Measurements of near‐surface shear in the ocean

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Abstract
In an effort to measure current shear very near the sea surface a string of vector‐measuring current meters suspended beneath a surface float was deployed 180 km off southern California at 34°N 121°30′W on November 26, 1981 and allowed to drift freely for over 6 days. This arrangement, which we call the current meter drifter (CMD), gave measurements at depths of 2.5, 5.5, 8.5, 11.5, 14.5, 25, and 63 m. During the first two days, when the winds were light (≃8 m/s) and variable in direction, a nearly uniform current shear was observed in the upper 15 m with a low‐frequency velocity difference of 5 cm/s between the instruments at 2.5 and 14.5 m. During the last four days, when the winds were brisk (> 12 m/s) and steady in direction from the NNW, a strong downwind shear of order 10−2 s−1 was observed in the upper 10 m with a velocity difference of ∼7 cm/s between the instruments at 2.5 and 8.5 m. During this same period the shear below 10 m was much smaller. The average currents during the CMD drift veer to the right of the wind stress with angular displacement increasing with depth. Time series of the velocity difference between 2.5 and 5.5 m compare very well with θ(t)u*κ−1 ln (5.5/2.5), where θ(t) is the wind direction vector (of unit magnitude), u* = (wind stress/ρ)½ is the friction velocity in water, and κ = 0.4 is von Karman's constant. On the other hand, a similar comparison of the velocity difference between 5.5 and 8.5 m to θ(t)u*κ−1In (8.5/5.5) is much poorer with observed velocity difference being much larger, possibly due to stable stratification effects. Possible errors in the measurements have been considered and estimated as less than the observed velocity differences. Near‐surface shears as large as the observed are very important in closing the momentum budget for the oceanic boundary layer.