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Guillaume Maze and John Marshall

Abstract

Analyzed fields of ocean circulation and the flux form of the potential vorticity equation are used to map the creation and subsequent circulation of low potential vorticity waters known as subtropical mode water (STMW) in the North Atlantic. Novel mapping techniques are applied to (i) render the seasonal cycle and annual-mean mixed layer vertical flux of potential vorticity (PV) through outcrops and (ii) visualize the extraction of PV from the mode water layer in winter, over and to the south of the Gulf Stream. Both buoyancy loss and wind forcing contribute to the extraction of PV, but the authors find that the former greatly exceeds the latter. The subsequent path of STMW is also mapped using Bernoulli contours on isopycnal surfaces.

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Charlène Feucher, Guillaume Maze, and Herlé Mercier

Abstract

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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Xavier Capet, Guillaume Roullet, Patrice Klein, and Guillaume Maze

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This study focuses on the description of an oceanic variant of the Charney baroclinic instability, arising from the joint presence of (i) an equatorward buoyancy gradient that extends from the surface into the ocean interior and (ii) reduced subsurface stratification, for example, as produced by wintertime convection or subduction. This study analyzes forced dissipative simulations with and without Charney baroclinic instability (C-BCI). In the former, C-BCI strengthens near-surface frontal activity with important consequences in terms of turbulent statistics: increased variance of vertical vorticity and velocity and increased vertical turbulent fluxes. Energetic consequences are explored. Despite the atypical enhancement of submesoscale activity in the simulation subjected to C-BCI, and contrary to several recent studies, the downscale energy flux at the submesoscale en route to dissipation remains modest in the flow energetic equilibration. In particular, it is modest vis à vis the global energy input to the system, the eddy kinetic energy input through conversion of available potential energy, and the classical inverse cascade of kinetic energy. Linear stability analysis suggests that the southern flank of the Gulf Stream may be conducive to oceanic Charney baroclinic instability in spring, following mode water formation and upper-ocean destratification.

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Guillaume Maze, Gael Forget, Martha Buckley, John Marshall, and Ivana Cerovecki

Abstract

The Walin water mass framework quantifies the rate at which water is transformed from one temperature class to another by air–sea heat fluxes (transformation). The divergence of the transformation rate yields the rate at which a given temperature range is created or destroyed by air–sea heat fluxes (formation). Walin’s framework provides a precise integral statement at the expense of losing spatial information. In this study the integrand of Walin’s expression to yield transformation and formation maps is plotted and used to study the role of air–sea heat fluxes in the cycle of formation–destruction of the 18° ± 1°C layer in the North Atlantic.

Using remotely sensed sea surface temperatures and air–sea heat flux estimates based on both analyzed meteorological fields and ocean data–model syntheses for the 3-yr period from 2004 to 2006, the authors find that Eighteen Degree Water (EDW) is formed by air–sea heat fluxes in the western part of the subtropical gyre, just south of the Gulf Stream. The formation rate peaks in February when the EDW layer is thickened by convection owing to buoyancy loss. EDW is destroyed by air–sea heat fluxes from spring to summer over the entire subtropical gyre. In the annual mean there is net EDW formation in the west to the south of the Gulf Stream, and net destruction over the eastern part of the gyre. Results suggest that annual mean formation rates of EDW associated with air–sea fluxes are in the range from 3 to 5 Sv (Sv ≡ 106 m3 s−1). Finally, error estimates are computed from sea surface temperature and heat flux data using an ensemble perturbation method. The transformation/formation patterns are found to be robust and errors mostly affect integral quantities.

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Ivana Cerovečki, Lynne D. Talley, Matthew R. Mazloff, and Guillaume Maze

Abstract

Subantarctic Mode Water (SAMW) is examined using the data-assimilating, eddy-permitting Southern Ocean State Estimate, for 2005 and 2006. Surface formation due to air–sea buoyancy flux is estimated using Walin analysis, and diapycnal mixing is diagnosed as the difference between surface formation and transport across 30°S, accounting for volume change with time. Water in the density range 26.5 < σθ < 27.1 kg m−3 that includes SAMW is exported northward in all three ocean sectors, with a net transport of (18.2, 17.1) Sv (1 Sv ≡ 106 m3 s−1; for years 2005, 2006); air–sea buoyancy fluxes form (13.2, 6.8) Sv, diapycnal mixing removes (−14.5, −12.6) Sv, and there is a volume loss of (−19.3, −22.9) Sv mostly occurring in the strongest SAMW formation locations. The most vigorous SAMW formation is in the Indian Ocean by air–sea buoyancy flux (9.4, 10.9) Sv, where it is partially destroyed by diapycnal mixing (−6.6, −3.1) Sv. There is strong export to the Pacific, where SAMW is destroyed both by air–sea buoyancy flux (−1.1, −4.6) Sv and diapycnal mixing (−5.6, −8.4) Sv. In the South Atlantic, SAMW is formed by air–sea buoyancy flux (5.0, 0.5) Sv and is destroyed by diapycnal mixing (−2.3, −1.1) Sv. Peaks in air–sea flux formation occur at the Southeast Indian and Southeast Pacific SAMWs (SEISAMWs, SEPSAMWs) densities. Formation over the broad SAMW circumpolar outcrop windows is largely from denser water, driven by differential freshwater gain, augmented or decreased by heating or cooling. In the SEISAMW and SEPSAMW source regions, however, formation is from lighter water, driven by differential heat loss.

