Poster Abstracts
Session 1 : Large and Meso-Scale Interactions at Mid-Latitude
Ana Aguiar Laboratoire de Physique des Océans, CNRS, Plouzané, France
Mediterranean outflow transports and entrainment estimates from observations and high-resolution modelling
We use a large set of hydrology and cross-slope direct current observations together with high resolution numerical simulations to revisit estimates of transports in the Gulf of Cadiz and to provide a three dimensional picture of the formation of the outflow and salinity fluxes from the Mediterranean into the intermediate layer of the Atlantic. In our model, the time-averaged Mediterranean undercurrent develops two cores of zonal velocity at lambda 8.5^W: one at 500 m (31.6-31.8 kg m^-3) and another around 1100 m (32.1-32.3 kg m^-3), with maximum westward velocity 0.36 m s^-1. A single well defined vein of saltier and warmer water (salinity maximum 36.9 psu) is found attached to the slope, centred at 1300 m (32.2-32.4 kg m^-3). The structure of the undercurrent in the observational sections is very similar, but instant velocity can reach 0.6 m s^-1 whereas the salinity peaks just above 36.5 psu. Estimates from both datasets indicate total westward salinity and heat fluxes of 1.3 psu Sv and 20times 10^12 J m^-3 Sv, respectively, near the Portimao Canyon. In observations, the transport is most intense between isopycnals 32.0-32.2 kg m^-3 but the salinity and heat fluxes are often most intense in 32.2-32.4 kg m^-3, reaching peak values of 0.6 psu Sv and 11times 10^12 J m^-3 Sv. At the Strait of Gibraltar, we estimate that the transport of the Mediterranean outflow (sigma1>33.15 kg m^-3) is about 0.65 Sv, 0.48 Sv of which correspond to pure Mediterranean water (S>38.4 psu). At the Portim\~ao Canyon, the results for westward transport (enclosed by isopycnals 31.6-32.6 kg m^-3) computed from both observations and numerical data are of 3.2-3.6 Sv and 3.5 Sv, respectively. This corresponds to almost a fivefold increase in volume as the outflow progresses along the northern boundary of the Gulf of Cadiz. By computing Lagrangian transports within a closed domain in the Gulf of Cadiz, we conclude that most of the volume entrained into the undercurrent is supplied through the south and southwest borders. After analysing the net transport in four density intervals equally spaced (0.2 kg m^-3) between isopycnals 31.8 and 32.6, we conclude that the diapycnal entrainment of NACW (from shallower levels) is about 1.1 Sv in total and distributed as 0.33, 0.36, 0.41 and 0.04 Sv, respectively. The bulk entrainment from and to intermediate levels (within isopycnals 31.6-32.6) is approximately 2 Sv.
Bernard Barnier Laboratoire de Glaciologie et de Géophysique de l’Environnement, CNRS, Grenoble, France
Irminger Rings: Could they make the heat of the subsurface boundary current available to the atmosphere in the Labrador Sea?
The role of strong ocean currents in the general circulation is intrinsically linked to the mesoscale turbulence they generate. In the Labrador Sea, the boundary current of the North Atlantic Subpolar Gyre is known to generate a great variety of eddies which have a strong impact on the seasonal cycle of deep convection, fluxing heat from the relatively warm water core of the boundary current into the interior of the Sea. This paper investigates the possible existence of an eddy-driven process that could connect the subsurface core of the boundary current to the atmosphere. The life cycle of Irminger Rings (IRs) in the Labrador Sea is investigated over several seasonal cycles in simulations carried out with a full primitive equation, eddy resolving (4 km resolution), circulation model driven by realistic air-sea fluxes. It is found that a local topographic feature off Cape Desolation (west coast of Greeland) generates IRs, which are the main source of high EKE leve! ls seen n orth of about 60°N in satellite altimetry. Model IRs characteristics are found to compare very well with recent observations from gliders. Like ocean rings, their peculiar potential vorticity structure (a negative core surrounded by a positive ring) insulates them from surrounding waters, and eddies survive several winters. Model IRs properties primarily evolve through surface exchanges with the atmosphere, especially heat loss, as suggested by recent observations. Lateral exchange of heat with ambient waters appears to be significantly smaller. Under the forcing conditions of our simulations, it takes about two winters to the atmosphere to extract the heat contained in the subsurface core of a ring and to bring it to a colder temperature comparable to that of the deep convection area. The ring usually collapses shortly after that. Therefore, the heat extracted by Irminger Rings from the boundary current is not given up to the interior ocean, but to the atmosphere. In tha! t sense, Irminger Rings could be seen as acting as a pipe making the heat of the subsurface boundary current available to the atmosphere.
