Internal Wave Turbulence Near a Texel Beach

Abstract

A summer bather entering a calm sea from the beach may sense alternating warm and cold water. This can be felt when moving forward into the sea (‘vertically homogeneous’ and ‘horizontally different’), but also when standing still between one’s feet and body (‘vertically different’). On a calm summer-day, an array of high-precision sensors has measured fast temperature-changes up to 1°C near a Texel-island (NL) beach. The measurements show that sensed variations are in fact internal waves, fronts and turbulence, supported in part by vertical stable stratification in density (temperature). Such motions are common in the deep ocean, but generally not in shallow seas where turbulent mixing is expected strong enough to homogenize. The internal beach-waves have amplitudes ten-times larger than those of the small surface wind waves. Quantifying their turbulent mixing gives diffusivity estimates of 10−4–10−3 m2 s−1, which are larger than found in open-ocean but smaller than wave breaking above deep sloping topography.

The Interaction of Large Amplitude Internal Seiches with a Shallow Sloping Lakebed: Observations of Benthic Turbulence in Lake Simcoe, Ontario, Canada

“…In this study temperature inversions were defined to occur when the temperature difference between two vertically separated thermistors at heights z 1 and z 2 exceeds T ( z 1 ) – T ( z 2 ) ≤ −4×10 −3 °C where the height of the thermistors are such that z 1 > z 2 . This threshold is twice as high as the accuracy of the temperature loggers, and also represents the average of the range of values used in other studies to identify density inversions, e.g., −5×10 −4 °C in [35] or −0.01°C in [15]. In addition, it is similar to the threshold of −3×10 −3 °C which was used in [4] to estimating the occurrence of 1m overturns and to which we compare our results to.…”

“…An estimate of the rate of dissipation of turbulent kinetic energy ( ε ) can be made from the Thorpe length scale L T usingwhere we have used the relationship that the Thorpe length scale L T is related to the energy containing Ozimodov scale, as [35], hence the factor of 0.64. The Thorpe length scale is obtained by reordering the raw temperature profiles into a stable monotonic profile without inversions and the length scale of displacements d is calculated as the vertical distance that an element must move to create a stable monotonic temperature profile [33], [36].…”

Comparison of Ellison and Thorpe scales from Eulerian ocean temperature observations

“…Data sets obtained using NIOZ high sampling rate thermistors [e.g., van Haren and Gostiaux , ] provide with a unique opportunity to study the temporal variability of turbulence in the ocean. These thermistors, deployed in a mooring, can measure temperature continuously and independently for up to 2 years, with a temporal resolution of 1 Hz and a vertical resolution set by the thermistor separation along the mooring line, usually about 1 m. A growing set of measurements has been collected in various locations, ranging from tidally dominated shores [ van Haren et al ., ] to seamounts [ van Haren and Gostiaux , ], to the open and deep ocean [ van Haren and Gostiaux , ]. In this paper, we will consider two data sets, which combine high temporal resolution with a reliable estimation of the vertical density profile.…”

Movements of the thermocline lead to high variability in benthic mixing in the nearshore of a large lake

“…Shear‐induced convection can cause a strong asymmetry in the level of bottom boundary layer turbulence and stratification between up‐ and down‐slope flow phases [ Lorke et al ., ; Cossu and Wells , ]. Shear‐induced convection has been studied in a number of oceanographic studies [see Simpson et al ., ; Rippeth et al ., ; Moum et al ., ; van Haren et al ., ], and has also been described in theoretical studies [ Becherer and Umlauf , ; Umlauf and Burchard , ].…”