Photogrammetric evaluation of the rate of distortion of vertical dye streaks within the main oceanic thermocline indicates that spiraling perturbation structures overlay the large-scale vertical shear. The data show pronounced shear structure with vertical scales of a meter or less at depths of hundreds of meters. The shapes as well as strengths of the observed shear structure have a strong dependence on depth. At 400 m the major variations in shear had vertical length scales of about 10 to 20 m; the velocity amplites were of the order of 1 cm s-1, and the predominant spiral pitch was consistent with downward propagation of energy by internal gravity waves. Shallower observations suggest a lateral intrusion as a predominant shear source.
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