Antoine Hochet Laboratoire de Physique des Océans, UBO, Brest, France
Vertical structure of large scale oceanic unsteady currents
A linear model based on the quasi-geostrophic equations is constructed in order to understand the vertical structure of Rossby waves and, more broadly, of anomalies resolved by altimeter data, roughly with periods longer than 20 days and with wavelength larger than 100km. Vertical boundary conditions are: no vertical flow at the bottom, and surface pressure imposed by sea surface height (SSH) at the top. We reconstruct the subsurface field with only sea surface height and climatological stratification data from the World Ocean Atlas. The solution is calculated in a periodic rectangular zone with a 3D discrete Fourier transform. The effect of the mean flow on Rossby waves is neglected, which we believe is a reasonable approximation for low latitudes. The model is then assessed with an idealized double gyre simulation (performed with MICOM). Results from this first part show that our linear Rossby wave model is able to give reasonable predictions of subsurface currents at low ! latitudes (below approximately 30°) and for relatively weak mean flow. However, the predictions degrade with stronger mean flow and high latitudes. We also compare the subsurface velocities calculated with our model using AVISO data and velocities from current meters. Results show that the model gives reasonably accurate results away from the top and bottom boundaries, side boundaries and far from western boundary currents. We found for the regions where the model is valid, an energy partition of the traditional modes of approximately 65 % in the barotropic mode and 25 % in the first baroclinic mode and remaining 10 % in higher modes.
Sandrine Djakouré
Numerical analysis of ocean circulation in the Northern Gulf of Guinea
with P. Penven, V. Koné, B. Bourlès
1. Institut de Recherche pour le Développement (IRD), LEGOS, Brest FRANCE.
2. Institut de Recherche pour le Développement (IRD), LPO, Brest FRANCE.
3. Centre de Recherche Océanologique (CRO), Abidjan COTE D'IVOIRE.
4. International Chair of Mathematical Physics and Applications, (ICMPA-UNESCO Chair CIPMA), University of Abomey-Calavi, Cotonou BENIN.
The ocean circulation and its variability in the Northern Gulf of Guinea has been found to modulate the amplitude of the African monsoon. Changes in Sea Surface Temperature due to coastal upwelling may also influence the regional climate. This upwelling is found along a zonal coast and its causes are still not clearly identified: local forcing (winds effect, Guinea Current, cape effect) or remote forcing (Kelvin waves generated at the equator). To document and study this particular coastal upwelling is thus relevant for climate dynamics and for local fisheries.
A modeling approach is used for a better understanding of the processes that lead to this coastal upwelling. A realistic configuration with the Regional Ocean Modeling System (ROMS) is built.
It is based on AGRIF (Adaptative Grid Refinement In Fortran) two-way nesting over the Tropical Atlantic (1/5) with a zoom in the Gulf of Guinea (1/15). Two different surface winds forcing are tested: COADS (Comprehensive Ocean Atmosphere Data Set) and the QuikSCAT scatterometer winds. The model is able to reproduce the mean circulation, the typical ocean patterns and their variability. According to observations from satellite and in situ data the QuikSCAT wind's are found to produce better results. Mesoscale cyclonic eddies seem to play a role on the regional dynamics. An idealistic configuration where the Cape Palmas and Cape of Three Points are removed is made to reveal their effects of the coastal upwelling. The model will also be used to investigate biogeochemical processes of the first trophic level in the Gulf of Guinea ecosystem.
