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Herlé Mercier and Kevin G. Speer

Abstract

Two moored arrays deployed in the Romanche Fracture Zone and Chain Fracture Zone in the equatorial Atlantic Ocean provide two-year-long time series of current and temperature in the Lower North Atlantic Deep Water and the Antarctic Bottom Water. Total time-averaged transport of Antarctic Bottom Water (potential temperature θ < 1.9°C) across the Mid-Atlantic Ridge amounts to 1.22 × 106 m3 s−1 eastward with a standard deviation of ±0.25 × 106 m3 s−1. A time-averaged transport of 0.36(± 0.23) × 106 m3 s−1 eastward is found for the Lower North Atlantic Deep Water in the 1.9° < θ < 2.1°C temperature range, but this may represent only a fraction of the total flow of this water mass across the ridge. Contributions of the Romanche Fracture Zone and Chain Fracture Zone to the Antarctic Bottom Water transport are similar, while the Chain Fracture Zone has the greater share of Lower North Atlantic Deep Water transport. Semiannual and annual periods are detected in the transport time series and together explain 24% of the Antarctic Bottom Water transport variance in the Romanche Fracture Zone. In the Chain Fracture Zone, Antarctic Bottom Water transport variance is dominated by fluctuations in the period band 10–20 days.

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Herlé Mercier and Alain Colin De Verdiére

Abstract

The Tourbillon experiment carried out in 1979–80 over the Porcupine Abyssal Plain in the eastern North Atlantic is the fist experiment providing data that allow an efficient description of the spatial structure of the mesoscale low-frequency flows in an eastern midlatitude basin. The eddies appear to scale in the horizontal as the internal Rossby radius of deformation and, although of an energy level comparable to those observed during MODE, they are intrinsically more nonlinear because of reduced horizontal scale. In much the same way the time scales are less than what is observed in western basin experiments MODE, Polymode III A, B suggesting some kind of turbulent dispersion relation with frequency proportional to wavenumber in the eddy energy-containing range. We observe in Tourbillon an equipartition of eddy kinetic and potential energy. The frequency kinetic-energy spectra have slopes of order −2 in a log–log form, hence are more white than in western basins (−2.5, −3). The distribution of eddy kinetic is highly intensified above the main pycnocline (more so at high rather than low frequencies). An empirical-orthogonal-function decomposition has been carried out in the vertical direction and indicates that the lowest frequencies (450–64 days) are vertically coherent, 72% of the total energy being described by only one EOF with baroclinic structure. The vertical coherence decreases with period. Finally the relevance of variable atmospheric driving to induce eddy motions is discussed.

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Charlène Feucher, Guillaume Maze, and Herlé Mercier

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A new objective algorithm for the characterization of the permanent pycnocline (OAC-P) in subtropical gyres is proposed. OAC-P is based on a pragmatic analysis of vertical density gradient features to identify the permanent pycnocline: OAC-P identifies the permanent pycnocline as the stratified layer found below surface mode waters. OAC-P provides the permanent pycnocline depth, unequivocally associated with a local maximum in the stratification, and top and bottom thicknesses, associated with upward and downward decreases in stratification, respectively. OAC-P uses half Gaussian curves as asymmetric nonlinear analytical models of the stratification peak. It is the first time that an algorithm is proposed to characterize objectively the permanent pycnocline for a region where handling the stronger stratification peak of the seasonal pycnocline is complex. A guideline for how to implement the OAC-P is given, with application to the North Atlantic Ocean Argo data as an example. OAC-P provides a detailed description of the mean structure of the North Atlantic subtropical permanent pycnocline. OAC-P detects a permanent pycnocline throughout the subtropical gyre north of the North Equatorial Current. The large-scale description of the permanent pycnocline depth structure as a classic bowl shape is captured however with much more detail. New regional information is provided. In particular, (i) there is only one region—the southern recirculation gyre of the Gulf Stream extension—where the permanent pycnocline is along an isopycnal surface and (ii) vertical asymmetries clearly discriminate one region from another.

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Nathalie Daniault, Pascale Lherminier, and Herlé Mercier

Abstract

The circulation and related transports at the southeast tip of Greenland are determined from direct current observations of a moored current meter array. The measurements cover a time span from June 2004 to June 2006. The net mean total southwestward transport of the East Greenland–Irminger Current from the midshelf (20 km off the coast at 60°N) to the 2070-m isobath (about 100 km offshore) was estimated as 17.3 Sv (Sv ≡ 106 m3 s−1) with an uncertainty of 1 Sv. The transport variability is characterized by a standard deviation of 3.8 Sv with a peak-to-peak amplitude up to 30 Sv. The seasonal variability has an amplitude of 1.5 Sv. Frequencies around 0.1 day−1 dominate the signal, although a variability at lower frequency (∼1 month−1) also appears in winter. The coherence between the observed transport variability and the wind stress curl variability over the Irminger Sea differs significantly from 0 at the 95% confidence level for periods greater than 5 days.

