arXiv subjects

Hans van Haren

Publications and source records attributed to Hans van Haren.

41 records · Page 3Linked to original sources

arXiv2019

Autumnal deep scattering layer from moored acoustic sensing in the subtropical Canary Basin

An enhanced acoustic scatterer reflectance layer was observed in the bathypelagic zone around 1650 m in the subtropical NE-Atlantic Ocean for about two months during autumn. It resembles a classic pattern of diapause-resting, possibly of large zooplankton, shrimp and/or Cyclothone, at great depths well below any sunlight penetration, which is more commonly found at higher latitudes. The observed slow sink and rise of about 2-5 m per day into and out of this deep layer is considerably slower than the more than 1000 m per day of diel vertical migration (DVM). During the two-month period of deep scattering, DVM is observed to be greatly reduced.

physics.ao-ph↗

arXiv2019

Open-ocean interior moored sensor turbulence estimates, below a Meddy

A one-year time series of moored high-resolution temperature T-sensor data from 1455 m depth on a 3900 m long line in about 5300 m of water in the NE-Atlantic Canary Basin are dominated by salinity (over-)compensated intrusions arising from the effects of Mediterranean outflow waters, which are commonly organized as Meddies. During the passage of a Meddy-core above the T-sensors, no intrusions were observed, thereby making it possible to use the temperature records to quantify turbulence parameters. The present data show that these ocean-interior turbulence estimates are from short-lived (less than 0.5 h) rather intense overturning cells with vertical scales of <5 m. Because the turbulence inertial subrange is found to extend into the internal wave band, the overturns are predominantly driven by shear associated with inertial currents. Kinetic energy, current shear and temperature variance peak at sub-inertial frequencies during the Meddy passage, suggesting wave trapping in the warm anti-cyclonic eddy and/or weakly stratified layers. The observations further show that internal wave displacements are coherent over vertical scales of up to 40 m during the presence of the Meddy compared with vertical coherence scales of less than 25 m during the more common no-Meddy conditions of double diffusion intrusions.

physics.ao-ph↗

arXiv2016

Estimates of the temperature flux-temperature gradient relation above a sea-floor

The relation between the flux of temperature (or buoyancy), the vertical temperature gradient and the height above the bottom, is investigated in an oceanographic context, using high-resolution temperature measurements. The model for the evolution of a stratified layer by Balmforth et al. (1998) is reviewed and adapted to the case of a turbulent flow above a wall. Model predictions are compared to the average observational estimates of the flux, exploiting a flux estimation method proposed by Winters & D'Asaro (1996). This estimation method enables the disentanglement of the dependence of the average flux on the height above the bottom and on the background temperature gradient. The classical N-shaped flux-gradient relation is found in the observations. Model and observations show similar qualitative behaviour, despite the strong simplifications used in the model. The results shed light on the modulation of the temperature flux by the presence of the boundary, and support the idea of a turbulent flux following a mixing-length argument in a stratified flow. Furthermore, the results support the use of Thorpe scales close to a boundary, if sufficient averaging is performed, suggesting that the Thorpe scales are affected by the boundary in a similar way as the mixing length.

physics.flu-dyn↗

arXiv2015

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.

physics.flu-dyn↗

arXiv2014

Comparison of Ellison and Thorpe scales from Eulerian ocean temperature observations

Ocean turbulence dissipation rate is estimated either by means of microstructure shear measurements, or by adiabatically reordering vertical profiles of density. The latter technique leads to the estimate of the Thorpe scale, which in turn can be used to obtain average turbulence dissipation rate by comparing the Thorpe scale to the Ozmidov scale. In both cases, the turbulence dissipation rate can be estimated using single vertical profiles from shipborne instrumentation. We present here an alternative method to estimate the length scale of overturns by using the Ellison length scale. The Ellison scale is estimated from temperature variance just beyond the internal wave band, measured by moored instruments. We apply the method to high resolution temperature data from two moorings deployed at different locations around the Josephine seamount (North Eastern Atlantic Ocean), in a region of bottom-intensified turbulence. The variance of the temperature time series just above the internal wave frequency band is well correlated with the Thorpe scale. The method is based on the time--frequency decomposition of variance called "maximum overlap discrete wavelet transform". The results show that the Ellison length scale can be a viable alternative to the Thorpe scale for indirectly estimating turbulence dissipation rate from moored instruments in the ocean if time resolution is sufficiently high. We suggest that fine structure contaminated temperature measurements can provide reliable information on turbulence intensity.

physics.flu-dyn↗