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Direct Measurements of Current Shear in the Tropical Pacific Ocean and Its Effect on Drift Buoy Performance

David S. BittermanNOAA/Atlantic Oceanographic and Meteorological Laboratory, Miami, Florida

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Donald V. HansenNOAA/Atlantic Oceanographic and Meteorological Laboratory, Miami, Florida

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Abstract

Measurements of ocean surface currents derived from drift buoy trajectories are subject to errors caused by slippage of the buoy relative to the surrounding water. This slippage error is caused by a number of forces acting on the buoy and drogue element, one of which is the current shear in the water. Idealized model calculations are used to exemplify some effects of vertical current shear on drogues, and on the performance of drogued buoy systems in current shear. It is shown that shear enhances the performance of drogues, and that long drogues should perform better than short drogues in shear, but shear also can induce slippage by adding drag force to the buoy hull.

To establish environmental design parameters, average and rms current shear values between 9.7 and 22.5 meters depth were computed from Doppler acoustic current profiler measurements from the tropical Pacific Ocean. Largest values of shear (∼0.025 s−1 rms) were found near the equator in the eastern Pacific as expected. Elsewhere the shear was generally less than 0.02 s−1, mostly less than 0.01 s−1. Average values of shear were generally less than 0.007 s−1.

Abstract

Measurements of ocean surface currents derived from drift buoy trajectories are subject to errors caused by slippage of the buoy relative to the surrounding water. This slippage error is caused by a number of forces acting on the buoy and drogue element, one of which is the current shear in the water. Idealized model calculations are used to exemplify some effects of vertical current shear on drogues, and on the performance of drogued buoy systems in current shear. It is shown that shear enhances the performance of drogues, and that long drogues should perform better than short drogues in shear, but shear also can induce slippage by adding drag force to the buoy hull.

To establish environmental design parameters, average and rms current shear values between 9.7 and 22.5 meters depth were computed from Doppler acoustic current profiler measurements from the tropical Pacific Ocean. Largest values of shear (∼0.025 s−1 rms) were found near the equator in the eastern Pacific as expected. Elsewhere the shear was generally less than 0.02 s−1, mostly less than 0.01 s−1. Average values of shear were generally less than 0.007 s−1.

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