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Bruno Ferron, Herlé Mercier, Kevin Speer, Ann Gargett, and Kurt Polzin

Abstract

The Romanche Fracture Zone is a major gap in the Mid-Atlantic Ridge at the equator, which is deep enough to allow significant eastward flows of Antarctic Bottom Water from the Brazil Basin to the Sierra Leone and Guinea Abyssal Plains. While flowing through the Romanche Fracture Zone, bottom-water properties are strongly modified due to intense vertical mixing. The diapycnal mixing coefficient in the bottom water of the Romanche Fracture Zone is estimated by using the finestructure of CTD profiles, the microstructure of high-resolution profiler data, and by constructing a heat budget from current meter data.

The finestructure of density profiles is described using the Thorpe scales L T. It is shown from microstructure data taken in the bottom water that the Ozmidov scale L O is related to L T by the linear relationship L O = 0.95L T, similar to other studies, which allows an estimate of the diapycnal mixing coefficient using the Osborn relation. The Thorpe scale and the diapycnal mixing coefficient estimates show enhanced mixing downstream (eastward) of the main sill of the Romanche Fracture Zone. In this region, a mean diapycnal mixing coefficient of about 1000 × 10−4 m2 s−1 is found for the bottom water.

Estimates of cross-isothermal mixing coefficient derived from the heat budgets constructed downstream of the current meter arrays deployed in the Romanche Fracture Zone and the nearby Chain Fracture Zone are in agreement with the finestructure estimates of the diapycnal mixing coefficient within the Romanche Fracture Zone. Although the two fracture zones occupy only 0.4% of the area covered by the Sierra Leone and Guinea Abyssal Plains, the diffusive heat fluxes across the 1.4°C isotherm in the Romanche and Chain Fracture Zones are half that found over the abyssal plains across the 1.8°C isotherm, emphasizing the role of these passages for bottom-water property modifications.

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Michel Arhan, Alain Colin De Verdiere, and Herlé Mercier

Abstract

Eulerian velocity measurements carried out along 48°N in the Atlantic Ocean Provide averaged velocities with definite large-scale structure. This warrants an analysis of these mean velocity vectors within the framework of steady and large-scale dynamics: the dominant part played by the bottom forcing in the planetary vorticity balance is demonstrated. Combined use of vertical velocities derived from fills balance and the mean velocity spirals allows us to estimate advection in the heat equation and residual cros-isopycnal velocities. At the westernmost mooring site (35°W) heat losses to the atmosphere account for these residuals, while at the other sites (20°, 25° and 30°W) lateral heat fluxes induced by mesoscale eddues must be invoked.

The hydrology of the region provides elements of comparison with the set of averaged velocities: qualitative through our knowledge of the motions of watermasses at this latitude, then more quantitative using geostrophic calculations. Beta-spiral inversions are carried out on climatological hydrographic data and the same dynamical analysis applied to the resulting velocity profiles. Some incompatibilities with the direct measurements are observed and possible masons for these differences discussed.

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Bruno Ferron, Florian Kokoszka, Herlé Mercier, and Pascale Lherminier

Abstract

A total of 96 finestructure and 30 microstructure full-depth vertical profiles were collected along the A25 Greenland–Portugal Observatoire de la Variabilité Interannuelle et Décennale en Atlantique Nord (OVIDE) hydrographic line in 2008. The microstructure of the horizontal velocity was used to calculate turbulent kinetic energy dissipation rates ε vmp, where vmp refers to the vertical microstructure profiler. The lowest dissipation values (ε vmp < 0.5 × 10−10 W kg−1) are found below 2000 m in the Iberian Abyssal Plain and in the center of the Irminger basin; the largest values (>5 × 10−10 W kg−1) are found in the main thermocline, around the Reykjanes Ridge, and in a 1000-m-thick layer above the bottom near 48°N. The finestructure of density was used to estimate isopycnal strain and that of the lowered acoustic Doppler current profiler to estimate the vertical shear of horizontal velocities. Strain and shear were used to estimate dissipation rates ε G03 () associated with the internal wave field. The shear-to-strain ratio correction term of the finescale parameterization ε G03 brings the fine- and microscale estimates of the dissipation rate into better agreement as found. The latitude/buoyancy frequency term slightly improves the parameterization for weakly stratified waters. Correction term ε G03 is consistent with ε vmp within a factor of 4.5 over 95% of the profiles. This good consistency suggests that most of the turbulent activity recorded in this dataset is due to the internal wave field. The canonical globally averaged diffusivity value of order 10−4 m2 s−1 needed to maintain the global abyssal stratification () is only reached on the flank of the Reykjanes Ridge and in the region around 48°N.