Thierry Penduff Laboratoire de Glaciologie et de Géophysique de l’Environnement, CNRS, Grenoble, France
Intrinsic variability at large spatio-temporal scales in the eddying ocean
Future ocean-atmosphere coupled models used for climate studies and projections will include eddying rather than laminar oceans. Based on seasonally- and interannually-forced global ocean/sea—ice simulations, this study shows that increasing resolution from 2° to 1/4° and to 1/12° leads to the emergence of a strong, intermittent, intrinsic (nonlinearly-driven), low-frequency (interannual-to-multidecadal) oceanic variability at mid-latitudes. We discuss the link between this low-frequency intrinsic variability and the chaotic character of the ocean circulation in the interannually-forced eddying regime. We will particularly focus on the Atlantic Meridional Overturning Circulation, Sea-Surface Height and Temperature, whose variability is being monitored, and whose direct forcing by the atmosphere is partly questioned by our results. The chaotic character of the intrinsic AMOC and SST low-frequency variabilities may, in turn, impact the atmosphere and the climate in fut! ure coupl ed simulations that resolve mesoscale eddies.
Sabrina Speich Ecole Normale Supérieure, Paris, France
Revisiting interocean exchanges within the global ocean circulation
Within the last 5-10 years high resolution numerical studies have highlighted the significant impact of mesoscale and submesoscale interactions and processes on lateral and vertical exchanges within the ocean and on air sea interactions. Eddying motions on these scales stir tracers, directly impacting meridional heat and freshwater fluxes, and organizing the biogeochemical system in terms of biodiversity, including the pelagic system and their associated predators (such as elephant seals and sea birds). Past studies, have demonstrated that eddy stirring is particularly effective at driving compensated T-S distributions, characterized by fine filaments in the temperature and salinity fields that have no impact on density. Also, recent works have revealed that strong Sea Surface Temperature (SST) fronts, on the scale of a Western Boundary Current, significantly affect not just the Marine Boundary Layer but also the troposphere. This has aroused renewed interest in air-sea inte! ractions at these fine scales.
Most of these studies have been and are still conducted in academical or simplified frameworks. Here, we present the state of the art of our knowledge and perspective studies on these scales that seems to profoundly impact the regional as well as the global ocean circulation via turbulent interocean exchanges. A particular emphasis will be given to the Indo-Atlantic exchanges south of Africa and within the Southern Ocean. This presentation will also show how important is the complementary approach of implementing both, models and observations, a concept that was so important in the approach to science of Lien.
The work is based on various Master, PhD studies and many collaborations within international projects that have been taking place since the early 2000s.
Session 2 : Meso and Sub-Mesoscale Turbulence
Hidenori Aiki Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan
A new expression for the form stress term in the vertically Lagrangian mean framework for the effect of surface waves on the upper-ocean circulation
with Richard J. Greatbatch (GEOMAR)
There is an ongoing discussion in the community concerning the wave-averaged momentum equations in the hybrid vertically Lagrangian and horizontally Eulerian (VL) framework and, in particular, the form stress term (representing the residual effect of pressure perturbations) that is thought to restrict the handling of higher-order waves in terms of a perturbation expansion. The present study shows that the traditional pressure-based form stress term can be transformed into a set of terms that do not contain any pressure quantities but do contain the time derivative of a wave-induced velocity. This wave-induced velocity is referred to as the pseudomomentum in the VL framework, as it is analogous to the generalized pseudomomentum in Andrews and McIntyre. This enables the second expression for the wave-averaged momentum equations in the VL framework (this time for the development of the total transport velocity minus the VL pseudomomentum) to be derived together with the vortex ! force. Th e velocity-based expression of the form stress term also contains the residual effect of the turbulent viscosity, which is useful for understanding the dissipation of wave energy leading to a transfer of momentum from waves to circulation. It is found that the concept of the virtual wave stress of Longuet-Higgins is applicable to quite general situations: it does not matter whether there is wind forcing or not, the waves can have slow variations, and the viscosity coefficient can vary in the vertical. These results provide a basis for revisiting the surface boundary condition used in numerical circulation models.
Anthony Bosse Laboratoire d’Océanographie et du Climat, UPMC, Paris, France
Submesoscale frontal processes at the margin of a deep convection area: a case study in the NW Mediterranean Sea
with Pierre Testor, Laurent Mortier, Pierre Damien, Claude Estournel, Patrick Marsaleix, Louis Prieur
The NW Mediterranean Sea is known to be one of the few places of the world ocean, where open-ocean deep convection can occur. This convection is triggered in winter by intense dry northerlies blowing over the ocean surface leading to strong buoyancy loss in the Gulf of Lions area. The deep convection takes place in the center of a cyclonic gyre whose northern part is the Northern Current (NC) and its southern part is associated with the North-Balearic Front (NBF). These two are related to density fronts that are especially enhanced during period of deep convection when the mixed layer depth reaches great depths offshore, thus leading to a rich submesoscale frontal activity.
