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Measurements of Turbulent Vertical Kinetic Energy in the Ocean Mixed Layer from Lagrangian Floats

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  • 1 Department of Marine Resources, National Sun Yat-sen University, Kaohsiung, Taiwan
  • | 2 Applied Physics Laboratory and School of Oceanography, University of Washington, Seattle, Washington
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Abstract

D'Asaro, in previous work using nearly neutrally buoyant Lagrangian floats in a wind forced mixed layer, found 〈w2〉 = Au2, where 〈w2〉 is the mean square vertical velocity and u∗ is the friction velocity estimated from shipboard meteorological measurements using bulk formulas. Depth profiles of A(z) = 〈w2〉(z)/u2 within the mixed layer showed a maximum value of A(z) of about 2, which is 1.75–2 times that measured in solid-wall turbulent boundary layers driven by a wall stress alone. This result implied that the ocean mixed layer was more energetic than shear-driven turbulent boundary layers driven by the same stress. Here these results are verified using observations of vertical velocity in the mixed layer from 72 float days of data from two Lagrangian floats in the North Pacific Ocean in the autumn of 2000. These floats were more neutrally buoyant than those used previously by D'Asaro, thus reducing possible biases. Wind stress was estimated from Quick Scatterometer satellite measurements and is thus subject to errors and biases different from those in D'Asaro's previous work. Despite these instrumental differences, the new results are very similar to those of the previous work, except that no corrections for internal wave velocities are needed. The values of 〈w21/2 and u∗ are correlated well, and the maximum value of A(z) is near 2.

Corresponding author address: Ruo-Shan Tseng, P.O. Box 59-161, Kaohsiung 804, Taiwan. Email: rstseng@mail.nsysu.edu.tw

Abstract

D'Asaro, in previous work using nearly neutrally buoyant Lagrangian floats in a wind forced mixed layer, found 〈w2〉 = Au2, where 〈w2〉 is the mean square vertical velocity and u∗ is the friction velocity estimated from shipboard meteorological measurements using bulk formulas. Depth profiles of A(z) = 〈w2〉(z)/u2 within the mixed layer showed a maximum value of A(z) of about 2, which is 1.75–2 times that measured in solid-wall turbulent boundary layers driven by a wall stress alone. This result implied that the ocean mixed layer was more energetic than shear-driven turbulent boundary layers driven by the same stress. Here these results are verified using observations of vertical velocity in the mixed layer from 72 float days of data from two Lagrangian floats in the North Pacific Ocean in the autumn of 2000. These floats were more neutrally buoyant than those used previously by D'Asaro, thus reducing possible biases. Wind stress was estimated from Quick Scatterometer satellite measurements and is thus subject to errors and biases different from those in D'Asaro's previous work. Despite these instrumental differences, the new results are very similar to those of the previous work, except that no corrections for internal wave velocities are needed. The values of 〈w21/2 and u∗ are correlated well, and the maximum value of A(z) is near 2.

Corresponding author address: Ruo-Shan Tseng, P.O. Box 59-161, Kaohsiung 804, Taiwan. Email: rstseng@mail.nsysu.edu.tw

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