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Herlé Mercier, Michel Ollitrault, and Pierre Yves Le Traon

Abstract

A nonlinear finite-difference inverse model is used for estimating the North Atlantic general circulation between 20° and 50°N. The inverse model with grid spacing 2° latitude and 2.5° longitude is based on hydrography and is in geostrophic and hydrostatic balances. The constraints of the inverse model are surface and subsurface float mean velocities; Ekman pumping derived from wind data; conservations of mass, heat, and salt; and the planetary vorticity equation at the reference level. The mass, heat, and salt conservations are applied in a vertically integrated form. The model does not have explicit mixing or air–sea flux terms. Vertical velocities result from the nondivergence of the 3D velocity field.

After inversion, float velocities, hydrographic data, and dynamical constraints are generally compatible within error bounds. A few float velocities are, however, rejected by the model mainly due to inadequate time or space sampling of the 2° latitude by 5° longitude boxes for which mean float velocities are computed.

The resulting circulation shows a maximum Gulf Stream transport close to 130 × 106 m3 s−1 at 64°W. Residuals of the vertically integrated heat and salt conservation constraints may be interpreted as air–sea fluxes and are of the right order of magnitude as compared to in situ measurements.

The float database used is already important particularly at the surface. However, its addition to the inversion does not change substantially the estimation by the model of integrated quantities, such as Gulf Stream transports, as compared to an inversion using hydrography and dynamical constraints alone. But floats significantly affect the estimation of the deep circulation increasing, for instance, the estimated velocity amplitude for the deep western boundary current flowing westward south of the Grand Banks.

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Claire Gourcuff, Pascale Lherminier, Herlé Mercier, and Pierre Yves Le Traon

Abstract

A method to estimate mass and heat transports across hydrographic sections using hydrography together with altimetry data in a geostrophic inverse box model is presented. Absolute surface velocities computed from Archiving, Validation, and Interpretation of Satellite Oceanographic data (AVISO) altimetry products made up of a combination of sea surface height measurements and geoid estimate are first compared to ship acoustic Doppler current profiler (S-ADCP) measurements of the Observatoire de la Variabilité Interannuelle et Décennale (OVIDE) project along hydrographic sections repeated every 2 yr in summer from Portugal to Greenland. The RMS difference between S-ADCP and altimetry velocities averaged on distances of about 100 km accounts for 3.3 cm s−1. Considering that the uncertainty of S-ADCP velocities is found at 1.5 cm s−1, altimetry errors are estimated at 3 cm s−1. Transports across OVIDE sections previously obtained using S-ADCP data to constrain the geostrophic inverse box model are used as a reference. The new method is found useful to estimate absolute transports across the sections, as well as part of their variability. Despite associated uncertainties that are about 50% larger than when S-ADCP is used, the results for the North Atlantic Current and heat transports, with uncertainties of 10%–15%, reproduce the already observed variability. The largest uncertainties are found in the estimates of the East Greenland Irminger Current (EGIC) transport (30%), induced by larger uncertainties associated with altimetry data at the western boundary.

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Clément Vic, Bruno Ferron, Virginie Thierry, Herlé Mercier, and Pascale Lherminier

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Internal waves in the semidiurnal and near-inertial bands are investigated using an array of seven moorings located over the Reykjanes Ridge in a cross-ridge direction (57.6°–59.1°N, 28.5°–33.3°W). Continuous measurements of horizontal velocity and temperature for more than 2 years allow us to estimate the kinetic energy density and the energy fluxes of the waves. We found that there is a remarkable phase locking and linear relationship between the semidiurnal energy density and the tidal energy conversion at the spring–neap cycle. The energy-to-conversion ratio gives replenishment time scales of 4–5 days on the ridge top versus 7–9 days on the flanks. Altogether, these results demonstrate that the bulk of the tidal energy on the ridge comes from near-local sources, with a redistribution of energy from the top to the flanks, which is endorsed by the energy fluxes oriented in the cross-ridge direction. Implications for tidally driven energy dissipation are discussed. The time-averaged near-inertial kinetic energy is smaller than the semidiurnal kinetic energy by a factor of 2–3 but is much more variable in time. It features a strong seasonal cycle with a winter intensification and subseasonal peaks associated with local wind bursts. The ratio of energy to wind work gives replenishment time scales of 13–15 days, which is consistent with the short time scales of observed variability of near-inertial energy. In the upper ocean (1 km), the highest levels of near-inertial energy are preferentially found in anticyclonic structures, with a twofold increase relative to cyclonic structures, illustrating the funneling effect of anticyclones.

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