  We present a detailed analysis of the temperature statistics in an
oceanographic observational dataset. The data are collected using a moored
array of thermistors, 100 m tall and starting 5 m above the bottom, deployed
during four months above the slopes of a Seamount in the north-eastern Atlantic
Ocean. Turbulence at this location is strongly affected by the semidiurnal
tidal wave. Mean stratification is stable in the entire dataset. We compute
structure functions, of order up to 10, of the distributions of temperature
increments. Strong intermittency is observed, in particular, during the
downslope phase of the tide, and farther from the solid bottom. In the lower
half of the mooring during the upslope phase, the temperature statistics are
consistent with those of a passive scalar. In the upper half of the mooring,
the temperature statistics deviate from those of a passive scalar, and evidence
of turbulent convective activity is found. The downslope phase is generally
thought to be more shear-dominated, but our results suggest on the other hand
that convective activity is present. High-order moments also show that the
turbulence scaling behaviour breaks at a well-defined scale (of the order of
the buoyancy length scale), which is however dependent on the flow state (tidal
phase, height above the bottom). At larger scales, wave motions are dominant.
We suggest that our results could provide an important reference for laboratory
and numerical studies of mixing in geophysical flows.