Since 2010, sustained observations of the circulation and water properties of the NW Mediterranean Sea have been carried out by gliders within the framework of the MOOSE project (Mediterranean Ocean Observatory System for the Environment: http://www.moose-network.fr/). During wintertime, they regularly sampled the NC, the deep convection zone as well as the NBF. In gliders data, the NC front and the NBF both exhibit clear evidences of vertical exchanges occurring along isopycnals of the density fronts. In order to investigate the dynamical characteristics of the front, we estimated of the Ertel potential vorticity based on the gliders data. In particular, we found layers of negative potential vorticity that can extend down to about 100m depth within the stratified part of the NC front. Under such specific conditions, symmetric instability might be able to growth and result in along isopycnal overturning circulations at a prescribed wave length, which is well correlated with! the glid er observations.
To further assess the ability of the glider to accurately estimate the potential vorticity at the front and the driving mechanisms being able to significantly extract potential vorticity and trigger frontal instabilities, we use the numerical model Symphonie in a regional configuration of the NW Mediterranean Sea at 1km horizontal resolution, which is able to well represent the deep convection processes, as well as submesoscale circulation features. In the model, we found that during episodes of strong northerlies potential vorticity is significantly extracted at the front as a result of down-front winds, especially in the western part of the Gulf of Lions. This leads to negative potential vorticity band extended in the area of dow-front winds down to about 100m depth that last for the short period of a few days. This is similar to the observations made by gliders and could thus explain the initiation of vertical exchanges of oceanic tracers at the front. These vertical circ! ulations might also have an impact on the heat transport of the NC and on the biogeochmistry of the area.
Pascale Bouruet-Aubertot Laboratoire d’Océanographie et du Climat, UPMC, Paris, France
Stratified inertial subrange inferred from in-situ measurements in the bottom boundary layer of Rockall channel
Deep-ocean high-resolution moored temperature data are analyzed with a focus on superbuoyant frequencies. A local Taylor hypothesis based on the horizontal velocity averaged over 2 h is used to infer horizontal wavenumber spectra of temperature variance. The inertial subrange extends over fairly low horizontal wavenumbers, typically within 2.e-3 and 2.e-1 cycles per minute (cpm). It is therefore interpreted as a stratified inertial subrange for most of this wavenumber interval, whereas in some cases the convective inertial subrange is resolved as well. Kinetic energy dissipation rate is inferred using theoretical expressions for the stratified inertial subrange. A wide range of values within 10^-9 and 4.10^-7 m2 s^-3 is obtained for time periods either dominated by semidiurnal tides or by significant subinertial variability.A scaling for that depends on the potential energy within the inertio-gravity waves (IGW) frequency band PE_IGW and the buoyancy frequency N is proposed for these two cases.When semidiurnal tides dominate, ’(PE_IGW N)^3/2, whereas ’PE_IGW N in the presence of significant subinertial variability. This result is obtained for energy levels ranging from1 to 30 times the Garrett–Munk energy level and is in contrast with classical finescale parameterization inwhich ;(PE_IGW)^2 that applies far from energy sources. The specificities of the stratified bottom boundary layer, namely a weak stratification, may account for this difference.
Pascale Bouruet-Aubertot Laboratoire d’Océanographie et du Climat, UPMC, Paris, France
Finescale structures around mesoscale eddies in the Arabian Sea.
In the Sea of Oman and in Arabian Sea, the mesoscale variability is largely influenced by dynamically deep-reaching eddies. The Persian Gulf Water (PGW hereafter), which is produced by evaporation in the Persian Gulf and which then flows at 250-300 m depth along the coasts of Oman, is advected by currents around the mesoscale eddies. Hydrological sections of these eddies show thin layers around these eddies; these layers tierh contain PGW (warm, salty water) or the Indian Central Water Water (which is cold and fresh). To determine the origine of this finescale structures we performed differents analyses on three sections of the Physindien 2011 experiment: a first one in the Gulf of Oman (RGO), a second one south of Ras Al Hadd (RAH - the southeastern tip of the Arabian Peninsula) and a third one (ROR) offshore of Ras Al Hadd, a section; section ROR crossed a small lens of PGW.
