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A. M. Thurnherr
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A. M. Thurnherr

Abstract

Lowered acoustic Doppler current profilers (LADCPs) are commonly used to measure full-depth velocity profiles in the ocean. Because LADCPs are lowered on hydrographic wires, elaborate data processing is required to remove the effects of instrument motion from the velocity measurements and to transform the resulting relative velocity profiles into a nonmoving reference frame. Two fundamentally different methods are used for this purpose: in the velocity inversion method, a set of linear equations is solved to separate the ocean and instrument velocities while simultaneously applying a combination of velocity-referencing constraints from navigational data, shipboard ADCP measurements, and bottom tracking. In the shear method, a gridded profile of velocity shear, which is not affected by instrument motion, is vertically integrated and referenced using a single constraint. The main goals of the present study consist in estimating the accuracy of LADCP-derived velocity profiles and determining which processing method performs better. To this purpose, 21 LADCP profiles collected during four surveys are compared to velocities measured simultaneously by nearby moored instruments at depths between 2000 and 3000 m. The LADCP data were processed with two slightly different publicly available implementations of the velocity inversion method, as well as with an implementation of the shear method that was extended to support multiple simultaneous velocity-referencing constraints. Regardless of the processing method, the overall rms LADCP velocity errors are <3 cm s−1 as long as multiple velocity-referencing constraints are imposed simultaneously. On the other hand, solutions referenced with a single constraint are associated with significantly greater errors. The two primary instrument characteristics that influence data quality are range and sampling rate. Dependence of the LADCP velocity errors on those two parameters was determined by reprocessing range-limited subsets and temporal subsamples of the LADCP data. Results indicate an approximately linear increase of the velocity errors with decreasing sampling rate. The relationship between velocity errors and instrument range is much less linear and characterized by a steep increase in velocity errors below a limiting range of ≈60 m. To improve the quality of the velocity data by increasing the instrument range, modern LADCP systems often include both upward- and downward-looking ADCPs. The data analyzed here indicate that the addition of a second ADCP can be as effective as doubling the range of a single-head LADCP system. However, in one of the datasets the errors associated with the profiles calculated from combined up- and down-looker data are significantly larger than the corresponding errors associated with the profiles calculated from the down-looker alone. The analyses carried out here indicate that the velocity errors associated with LADCP profiles can be significantly smaller than expected from previously published results and from the uncertainty estimates calculated by the velocity inversion method.

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A. M. Thurnherr

Abstract

In a paper published in 2002 in this journal, K. Polzin et al. derive corrections for spectra of vertical shear calculated from lowered acoustic Doppler current profiler (LADCP) velocity data. To illustrate and validate the corrections, they use velocities derived with a specific implementation of the shear method for LADCP processing that is no longer supported or widely used. In several recent publications, spectral corrections specific to this old processing method have been applied without modification to LADCP data processed with the more modern and much more widely used velocity-inversion method, which is associated with significantly less damping at high vertical wavenumbers than the older method. The purpose of this work is to derive and validate spectral corrections appropriate for different LADCP processing methods.

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A. M. Thurnherr and K. G. Speer

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It is commonly observed in hydrographic sections crossing midocean ridges that the isopycnals on the ridge flanks slope downward toward the crests. Although the observed vertical scales of isopycnal dipping are not consistent with steady diffusive boundary layers on slopes, the cross-flank density gradients can nevertheless be caused by diapycnal mixing acting on timescales of several years. The corresponding pressure gradients are usually inferred to be associated with cyclonic along-flank flows. Recent observations of southward flow along the highly corrugated western flank of the Mid-Atlantic Ridge near 20°S are inconsistent with this conceptual picture, however. Data from seven zonal cross-ridge sections and from four meridional along-ridge sections were used to analyze the hydrography on the flanks of the Mid-Atlantic Ridge between 2°N and 30°S. The majority of the hydrographic stations were occupied over deep cross-flank canyons, which extend over a total length of ∼90 000 km in the tropical and subtropical South Atlantic alone. The dipping of the isopycnal surfaces on the western ridge flank in the Brazil Basin is largely restricted to the canyons, where flow along the flank is topographically blocked. The magnitudes of the blocked apparent along-ridge transports are typically of order 1 Sv (1 Sv ≡ 106 m3 s−1) with values as high as 3 Sv, implying important consequences for circulation studies with both forward and inverse models. In the southern Brazil Basin the horizontal density gradients immediately above the blocking topography are reversed—that is, the densities increase toward the crest of the Mid-Atlantic Ridge, consistent with observations of southward flow along the flank. On the eastern flank in the Angola Basin there is a layer of crestward-increasing densities as well, but there it lies well above the ridge topography. Numerical solutions of the buoyancy equation indicate that the observed cross-flank density gradients of both signs are consistent with bottom-intensified diapycnal mixing, which causes a vertical buoyancy-flux dipole. Boundary mixing on slopes can therefore give rise to anticyclonic, as well as cyclonic, along-slope flows. The observed horizontal temperature and salinity gradients near the Mid-Atlantic Ridge in the South Atlantic cannot be accounted for by diapycnal mixing alone, on the other hand. The distributions of these properties are therefore largely determined by isopycnal processes.

