Abstract
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.