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Virginie Thierry and Yves Morel

Abstract

The authors investigate the influence of steep bottom topography on the propagation of a vortex in a two-layer quasigeostrophic model. The vortex is intensified in the upper layer and the planetary beta effect is taken into account.

The authors find that steep topography can scatter disturbances created by the upper-layer vortex displacement and maintain the lower-layer motion weak. It is thus shown that, when the vortex radius is smaller than a critical value, the vortex behaves as if the lower layer was at rest (or infinitely deep as in a reduced gravity model). If the radius is increased while holding the maximum vorticity of the vortex, the topographic Rossby waves—generated during the scattering process—have a stronger signature in the upper layer, and the vortex evolution begins to change in comparison with the reduced-gravity case. However, numerical experiments show that both the steep topography and reduced-gravity trajectories remain close up to a large radius, after which a vortex above a strong slope becomes unstable and is dispersed by topographic Rossby waves.

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Damien Desbruyères, Elaine L. McDonagh, Brian A. King, and Virginie Thierry
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Esther Portela, Nicolas Kolodziejczyk, Christophe Maes, and Virginie Thierry

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Using an Argo dataset and the ECCOv4 reanalysis, a volume budget was performed to address the main mechanisms driving the volume change of the interior water masses in the Southern Hemisphere oceans between 2006 and 2015. The subduction rates and the isopycnal and diapycnal water-mass transformation were estimated in a density–spiciness (στ) framework. Spiciness, defined as thermohaline variations along isopycnals, was added to the potential density coordinates to discriminate between water masses spreading on isopycnal layers. The main positive volume trends were found to be associated with the Subantarctic Mode Waters (SAMW) in the South Pacific and South Indian Ocean basins, revealing a lightening of the upper waters in the Southern Hemisphere. The SAMW exhibits a two-layer density structure in which subduction and diapycnal transformation from the lower to the upper layers accounted for most of the upper-layer volume gain and lower-layer volume loss, respectively. The Antarctic Intermediate Waters, defined here between the 27.2 and 27.5 kg m−3 isopycnals, showed the strongest negative volume trends. This volume loss can be explained by their negative isopyncal transformation southward of the Antarctic Circumpolar Current into the fresher and colder Antarctic Winter Waters (AAWW) and northward into spicier tropical/subtropical Intermediate Waters. The AAWW is destroyed by obduction back into the mixed layer so that its net volume change remains nearly zero. The proposed mechanisms to explain the transformation within the Intermediate Waters are discussed in the context of Southern Ocean dynamics. The στ decomposition provided new insight on the spatial and temporal water-mass variability and driving mechanisms over the last decade.

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Damien Desbruyères, Elaine L. McDonagh, Brian A. King, and Virginie Thierry

Abstract

The early twenty-first century’s warming trend of the full-depth global ocean is calculated by combining the analysis of Argo (top 2000 m) and repeat hydrography into a blended full-depth observing system. The surface-to-bottom temperature change over the last decade of sustained observation is equivalent to a heat uptake of 0.71 ± 0.09 W m−2 applied over the surface of Earth, 90% of it being found above 2000-m depth. The authors decompose the temperature trend pointwise into changes in isopycnal depth (heave) and temperature changes along an isopycnal (spiciness) to describe the mechanisms controlling the variability. The heave component dominates the global heat content increase, with the largest trends found in the Southern Hemisphere’s extratropics (0–2000 m) highlighting a volumetric increase of subtropical mode waters. Significant heave-related warming is also found in the deep North Atlantic and Southern Oceans (2000–4000 m), reflecting a potential decrease in deep water mass renewal rates. The spiciness component shows its strongest contribution at intermediate levels (700–2000 m), with striking localized warming signals in regions of intense vertical mixing (North Atlantic and Southern Oceans). Finally, the agreement between the independent Argo and repeat hydrography temperature changes at 2000 m provides an overall good confidence in the blended heat content evaluation on global and ocean scales but also highlights basin-scale discrepancies between the two independent estimates. Those mismatches are largest in those basins with the largest heave signature (Southern Ocean) and reflect both the temporal and spatial sparseness of the hydrography sampling.

