1.1 Sketch of the global meridional overturning circulation (MOC)
(Rahm-storf, 2002;Kuhlbrodt et al., 2007). . . . 1
1.2 Schematic diagram byMarshall and Speer (2012) of the upper and lower cell of the global MOC. . . 4
1.3 Flow, density, and dissipation distribution in a ridge-flank channel on the western flank of the MAR in the south Atlantic near 22◦S fromThurnherr et al. (2005). . . 5
1.4 Topography of the North Atlantic (Etopo 1 (NOAA)). The Lucky Strike segment is marked by a white dot. . . 9
1.5 Map by Wilson et al. (1995) of the French-American Zero-Angle Photon Spectrometer and Rocks (FAZAR) cruise study region. . . . 10
1.6 θ-S diagram from the eastern channel of the Lucky Strike segment. 11 1.7 Topography of the MAR from 33◦N to 40◦N. . . 12
1.8 Topography of the rift valley 400 km south of the Azores. . . 12
1.9 Section of potential densityσθ for the FAZAR study region by Wil-son et al. (1995). . . 13
1.10 θ-S diagram from World Ocean Database east and west of the MAR together with all profiles obtained in the course of this study. . . 14
1.11 Topography of the Lucky Strike segment. . . 15
1.12 Potential density σ2 obtained in the eastern channel of the LS seg-ment and the density difference from southern to northern basin. . . 17
1.13 Sketch of a hydraulic jump in uniform flow. . . 18
1.14 Sketch of hydraulic jumps of different amplitude by Chow (1959). . 19
1.15 Sketch of a hydraulic jump in a stratified shear flow. . . 20
1.16 The dependence of the flow and density profile from the parameter η. 21 1.17 The Froude Number F r∗ and F rc as funktion of the parameter η. . 23
2.1 Map of study site and position of measurements. . . 26
2.2 Picture of the AUV with MR and mounting. . . 30
2.3 Picture of the AUV with MR and mounting during recovery. . . 30
2.4 Spectra of acceleration signals. . . 31
2.5 Velocity differences of mooring derived and AUV derived flow ve-locities. . . 35
3.1 Example velocity shear spectra for elevated and low turbulence en-vironments. . . 39
List of Figures
3.2 Dissipation rate segment determined from the hybrid and from the
filtered spectrum. . . 41
3.3 Histogram of the dissipation rates. . . 42
4.1 Meridional distribution of the dissipation rate as function of pressure. 46 4.2 Meridional distribution of the dissipation rate as function of altitude. 47 4.3 Map of the dissipation rate. . . 48
4.4 Along-channel distribution of the box-averaged dissipation rate to-gether with vertical profiles of the dissipation rates obtained by St. Laurent and Thurnherr (2007). . . 49
4.5 Map of depth-averaged LADCP-derived velocities. . . 51
4.6 Section of station averaged along-channel LADCP derived velocity. 54 4.7 Section of station averaged across-channel LADCP derived velocity. 55 4.8 Richardson numbers along the channel. . . 56
4.9 Average cross-channel distribution of along-channel velocity. . . 58
4.10 Along-channel distribution of turbulent diffusivity. . . 60
4.11 Sketch for advection diffusion balance. . . 62
4.12 Meridional distribution of the density anomalies. . . 65
4.13 Temperature gradient wavenumber spectra. . . 68
4.14 Temperature gradient variance integrated over the inertial convec-tive subrange. . . 69
4.15 Dissipation rate and potential density along an AUV track segment. 71 4.16 Potential density derived from the AUV CTD in comparison with nearby lowered CTD observations. . . 72
4.17 Sketch of the along-channel and across-channel flow and density distribution. . . 74
5.1 Along-channel velocity from upstream and downstream moored AD-CPs. . . 78
5.2 Tidal fits to the along-channel velocity. . . 80
5.3 Mooring-based potential temperature time-series. . . 84
5.4 Lag correlation of the θ2 time-series with semi-diurnal tides. . . 85
5.5 Spectrum of isothermal displacement. . . 88
5.6 Lag correlation of the std of high-pass filtered potential temperature with low-pass filtered observed along-channel velocity. . . 90
5.7 Standard deviation of high-pass filtered θ2 time-series. . . 91
5.8 Along-channel velocity at the yoyo-station. . . 94
5.9 Mooring-based along-channel velocity during yoyo-Station. . . 96
5.10 Richardson Numbers for the yoyo-cast. . . 97
5.11 Density distribution obtained from tow-yo Sections. . . 98
5.12 Density distribution obtained from tow-yo Sections. . . 99
5.13 Mooring-derived along-channel velocity during tow-yo Sections. . . 100
5.14 Mooring-based along-channel velocity, semi-diurnal tide and time of lowered observations. . . 102
X
List of Figures 5.15 Depth averaged LADCP derived velocities averaged between 1800 m
depth and the sill depth. . . 104
5.16 Depth averaged LADCP derived velocities from the sill depth (2060 m) to the bottom. . . 105
5.17 Along-channel LADCP derived velocity for flood-tide. . . 106
5.18 Along-channel LADCP derived velocity for ebb-tide. . . 107
5.19 Across-channel section of along-channel LADCP derived velocity at the first across-channel section for flood- and ebb-tide. . . 109
5.20 Across-channel section of along-channel LADCP derived velocity at the second across-channel section for flood- and ebb-tide. . . 110
5.21 Dissipation rate and along channel flow 10 min averages. . . 113
5.22 Dissipation rate and along channel flow 10 min averages. . . 115
5.23 Dissipation rate and along channel flow 10 min averages. . . 116
5.24 Dissipation rates 10 min averages respect to pressure. . . 117
5.25 Sketch of the along-channel and across-channel flow and density distribution during flood-tide. . . 119
5.26 Sketch of the along-channel flow and density distribution during flood-tide for an undular and a weak hydraulic jump. . . 120
5.27 Sketch of the along-channel and across-channel flow and density distribution during weak along-channel flow conditions. . . 121
6.1 Along-channel velocity observed at the upstream and downstream mooring during the time of the yoyo-Station. . . 126
6.2 Along-channel flow anomaly at the yoyo-Station at D1. . . 127
6.3 Low-pass filtered mooring-based vertical velocity. . . 132
6.4 Lag correlation of std of high-pass filtered vertical velocity with low-pass filtered along-channel velocity, both mooring-based. . . 133
6.5 Time-series of std of high-pass filtered vertical velocity with low-pass filtered along-channel velocity, both mooring-based. . . 134
6.6 Along-channel and vertical velocity during AUV Dive 8. . . 137
A.1 Topography of the eastern channel of the Lucky Strike segment and station map. . . 147
A.2 Dissipation rate relative uncertainty. . . 148
A.3 Potential density and LADCP-derived velocities from the CTD/LADCP yo-yo cast byThurnherr (2011). . . 148
A.4 Buoyancy frequency and buoyancy frequency variability along the channel. . . 149
A.5 Histogram of the buoyancy frequency variability. . . 149
A.6 Dissipation rate and along channel flow maps of 10 min averages. . 150
A.7 Dissipation rate and along channel flow maps of 10 min averages. . 151
A.8 Dissipation rate and along channel flow maps of 10 min averages. . 151