Mixing, Hydrography, and Flow in the eastern Channel of the Lucky Strike Segment

sandra.tippenhauer [ at ] awi.de


Diapycnal mixing in the deep ocean is known to be much stronger in the vicinity of rough topography of mid-ocean ridges than over abyssal plains. In this thesis a microstructure probe attached to an autonomous underwater vehicle (AUV) was used to infer the spatial distribution of the dissipation rate of turbulent kinetic energy in the central valley of the Mid-Atlantic Ridge. This represents the first successful realization of a horizontal, AUV-based, deep-ocean microstructure survey. The study focused on a channel with unidirectional, partially supercritical sill overflow. The density was found to decrease along the channel following the mean flow of 3 to 8 cm/s. The magnitude of the dissipation rate was distributed asymmetrically relative to the position of the sill. Elevated dissipation rates were present in a segment 1 to 4 km downstream of the sill, reaching 1 · 10^−7 W/kg. Flow speeds of more than 20 cm/s and elevated density finestructure were observed within this segment. The average along-channel flow was found to be strongly modulated by the semidiurnal M2 tidal flow. Supercritical flow down the lee slope of the sill was observed during strong flow velocity conditions, and a hydraulic jump is expected to occur downstream of the sill during these phases. Consistently, upward displacement of isopycnals was observed in the area where the velocity distribution suggested the presence of a hydraulic jump. Indications for upstream propagating hydraulic jumps were found during phases of decreasing flow velocities. Upstream propagating hydraulic jumps offer a possibility of inducing turbulent mixing closer to the sill or even upstream of it. The distributions of the flow, density and mixing rate provide a consistent picture of the fundamental physical mechanisms controlling the mixing in this deep ocean channel, i.e. tidally modulated, jet unidirectional sill overflow with a hydraulic jump inducing turbulent mixing downstream. These results indicate deep-ocean mixing to depend heavily on the local bottom topography and flow conditions. Although one particular channel was studied, the fundamental physical mechanisms identified in this thesis are expected to be applicable to other, similar channels. Furthermore, the results nicely illustrate that horizontally-profiling AUV-based observations may be an efficient tool to study deep-ocean turbulence over complex terrain where free-falling and lowered turbulence measurements are inefficient and time-consuming.

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Tippenhauer, S. (2015): Mixing, Hydrography, and Flow in the eastern Channel of the Lucky Strike Segment PhD thesis,

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