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Gaël Forget, Guillaume Maze, Martha Buckley, and John Marshall

Abstract

The seasonal cycle in the volume and formation rate of Eighteen Degree Water (EDW) in the North Atlantic is quantified over the 3-yr period from 2004 to 2006. The EDW layer is defined as all waters that have a temperature between 17° and 19°C. The study is facilitated by a synthesis of various observations—principally Argo profiles of temperature and salinity, sea surface temperature, and altimetry—using a general circulation model as an interpolation tool. The winter increase in EDW volume is most pronounced in February, peaking at about 8.6 Svy, where 1 Svy ≈ 3.15 × 1013 m3 corresponding to a 1 Sv (Sv ≡ 106 m3 s−1) flow sustained for one year. This largely reflects winter EDW formation due to air–sea heat fluxes. Over the remainder of the year, newly created EDW is consumed by air–sea heat fluxes and ocean mixing, which roughly contribute ⅔ and ⅓, respectively. The authors estimate a net annual volume increase of 1.4 Svy, averaged over the 3-yr period. It is small compared to the amplitude of the seasonal cycle (8.6 Svy) and annual formation due to air–sea fluxes (4.6 Svy). The overall EDW layer volume thus appears to fluctuate around a stable point during the study period. An estimate of the full EDW volume budget is provided along with an uncertainty estimate of 1.8 Svy, and largely resolves apparent conflicts between previous estimates.

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Guillaume Maze, Fabio D’Andrea, Alain Colin de Verdière, and Patrice Klein

Abstract

The stationary atmospheric response to an idealized sea surface temperature anomaly (SSTA) is studied with a quasigeostrophic atmospheric model of the Southern Hemisphere. Sensitivity of the stationary response to the midlatitude SSTA location is determined and responses are decomposed on vertical modes.

The SSTA almost directly forces baroclinic responses, inducing warm-air anomalies 40°–50° downstream, eastward, to the SSTA. These baroclinic responses arise from an equilibrium between the SSTA-induced anomalous vortex stretching and (i) advection by the quasi-stationary flow and (ii) dissipation by high-frequency eddies.

The barotropic response consists of a midlatitude ridge (trough) and a South Pole trough (ridge) for SSTAs localized from the Drake Passage to the western Indian Ocean (from south of Australia to the center of the Pacific Ocean). This response can be further decomposed into (i) a zonally asymmetric component, a quasi-stationary wave train forced by a barotropic ridge downstream of the SSTA; and (ii) a zonal-mean component similar to a meridional shift of westerlies and hence a southern annular mode (SAM)-like pattern. The former component is phase locked with the SSTA position, while the latter has a phase that depends on the relative SSTA position with regard to the background quasi-stationary wave pattern. The study shows that the barotropic downstream ridge response is responsible for modifying the low-frequency eddy–mean flow interactions through relative vorticity fluxes and inducing the bipolar projection of the zonal-mean response.

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Emily Shuckburgh, Guillaume Maze, David Ferreira, John Marshall, Helen Jones, and Chris Hill

Abstract

The modulation of air–sea heat fluxes by geostrophic eddies due to the stirring of temperature at the sea surface is discussed and quantified. It is argued that the damping of eddy temperature variance by such air–sea fluxes enhances the dissipation of surface temperature fields. Depending on the time scale of damping relative to that of the eddying motions, surface eddy diffusivities can be significantly enhanced over interior values. The issues are explored and quantified in a controlled setting by driving a tracer field, a proxy for sea surface temperature, with surface altimetric observations in the Antarctic Circumpolar Current (ACC) of the Southern Ocean. A new, tracer-based diagnostic of eddy diffusivity is introduced, which is related to the Nakamura effective diffusivity. Using this, the mixed layer lateral eddy diffusivities associated with (i) eddy stirring and small-scale mixing and (ii) surface damping by air–sea interaction is quantified. In the ACC, a diffusivity associated with surface damping of a comparable magnitude to that associated with eddy stirring (∼500 m2 s−1) is found. In frontal regions prevalent in the ACC, an augmentation of surface lateral eddy diffusivities of this magnitude is equivalent to an air–sea flux of 100 W m−2 acting over a mixed layer depth of 100 m, a very significant effect. Finally, the implications for other tracer fields such as salinity, dissolved gases, and chlorophyll are discussed. Different tracers are found to have surface eddy diffusivities that differ significantly in magnitude.

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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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Nikolay P. Nezlin, Mathieu Dever, Mark Halverson, Jean-Michel Leconte, Guillaume Maze, Clark Richards, Igor Shkvorets, Rui Zhang, and Greg Johnson

Abstract

This study demonstrates the long-term stability of salinity measurements from Argo floats equipped with inductive conductivity cells, which have extended float lifetimes as compared to electrode-type cells. New Argo float sensor payloads must meet the demands of the Argo governance committees before they are implemented globally. Currently, the use of CTDs with inductive cells designed and manufactured by RBR, Ltd., has been approved as a Global Argo Pilot. One requirement for new sensors is to demonstrate stable measurements over the lifetime of a float. To demonstrate this, data from four Argo floats in the western Pacific Ocean equipped with the RBRargo CTD sensor package are analyzed using the same Owens–Wong–Cabanes (OWC) method and reference datasets as the Argo delayed-mode quality control (DMQC) operators. When run with default settings against the standard DMQC Argo and CTD databases, the OWC analysis reveals no drift in any of the four RBRargo datasets and, in one case, an offset exceeding the Argo target salinity limits. Being a statistical tool, the OWC method cannot strictly determine whether deviations in salinity measurements with respect to a reference hydrographic product (e.g., climatologies) are caused by oceanographic variability or sensor problems. So, this study furthermore investigates anomalous salinity measurements observed when compared with a reference product and demonstrates that anomalous values tend to occur in regions with a high degree of variability and can be better explained by imperfect reference data rather than sensor drift. This study concludes that the RBR inductive cell is a viable option for salinity measurements as part of the Argo program.

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