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Article
Article Type:
Research Article

The Role of Intermittency in Internal-Wave Shear Dispersion

Eric Kunze NorthWest Research Associates, Redmond, Washington

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Miles A. Sundermeyer School for Marine Science and Technology, University of Massachusetts Dartmouth, New Bedford, Massachusetts

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Print Publication:
01 Dec 2015
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LatMix: Studies of Submesoscale Stirring and Mixing
DOI:
https://doi.org/10.1175/JPO-D-14-0134.1
Page(s):
2979–2990
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Abstract

This paper revisits a long-standing discrepancy between (i) 1–5-km isopycnal diffusivities of O(1) m2 s−1 based on dye spreading and (ii) inferences of O(0.1) m2 s−1 from internal-wave shear dispersion Kh ~ 〈Kz〉〈〉/f2 in several studies in the stratified ocean interior, where 〈Kz〉 is the bulk average diapycnal diffusivity, 〈〉 the finescale shear variance, and f the Coriolis frequency. It is shown that, taking into account (i) the intermittency of shear-driven turbulence, (ii) its lognormality, and (iii) its correlation with unstable finescale near-inertial shear, internal-wave shear dispersion cannot necessarily be discounted based on available information. This result depends on an infrequent occurrence of turbulence bursts, as is observed, and a correlation between diapycnal diffusivity Kz and the off-diagonal vertical strain, or the vertical gradient of horizontal displacement, |χz| = |∫Vz dt|, which is not well known and may vary from region to region. Taking these factors into account, there may be no need to invoke additional submesoscale mixing mechanisms such as vortical-mode stirring or internal-wave Stokes drift to explain the previously reported discrepancies.

Corresponding author address: Eric Kunze, NorthWest Research Associates, 4126 148th Ave. NE, Redmond, WA 98052. E-mail: kunze@nwra.com

This article is included in the LatMix: Studies of Submesoscale Stirring and Mixing Special Collection.

Abstract

This paper revisits a long-standing discrepancy between (i) 1–5-km isopycnal diffusivities of O(1) m2 s−1 based on dye spreading and (ii) inferences of O(0.1) m2 s−1 from internal-wave shear dispersion Kh ~ 〈Kz〉〈〉/f2 in several studies in the stratified ocean interior, where 〈Kz〉 is the bulk average diapycnal diffusivity, 〈〉 the finescale shear variance, and f the Coriolis frequency. It is shown that, taking into account (i) the intermittency of shear-driven turbulence, (ii) its lognormality, and (iii) its correlation with unstable finescale near-inertial shear, internal-wave shear dispersion cannot necessarily be discounted based on available information. This result depends on an infrequent occurrence of turbulence bursts, as is observed, and a correlation between diapycnal diffusivity Kz and the off-diagonal vertical strain, or the vertical gradient of horizontal displacement, |χz| = |∫Vz dt|, which is not well known and may vary from region to region. Taking these factors into account, there may be no need to invoke additional submesoscale mixing mechanisms such as vortical-mode stirring or internal-wave Stokes drift to explain the previously reported discrepancies.

Corresponding author address: Eric Kunze, NorthWest Research Associates, 4126 148th Ave. NE, Redmond, WA 98052. E-mail: kunze@nwra.com

This article is included in the LatMix: Studies of Submesoscale Stirring and Mixing Special Collection.

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