On the Relationship Between Ocean Transparency and the Depth of Sonic Scattering Layers in the North Atlantic

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Abstract
Using some 5060 observations of Secchi-disc transparency from all available sources, a chart of ocean transparency was constructed for the Atlantic between latitudes15°S and 60°N. Although the inherent inaccuracy and poor comparability of Secchi-disc observations prohibit the more detailed charting of ocean transparency by months, or even by seasons, it is thought that the broad features of the annual average distribution of transparency are meaningful and relatively unbiassed by seasonal changes. Echo-sounder records at 10 kHz conducted at 3° intervals of latitude between 10°N and 68°N (NAVADO Atlantic surveys) have shown the existence of five main sonic scattering layers in the North Atlantic, of which two are of widespread geographical occurrence. A striking association is found between ocean transparency and the maximum diel migration-depth attained by the two principal scattering layers, suggesting that the principal factor determining the extent of scattering layer migration in the North Atlantic is that of light penetration.

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