Microstructure measurements and estimates of entrainment in the Denmark Strait overflow plume

Abstract

To examine processes controlling the entrainment of ambient water into the Denmark Strait overflow (DSO) plume/gravity current, measurements of turbulent dissipation rate were carried out by a quasi-free-falling (tethered) microstructure profiler (MSP). The MSP was specifically designed to collect data on dissipation-scale turbulence and fine thermohaline stratification in an ocean layer to depths of 3500 m. The task was to perform microstructure measurements in the DSO plume in the lower 300 m depth interval including the bottom mixed layer and the interfacial layer below the non-turbulent ambient water. The MSP was attached to a Rosette water sampler rack equipped with a SeaBird CTDO and an RD Instruments Lowered Acoustic Doppler Current Profiler (LADCP). At a chosen depth, the MSP was remotely released from the rack to perform measurements in a quasi-free-falling mode. <br><br> Using the measured vertical profiles of dissipation, the entrainment rate as well as the bottom and interfacial stresses were estimated in the DSO plume at a location 200 km downstream of the sill at depths up to 1771 m. Dissipation-derived estimates of entrainment were found to be much smaller than bulk estimates of entrainment calculated from the downstream change of the mean properties in the plume, suggesting the lateral stirring due to meso-scale eddies rather than diapycnal mixing as the main contributor to entrainment. Dissipation-derived bottom stress estimates are argued to be roughly one-third the magnitude of those from log velocity profiles. In the interfacial layer, the Ozmidov scale calculated from turbulence dissipation rate and buoyancy frequency was found to be linearly proportional to the overturning scale extracted from conventional CTD data (the Thorpe scale), with a proportionality constant of 0.76, and a correlation coefficient of 0.77

Turbulent Scales Observed in a River Plume Entering a Fjord

“…Within the plume layer itself are also strong density gradients ( N 2 ≈10 −2 s −2 ) (Figure b), which are comparable to the typical maximum stratification across the pycnocline in many coastal plumes such as the Connecticut, Merrimack, and Hudson Rivers (Jurisa et al, ; MacDonald et al, ; O'Donnell et al, ), further limiting the size of L T . The results presented here do not provide an alternate coefficient to the generally accepted value of 0.8, which correlates L T and L O (Dillon, ), nor do they attempt to disprove the linear relationship between the fundamental length scales, which has been supported in many oceanic environments (Ferron et al, ; Moum, ; Paka et al, ). However, the results do highlight that care must be taken when assuming a ratio between L T and L O in an environment where physical boundaries impact their vertical extent.…”

“…Having resolved L T as an indicator of vertical mixing, it is possible to then derive an estimate of turbulence dissipation rate from the overturning length scale (Moum, 1996): Dillon (1982). This ratio has been supported by subsequent observations (Ferron et al, 1998;Moum, 1996;Paka et al, 2013;Stansfield et al, 2001) and is commonly used to infer in the ocean due to the relative ease with which overturns are measured using standard oceanographic instrumentation (Finnigan et al, 2002;Fer, 2004;North et al, 2018). However, a number of other studies have proposed coefficients ranging from 0.6 to 1.2 (Mater et al, 2015;Smyth, 2000) and Ferron et al (1998) found that variability in the ratio can be 60%.…”

Entrainment and Energy Transfer Variability Along the Descending Path of the Denmark Strait Overflow Plume

“…The characteristics of the high‐transport flow (high shear and dissipation, and low Ri ( z )) suggest that shear instabilities are forming in the IL, increasing surface areas and decreasing diffusion lengths, leading to enhanced mixing and thus driving entrainment of overlying water. As described by Paka et al (), the presence of these same indicators in the BL does not indicate similar processes. Instead, the well‐mixed BL likely forms through convection, as the fast upper section of the BL overtakes the slower bottom section (see profiles in Figure ).…”

“…The depth‐dependent dissipation rate of kinetic energy ( ) was calculated from as: where c L is a constant that was set to 0.76, an appropriate value for the Denmark Strait overflow plume determined by Paka et al () using microstructure and CTD profiles. Where no were detected, was set to a typical background value of W kg −1 determined by Paka et al (). To clearly identify depths with high dissipation rates, plots of only show values above W kg −1 .…”

“…The latter is the dominant process further downstream (e.g., Girton & Sanford, ; Koszalka et al, ; Voet & Quadfasel, ), but within the first 200 km the contribution of the two processes is unclear. Studies along this initial section are limited and the only direct turbulence measurements were taken with a microstructure profiler at a single location 200 km downstream of the sill (Paka et al, ). The resulting entrainment rates were small compared to previous bulk estimates, suggesting that lateral stirring from eddies controlled entrainment, not vertical mixing.…”