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A. M. Thurnherr, I. Goszczko, and F. Bahr

Abstract

Data collected with acoustic Doppler current profilers installed on CTD rosettes and lowered through the water column [lowered ADCP (LADCP) systems] are routinely used to derive full-depth profiles of ocean velocity. In addition to the uncertainties arising from random noise in the along-beam velocity measurements, LADCP-derived velocities are commonly contaminated by bias errors due to imperfectly measured instrument attitude (heading, pitch, and roll). Of particular concern are the heading measurements, because it is not usually feasible to calibrate the internal ADCP compasses with the instruments installed on a CTD rosette, away from the magnetic disturbances of the ship. Heading data from dual-headed LADCP systems, which consist of upward- and downward-pointing ADCPs installed on the same rosette, commonly indicate heading-dependent compass errors with amplitudes exceeding 10°. In an attempt to reduce LADCP velocity errors, several dozen profiles of simultaneous LADCP and magnetometer/accelerometer data were collected in the Gulf of Mexico. Agreement between the LADCP profiles and simultaneous shipboard velocity measurements improves significantly when the former are processed with external attitude measurements. Another set of LADCP profiles with external attitude data was collected in a region of the Arctic Ocean where the horizontal geomagnetic field is too weak for the ADCP compasses to work reliably. Good agreement between shipboard velocity measurements and Arctic LADCP profiles collected at magnetic dip angles exceeding and processed with external attitude measurements indicate that high-quality velocity profiles can be obtained close to the magnetic poles.

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A. M. Thurnherr, L. C. St. Laurent, K. G. Speer, J. M. Toole, and J. R. Ledwell

Abstract

To close the global overturning circulation, the production and sinking of dense water at high latitudes must be balanced elsewhere by buoyancy gain and upward vertical motion. Hydrographic and microstructure observations from the Brazil Basin in the South Atlantic Ocean indicate that most of the abyssal mixing there takes place on the topographically rough flank of the midocean ridge. In previous studies it has been suggested that the high level of abyssal mixing observed on the ridge flank is primarily caused by breaking internal waves forced by tidal currents. Here, the results from a detailed analysis of velocity, hydrographic, and microstructure data from a ridge-flank canyon are presented. Two-year-long current-meter records indicate that within the canyon there is a significant along-axial mean flow down the density gradient toward the ridge crest. Five hundred meters above the canyon floor the kinetic energy in the subinertial band exceeds that associated with the semidiurnal tides by approximately a factor of 2. The mean dissipation of kinetic energy inside the canyon exceeds that above the ridge-flank topography by approximately a factor of 5. The largest dissipation values were observed downstream of a narrow, 1000-m-high sill that extends across the full width of the canyon. Along the entire canyon, there is a strong association between the presence of sills and along-axial density gradients, while there is no similar association between the presence of depressions and density gradients. Together, these observations suggest that sill-related mixing contributes at least as much to the diapycnal buoyancy flux in the canyon as tidally forced internal-wave breaking, which is not expected to be associated preferentially with sills. While only ≈15% of the interfacial area between Antarctic Bottom Water and North Atlantic Deep Water in the Brazil Basin lie inside canyons, the available data suggest that approximately one-half of the diapycnal buoyancy fluxes take place there. In comparison, the region above the ridge-flank topography accounts for about one-third of the buoyancy fluxes. The apparent importance of sill-related processes for mixing in ridge-flank canyons is therefore of global significance, especially considering that such canyons occur on average every 50 km along 2/3 of the global midocean ridge system, and that sills partially block the canyon axes every few tens of kilometers.