A wavelet analysis was used to determine the horizontal and vertical length scales of these homogeneous layers. Some are consistent with thermohaline staircases, associated with salt fingering (wavelengths of 2 to 16 meters) and some have the same width all around the eddies (wavelength of 32 to 64 meters), in agreement with the work of Nguyen et al. (2012). The Turner angle was used to identify the mechanism at the origin of these structures : double diffusion or salt fingering.
Then, the spectra of isopycnics slopes were calculated (following Holbrook et al., 2013) to identify the type of turbulence at these scales : inertial-convective or inertial-diffusive. The limit between this two modes is manifested by a slope break in the spectrum. The power law in our data is k^{1/3} which corresponds to inertial-convective turbulence.
Then, to determine the regimes between differents scales, we compute the kinetic energy spectra in barotropic and baroclinic modes and the potential energy spectra, with our date. The power laws for our sections show a limit between scales above and below the deformation radius. In some sections, we found two slope breaks, one between the inverse cascade of energy to large scale (energy spectrum k_h^{-5/3}) and the cascade of enstrophy to small scales (energy spectrum k_h^{-3}). The second limit appears to lie between the mesoscale and submesoscale, where energy spectrum returns to k_h^{-5/3}.
Laura Cimoli Laboratoire de Physique des Oceans, UBO, Brest, France
Impacts of the shelf bathymetry on the cross shelf transport and coastal current stability
A numerical approach aimed at simulating coastal current dynamics within an idealized coastal channel model is here presented. This study falls under the framework of the French project SYNBIOS (Submeso scale dYNamics and BIOlogy on steep Slopes) and focuses on the impacts of the shelf bathymetry on the surface current stability. Its main objectives are: 1) to evaluate how the surface current characteristics and stability are modified when the bottom slope varies, and 2) to evaluate the typical size and intensity of meso and submeso scale eddies generated by unstable currents. Simulations are run with the regional model ROMS_AGRIF (Regional Oceanic Modeling System), with a configuration developed at LPO (Laboratoire de Physique des Oceans) to reproduce an idealized coastal area. A sensitivity assessment is performed studying the evolution of a coastal current jet with an initial velocity of 0.35 m/s. Coastal current sensitivity is explored by considering many different changes in the input parameters. In detail they are: width of the shelf, slope (starting from a flat bottom condition and increasing its value), total depth of the shelf, distance of the current jet from the coast. The growth rate of the current instability is estimated from the exponential growth of the perturbation kinetic energy (i.e. cross-shore kinetic energy) renormalized by the along shore kinetic energy (i.e. the average kinetic energy in the along-shore direction). Their ratio returns the dimensionless kinetic energy anomaly, which gives us information about the instability! and the formation of eddies. A first set of experiments shows that both the slope and the distance of the current jet from the coast clearly affect the stability of the current. Starting from a flat bottom profile and keeping the other parameters constant, an increasing in the slope value corresponds to a shift from an unstable current to a stable one, indicating that the slope has the potential capability to stabilize the current. The effect of the slope is also function of the distance from the coast: if the current jet is located too far from the maximum slope, the stabilizating effect generated by that specific bathymetry is lost. In this on-going study we are now performing new statistical analysis to compare the different experiments, focusing on the growth rate of the instability, the wavelength, and the surface vorticity. The future work will focus on these and new variables to compare the experiments and to quantify the typical size and intensity of the meso and submeso scale eddies.
Dhouha Ferjani Laboratoire de Physique des Oceans, UBO, Brest, France
Observed temporal correlations between mesoscale currents and near inertial wave and non-wave motions in the World Ocean.
Not all motions with near-inertial frequency are internal gravity waves. Here we use the polarization relations and rotary spectrum, applied to 5000 current meter records, representing 58000 months of data, throughout the World Ocean to differentiate between motions that are consistent or not with linear internal waves. A remarkable contrast was found between the wave and the non-wave components' temporal correlation with the monthly mean currents. The near-inertial internal wave energy was found to be weakly anticorrelated with the mesoscale energy while the near-inertial non-wave motion was positively correlated with the mesoscale energy as were the two counter-rotating near-inertial components.
