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Shear at the Base of the Oceanic Mixed Layer Generated by Wind Shear Alignment

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  • 1 School of Ocean Sciences, Bangor University, Menai Bridge, Wales, and Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, United Kingdom
  • | 2 School of Ocean Sciences, Bangor University, Menai Bridge, Wales, United Kingdom
  • | 3 National Oceanography Centre, Southampton, United Kingdom
  • | 4 Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California
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Abstract

Observations are used to evaluate a simple theoretical model for the generation of near-inertial shear spikes at the base of the open ocean mixed layer when the upper ocean displays a two-layer structure. The model predicts that large changes in shear squared can be produced by the alignment of the wind and shear vectors. A climatology of stratification and shear variance in Drake Passage is presented, which shows that these assumptions are most applicable to summer, fall, and spring but are not highly applicable to winter. Temperature, salinity, and velocity data from a high spatial resolution cruise in Drake Passage show that the model does not predict all large changes in shear variance; the model is most effective at predicting changes in shear squared when it arises owing to near-inertial wind-driven currents without requiring a rotating resonant wind stress. The model is also more effective where there is a uniform mixed layer above a strongly stratified transition layer. Rotary spectral and statistical analysis of an additional 242 Drake Passage transects from 1999 to 2011 confirmed the presence of this shear-spiking mechanism, particularly in summer, spring, and fall when stratification is stronger.

Corresponding author address: Liam Brannigan, AOPP, Clarendon Lab, Dept. of Physics, University of Oxford, Oxford OX1 3PU, United Kingdom. E-mail: brannigan@atm.ox.ac.uk

Abstract

Observations are used to evaluate a simple theoretical model for the generation of near-inertial shear spikes at the base of the open ocean mixed layer when the upper ocean displays a two-layer structure. The model predicts that large changes in shear squared can be produced by the alignment of the wind and shear vectors. A climatology of stratification and shear variance in Drake Passage is presented, which shows that these assumptions are most applicable to summer, fall, and spring but are not highly applicable to winter. Temperature, salinity, and velocity data from a high spatial resolution cruise in Drake Passage show that the model does not predict all large changes in shear variance; the model is most effective at predicting changes in shear squared when it arises owing to near-inertial wind-driven currents without requiring a rotating resonant wind stress. The model is also more effective where there is a uniform mixed layer above a strongly stratified transition layer. Rotary spectral and statistical analysis of an additional 242 Drake Passage transects from 1999 to 2011 confirmed the presence of this shear-spiking mechanism, particularly in summer, spring, and fall when stratification is stronger.

Corresponding author address: Liam Brannigan, AOPP, Clarendon Lab, Dept. of Physics, University of Oxford, Oxford OX1 3PU, United Kingdom. E-mail: brannigan@atm.ox.ac.uk
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