H. van Haren

NIOZ3: Independent Temperature Sensors Sampling Yearlong Data at a Rate of 1 Hz

Some 110 independent sensors form the NIOZ3-thermistor ldquostringrdquo to study waves in the ocean interior sampling at a rate of 1 Hz during at least one year. The string operates without connecting cables between the newly designed sensors, which are programmed and synchronized via induction. The accuracy of previous custom-made high-sampling rate thermistor strings is maintained, being better than 1 mK. This is demonstrated here using data from three recent field trials, two above seamounts and one in the ocean interior that occasionally show vigorous (nonlinear) internal wave motions.

Temperature statistics above a deep-ocean sloping boundary

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.

Convective mixing by internal waves in the Puerto Rico Trench

A2.4 km long deep-sea mooringwas deployed for 14 months in the Puerto Rico Trench, the deepestpart of the Atlantic Ocean. Below its top buoyancy package, the mooring line held a 200 m long stringof high-resolution temperature sensors and a current meter. Over the instrumented range between6,004 and 6,204 m, waters are very weakly stratified, with local buoyancy frequency equaling about1.9 times the semidiurnal tidal frequency. Besides quiescent waters, the detailed observations showregular vertical turbulent overturning associated with periodic warming and driven by internal tidaland inertial waves. During such episodes, the 4-day and 200 m vertically averaged dissipation rateincreases from approximately 10−11 up to 10−9 m2 s−3, and eddy diffusivity increases from 5 ×10−5 up to 4 × 10−3 m2 s−1. No large-scale shear-driven Kelvin-Helmholtz billows are observed,and free convection seems the dominant primary turbulent overturning mechanism affecting materialredistribution and life in the weakly stratified environment.