Measurement of small-scale oceanic motions with neutrally-buoyant flloats

The movement of water in the ocean waa studied by following 1 3 motions of instrumented neutrally-buoyant floats. Measurements were made at various depths in the Caribbean Sea and in the Atlantic Ocean several hundred miles east of Bermuda. There was a continuous vertical bobbing of the floats, and two floats at nearly the same depth separated at an uneven rate in time. Near the surface, vertical displacements of approximately 3 m took place during periods of 10 to 30 minutes, and movements of approximately 30 m took place over a period of tens of hours. The slow large-amplitude motions were probably forced by semidiurnal tides in the Caribbean. Vertical movements at depths near 1000 m in both operating areas amounted to approximately 10 m over periods of 1 to 2 hours.

Floats at their equilibrium depths do not faithfully follow the movements of the water environment. An approximate solution is derived for the response of a float to an internal wave. It was found that a resonance causes the float movements to exceed the water displacements at the shorter periods whereas the stiffness of the float tends to keep it from moving in deep water where the density gradient is small.