Temperature statistics above a deep-ocean sloping boundary

TL;DR

This study analyzes temperature fluctuations in deep-ocean slopes, revealing turbulence characteristics, intermittency, and the influence of tidal phases, with implications for understanding mixing processes in geophysical flows.

Contribution

It provides detailed temperature statistics and turbulence analysis above a seamount, highlighting the effects of tidal phases and flow regimes on scalar intermittency and turbulence scaling.

Findings

Intermittency varies with tidal phase and height above bottom.

Temperature behaves as a passive scalar in the lower mooring.

Convective activity influences temperature statistics in the upper mooring.

Abstract

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.