Abstract. A 400 m long array with 201 high-resolution NIOZ temperature
sensors was deployed above a north-east equatorial Pacific hilly abyssal
plain for 2.5 months. The sensors sampled at a rate of 1 Hz. The lowest
sensor was at 7 m above the bottom (m a.b.). The aim was to study internal
waves and turbulent overturning away from large-scale ocean topography.
Topography consisted of moderately elevated hills (a few hundred metres),
providing a mean bottom slope of one-third of that found at the Mid-Atlantic
Ridge (on 2 km horizontal scales). In contrast with observations over
large-scale topography like guyots, ridges and continental slopes, the
present data showed a well-defined near-homogeneous “bottom boundary
layer”. However, its thickness varied strongly with time between < 7 and
100 m a.b. with a mean around 65 m a.b. The average thickness exceeded
tidal current bottom-frictional heights so that internal wave breaking
dominated over bottom friction. Near-bottom fronts also varied in time (and
thus space). Occasional coupling was observed between the interior internal
wave breaking and the near-bottom overturning, with varying up- and down-
phase propagation. In contrast with currents that were dominated by the
semidiurnal tide, 200 m shear was dominant at (sub-)inertial frequencies.
The shear was so large that it provided a background of marginal stability
for the straining high-frequency internal wave field in the interior. Daily
averaged turbulence dissipation rate estimates were between 10−10 and
10−9 m2 s−3, increasing with depth, while eddy diffusivities
were of the order of 10−4 m2 s−1. This most intense
“near-bottom” internal-wave-induced turbulence will affect the resuspension
of sediments.
Numerical simulation of the three-dimensional wave-induced currents on unstructured grid