The oceanʹs internal motion: A short overview of NIOZ thermistor string observations

Detailed observations of the variability in space and time of motions that dominate redistribution of material and heat are rare. However, these motions are vital for life in seas and ocean. Modern electronics have allowed the manufacturing of 1-Hz high-sampling rate, < 1-mK precision, 6000-m depth-rated temperature sensors with the potential of 1-year uninterrupted stand-alone operation. These sensors have been specifically developed for use in a vertical array of many O(100), to study dynamic processes like fronts and internal waves in shallow seas and deep ocean. Under conditions of sufficient spatial vertical resolution, O(0.1–1 m), and temperature acting as a proper tracer for density variations, the sensors are excellent in estimating turbulence parameters generated by such processes. In the ocean interior, they reveal continuous internal wave variability and step-like vertical layering in temperature, but very little turbulence. Above sloping topography, small- and large-scale overturnings yield turbulence parameter values which vary by up to four orders of magnitude as a function of time. Largest values are observed in bursts lasting typically 500–1000 s and associated with nonlinear internal wave passages. These bursts occur irregularly in a tidal phase. When extrapolated to the ocean at large, sufficient mixing is observed above sloping boundaries to maintain the overall vertical density stratification.