4 Turbulence, Density and Flow Distribution along the Channel
4.5 Summary and Discussion
by the large horizontal extent (about 500 m) of the density anomaly that should be significantly reduced in the case of a turbulent overturn. Thorpe scales were also computed for overturns observed in individual CTD profiles (Fig. 4.16 lower panel). Most of them were consistent with the Ozmidov scale. Only one CTD profile showed a Thorpe scale of 47 m, again twice as large as the Ozmidov scale.
The question emerges which process might be responsible for the vertical displace-ment of the near-bottom waters that probably was inconsistent with a turbulent overturn. An explanation for the high-density, high-turbulent patch (Fig. 4.15) could be a horizontally sampled hydraulic jump which is likely to exist in the same area where the patch was observed. It is unclear how turbulence and density would be distributed inside a hydraulic jump on a constant pressure level. The study by Legg and Huijts (2006) shows a density distribution (their Fig. 7) which could, if sampled horizontally, result in a distribution comparable to the one obtained here (Fig. 4.15). Unfortunately no velocity record is available at the time and location of the patch, the hydraulic jump hypothesis cannot be supported by in-situ veloc-ity measurements.
However, the differences encountered during both passings indicate temporal vari-ability of the density and turbulence field. Temporal varivari-ability induced by tides has been observed in the rift valley of the MAR by Keller et al.(1975) and could explain the above described differences. A tidal analysis was conducted which is reported in Section 5.
4.5 Summary and Discussion
With the new AUV-MR system the horizontal distribution of the dissipation rate and the diffusivity along the deep ocean channel over complex topography was derived in the context of this thesis. From the measurements presented here a clear pattern of the along-channel distribution of the dissipation rate emerged, with highest dissipation rates being present in a segment 1 to 4 km downstream of the sill, and lower values both upstream and downstream of it. The same asym-metry was found in small-scale density variability confirming the distribution of the mixing from an independent measurement parameter. The question how the dissipation rate is distributed along the channel, across the sill, which as a result of a sparse data set remained open from the study of St. Laurent and Thurnherr (2007) thus was answered here. Indications for single mixing events upstream of the sill were found also in this study and were consistent with near critical reflec-tion of the barotropic M2 tide at the steep bottom. At least, some of the elevated mixing found by St. Laurent and Thurnherr (2007) upstream of the sill might be explained by the same mechanisms.
Elevated mixing downstream of the sill as shown in this thesis is consistent with a mean overflow across the sill (Alford et al., 2013) which was observed by St. Lau-rent and Thurnherr (2007) and was confirmed here. A sketch of the observed flow conditions in the eastern channel of the Lucky Strike segment was developed to demonstrate the processes (Fig. 4.17).
4 Turbulence, Density and Flow Distribution along the Channel
The mean overflow across the sill fuels the mixing downstream. A hydraulic jump was observed in the study by St. Laurent and Thurnherr (2007) as well as in this thesis. Froude Numbers were in the range indicating a small hydraulic jump i.e.
an ‘undular’ or ‘weak’ hydraulic jump. If the jump would be an undular hydraulic jump it would be transient i.e. disintegrate due to upstream propagation of intern waves (Section 1.4). Furthermore, enhanced variability would be expected down-stream of the jump due to the undular waves of the jump (Section 1.4). At this point it is unclear whether the hydraulic jump is a permanent feature. This is further discussed in Section 5.
Temporal variability of a hydraulic jump might also be induced by variability of the along-channel velocity magnitude. The analysis of temperature gradient spec-tra in conjunction with the density and dissipation measurements in Section 4.4.3 indicated temporal variability which might be induced by tides. A strong influence of semi-diurnal tides was reported byKeller et al.(1975) for a sill south of the LS segment. Tidal analysis of the data set underlying this thesis is conducted in the following Section.
The density distribution downstream of the hydraulic jump is dependent on the jump amplitude and can not be further determined here. It is unclear whether isopycnals oscillate due to waves of an undular jump, whether isopycnals bend downward due to the downward sloping bottom (as indicated in the sketch in Fig. 4.17) or whether they should be at a constant depth level downstream of the hydraulic jump. As the distance between station D3 and D4 was quite large (>1 km) and no further downstream station was obtained, possible oscillations of isopycnals cannot be verified.
Figure 4.17: Sketch of the along-channel (left) and across-channel (middle and right) flow and density distribution. The location of the cross channel sections are indicated in the along-channel section by the mooring positions. The flow is indicated by background colors, while contour-lines indicate isopycnals. The white spirals indicate mixing. Also indicated are the mooring positions in the along and across-channel section as well as their instrumentation. They were all equipped like indicated at the upstream mooring (one ADCP and two MicroCats). Note that the jump amplitude is overdone and that the sizes of all features are not to scale.
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4.5 Summary and Discussion The small scale analysis of the temperature variability showed the turbulent sub-range to extend to length scales of a few hundred meters which was also observed by Klymak and Moum (2007b). Their study was conducted in the upper ocean near the Hawaiian Ridge with an average stratification of 4.5·10−31/s, while the results presented here apply to a narrow channel with weaker stratification of about 0.5·10−3 to 1·10−31/s. The consistency of the temperature gradient wavenumber spectra in the inertial-convective subrange of turbulence with the 2/3 dependence could be of interest for future studies. If the inertial-convective subrange of tur-bulence is fully resolved, the diffusivity for temperature can be assumed to equal the diffusivity for density and changes in the stratification can be neglected. This relationship could be used to infer the dissipation rate in low turbulence envi-ronments where the AUV induced noise inhibits the use of velocity shear based measurements. Despite this option, the mounting of the MR could be improved such that the AUV induced noise would be reduced.
In this Section it was demonstrated that AUV based velocity shear observations are suitable for the deep ocean above rough topography in constricted channels.
This widens the measurement range for horizontal profiling to areas where free-falling and lowered measurements are inefficient and partly lack the possibility to sample at exact, pre-defined positions which might be crucial for understanding flow-topography interaction. The importance of the bottom topography for mixing has recently been shown byAlford et al.(2013) in the Pacific Ocean. Resolving and understanding the spatially variable deep-ocean mixing is essential as it provides buoyancy to deep-ocean waters causing upwelling as part of the meridional over-turning circulation. With AUV-based microstructure mapping the understanding of such processes could be advanced in future studies. The observations presented here can serve as test case for numerical modeling efforts of topography-induced, deep-ocean mixing in the presence of a time mean flow.