak at all levels, but variability across the front, as reflected by the shape of the thermal profile, is sufficient to complicate sound transmission. A useful definition for the purpose of naval operations can be stated as: A tactically significant front is any discontinuity in the ocean which significantly alters the pattern of sound transmission and propagation loss. Thus, a rapid change in the depth of the sound channel, a difference in the sonic-layer depth, or a temperature inversion would denote the presence of a front.

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