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L. St. Laurent, A. C. Naveira Garabato, J. R. Ledwell, A. M. Thurnherr, J. M. Toole, and A. J. Watson

Abstract

Direct measurements of turbulence levels in the Drake Passage region of the Southern Ocean show a marked enhancement over the Phoenix Ridge. At this site, the Antarctic Circumpolar Current (ACC) is constricted in its flow between the southern tip of South America and the northern tip of the Antarctic Peninsula. Observed turbulent kinetic energy dissipation rates are enhanced in the regions corresponding to the ACC frontal zones where strong flow reaches the bottom. In these areas, turbulent dissipation levels reach 10−8 W kg−1 at abyssal and middepths. The mixing enhancement in the frontal regions is sufficient to elevate the diapycnal turbulent diffusivity acting in the deep water above the axis of the ridge to 1 × 10−4 m2 s−1. This level is an order of magnitude larger than the mixing levels observed upstream in the ACC above smoother bathymetry. Outside of the frontal regions, dissipation rates are O(10−10) W kg−1, comparable to the background levels of turbulence found throughout most mid- and low-latitude regions of the global ocean.

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A. M. Thurnherr, L. Clément, L. St. Laurent, R. Ferrari, and T. Ijichi

Abstract

Closing the overturning circulation of bottom water requires abyssal transformation to lighter densities and upwelling. Where and how buoyancy is gained and water is transported upward remain topics of debate, not least because the available observations generally show downward-increasing turbulence levels in the abyss, apparently implying mean vertical turbulent buoyancy-flux divergence (densification). Here, we synthesize available observations indicating that bottom water is made less dense and upwelled in fracture zone valleys on the flanks of slow-spreading midocean ridges, which cover more than one-half of the seafloor area in some regions. The fracture zones are filled almost completely with water flowing up-valley and gaining buoyancy. Locally, valley water is transformed to lighter densities both in thin boundary layers that are in contact with the seafloor, where the buoyancy flux must vanish to match the no-flux boundary condition, and in thicker layers associated with downward-decreasing turbulence levels below interior maxima associated with hydraulic overflows and critical-layer interactions. Integrated across the valley, the turbulent buoyancy fluxes show maxima near the sidewall crests, consistent with net convergence below, with little sensitivity of this pattern to the vertical structure of the turbulence profiles, which implies that buoyancy flux convergence in the layers with downward-decreasing turbulence levels dominates over the divergence elsewhere, accounting for the net transformation to lighter densities in fracture zone valleys. We conclude that fracture zone topography likely exerts a controlling influence on the transformation and upwelling of bottom water in many areas of the global ocean.

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A. M. Thurnherr, K. J. Richards, C. R. German, G. F. Lane-Serff, and K. G. Speer

Abstract

High levels of diapycnal mixing and geothermal heating near midocean ridges contribute to the buoyancy fluxes that are required to close the global circulation. In topographically confined areas, such as the deep median valleys of slow-spreading ridges, these fluxes strongly influence the local hydrography and dynamics. Data from a segment-scale hydrographic survey of the rift valley of the Mid-Atlantic Ridge and from an array of current meters deployed there during an entire year are analyzed in order to characterize the dominant hydrographic patterns and dynamical processes. Comparison with historic hydrographic data indicates that the temporal variability during the last few decades has been small compared to the observed segment-scale gradients. The rift valley circulation is characterized by inflow from the eastern ridge flank and persistent unidirectional along-segment flow into a cul-de-sac. Therefore, most of the water flowing along the rift valley upwells within the segment with a mean vertical velocity >10−5 m s−1. The observed streamwise hydrographic gradients indicate that diapycnal mixing dominates the rift valley buoyancy fluxes by more than an order of magnitude, in spite of the presence of a large hydrothermal vent field supplying several gigawatts of heat to the water column. Hydrographic budgets in the rift valley yield diffusivity values of order 5 × 10−3 m2 s−1, consistent with estimates derived from statically unstable overturns, the largest of which were observed downstream of topographic obstacles in the path of the along-segment flow. This suggests vertical shear associated with cross-sill flows as the dominant contributor to the mechanical mixing in the rift valley.

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