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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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Fabienne Gaillard, Thierry Reynaud, Virginie Thierry, Nicolas Kolodziejczyk, and Karina von Schuckmann

Abstract

The In Situ Analysis System (ISAS) was developed to produce gridded fields of temperature and salinity that preserve as much as possible the time and space sampling capabilities of the Argo network of profiling floats. Since the first global reanalysis performed in 2009, the system has evolved, and a careful delayed-mode processing of the 2002–12 dataset has been carried out using version 6 of ISAS and updating the statistics to produce the ISAS13 analysis. This last version is now implemented as the operational analysis tool at the Coriolis data center. The robustness of the results with respect to the system evolution is explored through global quantities of climatological interest: the ocean heat content and the steric height. Estimates of errors consistent with the methodology are computed. This study shows that building reliable statistics on the fields is fundamental to improve the monthly estimates and to determine the absolute error bars. The new mean fields and variances deduced from the ISAS13 reanalysis and dataset show significant changes relative to the previous ISAS estimates, in particular in the Southern Ocean, justifying the iterative procedure. During the decade covered by Argo, the intermediate waters appear warmer and saltier in the North Atlantic and fresher in the Southern Ocean than in World Ocean Atlas 2005 long-term mean. At interannual scale, the impact of ENSO on the ocean heat content and steric height is observed during the 2006/07 and 2009/10 events captured by the network.

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Fabienne Gaillard, Emmanuelle Autret, Virginie Thierry, Philippe Galaup, Christine Coatanoan, and Thomas Loubrieu

Abstract

Argo floats have significantly improved the observation of the global ocean interior, but as the size of the database increases, so does the need for efficient tools to perform reliable quality control. It is shown here how the classical method of optimal analysis can be used to validate very large datasets before operational or scientific use. The analysis system employed is the one implemented at the Coriolis data center to produce the weekly fields of temperature and salinity, and the key data are the analysis residuals. The impacts of the various sensor errors are evaluated and twin experiments are performed to measure the system capacity in identifying these errors. It appears that for a typical data distribution, the analysis residuals extract 2/3 of the sensor error after a single analysis. The method has been applied on the full Argo Atlantic real-time dataset for the 2000–04 period (482 floats) and 15% of the floats were detected as having salinity drifts or offset. A second test was performed on the delayed mode dataset (120 floats) to check the overall consistency, and except for a few isolated anomalous profiles, the corrected datasets were found to be globally good. The last experiment performed on the Coriolis real-time products takes into account the recently discovered problem in the pressure labeling. For this experiment, a sample of 36 floats, mixing well-behaved and anomalous instruments of the 2003–06 period, was considered and the simple test designed to detect the most common systematic anomalies successfully identified the deficient floats.

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Serge Le Reste, Vincent Dutreuil, Xavier André, Virginie Thierry, Corentin Renaut, Pierre-Yves Le Traon, and Guillaume Maze

Abstract

The international Argo program, consisting of a global array of more than 3000 free-drifting profiling floats, has now been monitoring the upper 2000 m of the ocean for several years. One of its main proposed evolutions is to be able to reach the deeper ocean in order to better observe and understand the key role of the deep ocean in the climate system. For this purpose, Ifremer has designed the new “Deep-Arvor” profiling float: it extends the current operational depth down to 4000 m, and measures temperature and salinity for up to 150 cycles with CTD pumping continuously and 200 cycles in spot sampling mode. High-resolution profiles (up to 2000 points) can be transmitted and data are delivered in near–real time according to Argo requirements. Deep-Arvor can be deployed everywhere at sea without any preballasting operation and its light weight (~26 kg) makes its launching easy. Its design was done to target a cost-effective solution. Predefined spots have been allocated to add an optional oxygen sensor and a connector for an extra sensor. Extensive laboratory tests were successful. The results of the first at-sea experiments showed that the expected performances of the operational prototypes had been reached (i.e., to perform up to 150 cycles). Meanwhile, the industrialization phase was completed in order to manufacture the Deep-Arvor float for the pilot experiment in 2015. This paper details all the steps of the development work and presents the results from the at-sea experiments.

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M. Susan Lozier, Sheldon Bacon, Amy S. Bower, Stuart A. Cunningham, M. Femke de Jong, Laura de Steur, Brad deYoung, Jürgen Fischer, Stefan F. Gary, Blair J. W. Greenan, Patrick Heimbach, Naomi P. Holliday, Loïc Houpert, Mark E. Inall, William E. Johns, Helen L. Johnson, Johannes Karstensen, Feili Li, Xiaopei Lin, Neill Mackay, David P. Marshall, Herlé Mercier, Paul G. Myers, Robert S. Pickart, Helen R. Pillar, Fiammetta Straneo, Virginie Thierry, Robert A. Weller, Richard G. Williams, Chris Wilson, Jiayan Yang, Jian Zhao, and Jan D. Zika

Abstract

For decades oceanographers have understood the Atlantic meridional overturning circulation (AMOC) to be primarily driven by changes in the production of deep-water formation in the subpolar and subarctic North Atlantic. Indeed, current Intergovernmental Panel on Climate Change (IPCC) projections of an AMOC slowdown in the twenty-first century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep-water formation. The motivation for understanding this linkage is compelling, since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic Program (OSNAP), to provide a continuous record of the transbasin fluxes of heat, mass, and freshwater, and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014, and the first OSNAP data products are expected in the fall of 2017.

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