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Interacting wind- and tide-forced boundary-layers in a large strait
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  • Arnaud François Valcarcel,
  • Craig L. Stevens,
  • Joanne O'Callaghan,
  • Sutara Havelin Suanda
Arnaud François Valcarcel
University of Otago / National Institute for Water & Atmospheric Research, University of Otago / National Institute for Water & Atmospheric Research

Corresponding Author:arnaud.valcarcel@niwa.co.nz

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Craig L. Stevens
National Institute for Water & Atmospheric Research, Wellington, New Zealand, National Institute for Water & Atmospheric Research, Wellington, New Zealand
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Joanne O'Callaghan
National Institute of Water and Atmospheric Research, National Institute of Water and Atmospheric Research
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Sutara Havelin Suanda
University of North Caorlina Wilmington, University of North Caorlina Wilmington
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Observations of the spatio-temporal structure of turbulent mixing in a large, energetic strait were used to examine the interactions between wind- and tidally-forced boundary layers in a coastal environment. Te Moana-o-Raukawa (Cook Strait) of Aotearoa (New Zealand) is a relatively wide, energetic strait, known to experience substantial tidal currents and wind stress. A turbulence-enabled ocean glider mission measured O(40,000) turbulence samples that passed QAQC including the use of a vehicle-mounted speed through water sensor. The observations were compared to one-dimensional models of turbulence to understand the mechanisms that regulates the vertical structure of mixing. Tidal flows of O(1 m/s) and wind speeds of O(10 m/s) enhance dissipation to ε=O(10^{-5} W/kg) through boundary drag, shear-driven production of turbulent kinetic energy (P) and to a minor extent buoyancy flux (G). The benthic and wind-driven boundary layers behaved reasonably predictably when considering a 1D perspective. The interaction between the two boundary layers depended on mid-water column stratification which is to a large degree an externally-prescribed condition. Transient stratification can stabilize the mean flow (median Ri_g=0.6(>1/4)) and reduce both turbulence intensity (Re_b) and diapycnal diffusivity (K_z) by up to two orders of magnitude in the middle of the water column, insulating bottom and surface mixing-layers. Mid-water dissipation rate levels tend to be associated with marginal dynamical stability (median Ri_g=0.22(~1/4)) and canonical mixing efficiency (median R_f=0.17), while elevated levels are connected to unstable mean flow conditions (median Ri_g=0.14(<1/4)) and reduced mixing efficiency (median R_f=0.1(<0.17)) that promotes turbulence growth.