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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, 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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Xue Fan, Uwe Send, Pierre Testor, Johannes Karstensen, and Pascale Lherminier

Abstract

Mesoscale anticyclonic eddies in the Irminger Sea are observed using a mooring and a glider. Between 2002 and 2009, the mooring observed 53 anticyclones. Using a kinematic model, objective estimates of eddy length scales and velocity structure are made for 16 eddies. Anticyclones had a mean core diameter of 12 km, and their mean peak observed azimuthal speed was 0.1 m s−1. They had core salinities and potential temperatures of 34.91–34.98 and 4.48°–5.34°C, respectively, making them warm and salty features. These properties represent a typical salinity anomaly of 0.03 and a temperature anomaly of 0.28°C from noneddy values. All eddies had small (≪1) Rossby numbers. In 2006, the glider observed two anticyclones having diameters of about 20 km and peak azimuthal speeds of about 0.3 m s−1. Similar salinity anomalies were detected throughout the Irminger Sea by floats profiling in anticyclones. Two formation regions for the eddies are identified: one to the west of the Reykjanes Ridge and the other off the East Greenland Irminger Current near Cape Farewell close to the mooring. Observations indicate that eddies formed in the former region are larger than eddies observed at the mooring. A clear increase in eddy salinity is observed between 2002 and 2009. The observed breakup of these eddies in winter implies that they are a source of salt for the central gyre. The anticyclones are similar to those found in both the Labrador Sea and Norwegian Sea, making them a ubiquitous feature of the subpolar North Atlantic basins.

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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

Abstract

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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Bruno Ferron, Florian Kokoszka, Herlé Mercier, Pascale Lherminier, Thierry Huck, Aida Rios, and Virginie Thierry

Abstract

The variability of the turbulent kinetic energy dissipation due to internal waves is quantified using a finescale parameterization applied to the A25 Greenland–Portugal transect repeated every two years from 2002 to 2012. The internal wave velocity shear and strain are estimated for each cruise at 91 stations from full depth vertical profiles of density and velocity. The 2002–12 averaged dissipation rate 〈ε 2002–2012〉 in the upper ocean lays in the range 1–10 × 10−10 W kg−1. At depth, 〈ε 2002–2012〉 is smaller than 1 × 10−10 W kg−1 except over rough topography found at the continental slopes, the Reykjanes Ridge, and in a region delimited by the Azores–Biscay Rise and Eriador Seamount. There, the vertical energy flux of internal waves is preferentially oriented toward the surface and 〈ε 2002–2012〉 is in the range 1–20 × 10−10 W kg−1. The interannual variability in the dissipation rates is remarkably small over the whole transect. A few strong dissipation rate events exceeding the uncertainty of the finescale parameterization occur at depth between the Azores–Biscay Rise and Eriador Seamount. This region is also marked by mesoscale eddying flows resulting in enhanced surface energy level and enhanced bottom velocities. Estimates of the vertical energy fluxes into the internal tide and into topographic internal waves suggest that the latter are responsible for the strong dissipation events. At Eriador Seamount, both topographic internal waves and the internal tide contribute with the same order of magnitude to the dissipation rate while around the Reykjanes Ridge the internal tide provides the bulk of the dissipation rate.

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Aurelien L. Ponte, Patrice Klein, Xavier Capet, Pierre-Yves Le Traon, Bertrand Chapron, and Pascale Lherminier

Abstract

High-resolution numerical experiments of ocean mesoscale eddy turbulence show that the wind-driven mixed layer (ML) dynamics affects mesoscale motions in the surface layers at scales lower than O(60 km). At these scales, surface horizontal currents are still coherent to, but weaker than, those derived from sea surface height using geostrophy. Vertical motions, on the other hand, are stronger than those diagnosed using the adiabatic quasigeotrophic (QG) framework. An analytical model, based on a scaling analysis and on simple dynamical arguments, provides a physical understanding and leads to a parameterization of these features in terms of vertical mixing. These results are valid when the wind-driven velocity scale is much smaller than that associated with eddies and the Ekman number (related to the ratio between the Ekman and ML depth) is not small. This suggests that, in these specific situations, three-dimensional ML motions (including the vertical velocity) can be diagnosed from high-resolution satellite observations combined with a climatological knowledge of ML conditions and interior stratification.

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Anastasia Falina, Artem Sarafanov, Herlé Mercier, Pascale Lherminier, Alexey Sokov, and Nathalie Daniault

Abstract

Hydrographic data collected in the Irminger Sea in the 1990s–2000s indicate that dense shelf waters carried by the East Greenland Current south of the Denmark Strait intermittently descend (cascade) down the continental slope and merge with the deep waters originating from the Nordic Seas overflows. Repeat measurements on the East Greenland shelf at ~200 km south of the Denmark Strait (65°–66°N) reveal that East Greenland shelf waters in the Irminger Sea are occasionally as dense (σ 0 > 27.80) as the overflow-derived deep waters carried by the Deep Western Boundary Current (DWBC). Clear hydrographic traces of upstream cascading of dense shelf waters are found over the continental slope at 64.3°N, where the densest plumes (σ 0 > 27.80) originating from the shelf are identified as distinct low-salinity anomalies in the DWBC. Downstream observations suggest that dense fresh waters descending from the shelf in the northern Irminger Sea can be distinguished in the DWBC up to the latitude of Cape Farewell (~60°N) and that these waters make a significant contribution to the DWBC transport.

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Gilles Reverdin, Jean-Claude Gascard, Bernard Le Cann, Louis Prieur, Michel Assenbaum, and Pascale Lherminier

Abstract

An anticyclonic mode water vortex and its environment were investigated from November 2000 to September 2001 in the northeast Atlantic (near 43.5°N, 15°–19°W) with neutrally buoyant drifting floats, moored current meters, satellite altimetric sea surface height, and several hydrological surveys and sections. These observations reveal a coherent inner core (∼30 km in diameter) made of very oxygenated northeast Atlantic central waters (11°–12.7°C and 35.5–35.7 on the 1978 practical salinity scale) from 150 m down to about 750-m depth. The core presents high relative vorticity (up to approximately −0.5 times the Coriolis frequency f ) within at least 10 km of its center, near 400–700 m. Peak velocity along the core rim is located deeper than 600 m bordering the deepest and densest (σθ = 27.175 kg m−3) northeast Atlantic mode water found during the Programme Océan Multidisciplinaire Méso Echelle (POMME) project. This water likely originates north of 47°N, where it could have been in contact with the sea surface in early 1999. Below the core, large near-inertial internal waves are found. At least during spring and summer 2001, the core was embedded in a much larger anticyclonic eddy that extends to 100 km from its center, with azimuthal velocity decreasing from the sea surface to 1500 m. This eddy does not trap floats for a long time and is associated with a sea level anomaly on the order of 10 cm. From January through August 2001, both the core and the larger eddy moved anticyclonically around a shallow part of the Azores–Biscay ridge. The core trajectory also exhibits smaller anticyclonic loops on shorter time scales, suggesting that at least at times it is not located at the center of the larger eddy.

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