Nicolas Kolodziejczyk Laboratoire d’Océanographie et du Climat, IPSL, Paris, Paris
Observation of the surface horizontal thermohaline variability at meso- to submesoscales in the North-Eastern Subtropical Atlantic
The seasonal variability of the surface horizontal thermohaline structure is investigated in the north Atlantic Surface Salinity Maximum (SSM) at length scales from 5 km to hundred km. The near surface temperature and salinity data from merchant ship thermosalinograph (TSG) transects across the Atlantic were used to compute the horizontal temperature, salinity and density fluctuation, and the density ratio in the northern subtropical-tropical Atlantic. During late winter, in north-eastern SSM, the thermohaline compensation is observed for wavelengths from 5-10 km to more than 200 km. In spite of large and sharp surface thermohaline fronts a very weak density surface horizontal gradient is observed. In contrast, in the southern SSM, at large scale (>200 km) the density ratio is controlled by the salinity gradient and the horizontal density gradient is sharpened by a constructive contribution of opposite salinity and temperature gradients. During summer in the north-eastern! SSM, the salinity and temperature are no more compensated due to strong atmospheric heating the upper ocean. In the north-eastern SSM, the wavenumber spectra of temperature, salinity and density are computed for late winter and summer TSG transects. During winter, steep density spectra at scale between 100-5 km is consistent with interior quasi-geostrophic turbulence with a slope close to k-3. In contrast, during summer, the density spectra between 200-5 km are more flatter.
Pierre L'Hégaret Laboratoire de Physique des Oceans, UBO, Brest, France
Seasonal mesoscale variability in the Northern Arabian Sea
The ocean circulation around the Arabian Peninsula is mainly dominated by the monsoon, from Southwest in winter, Northeast in summer. During the intermonsoon, the mesoscale variability is driven by eddies with a strong vertical influence. We focus here on the Northern Arabian Sea, which is connected to a strong evaporation basin, the Persian Gulf, via the Sea of Oman. The warm and salty water produced in the Persian Gulf (PGW for Persian Gulf Water) is advected around the eddies with differents paths for each seasons. Using monthly meaned altimetric and wind maps, climatological data for temperature and salinity, we describe the onset and evolution of the mesoscale eddies and other features in the Northern Arabian Sea. Those structures have a deep reaching impact on the water masses. From here we present the spring 2011 intermonsoon, during the PhysIndien experiment directed by the SHOM. EOFs extracted from altrimetric data show that this period is representative of the averaged situation. With a higher spatial resolution we focus on the advection of the PGW by the mesoscale gyres. Three of them, one cylonic and two anticyclonics, stay stationnary through the season along the Omani coast. They advect diluted PGW far South from its known climatological extension. As well they send the PGW vein through the Iranian coast and break the current in submesoscale structures; filaments and a lens of PGW were recorded during the experiment.
Francesco Ragone Meteorologisches Institut, Universität Hamburg, Hamburg, Germany
Surface Quasi and Semi-Geostrophic Dynamics
The Surface Quasi-Geostrophic approximation (SQG) has been used in order to investigate the formation of sharp fronts and singularities in a class of analogous equations that include the three dimensional incompressible Euler equations. The Semi-Geostrophic approximation (SG) on the other hand has been introduced in order to study frontogenesis in the atmosphere and is known to feature finite-time singular solutions. A Surface Semi-Geostrophic approximation (SSG) can be formulated imposing on SG analogous boundary conditions as in SQG. In this work we compare numerically Euler, SQG and SSG dynamics as models for submesoscale processes in the ocean, with focus on the formation of instabilities and quasi-singular solutions.
Guillaume Roullet Laboratoire de Physique des Oceans, UBO, Brest, France
2D turbulence with complicated boundaries
with J.C. McWilliams (UCLA) and C. Vic (LPO)
We examine the consequences of lateral viscous boundary layers on the 2D turbulence that arise in domains with complicated boundaries (headlands, bays etc). The study is carried out numerically with LES. The numerics has been carefully designed to ensure global conservation laws and minimize the extent of dissipation scales. The turbulence dramatically differs from the classical biperiodic case. The boundary layer detachments act as small-scale energy sources exciting the inverse cascade of energy. The detachment, very intermittent in time, always yields a net energy dissipation. In free decay, the final state depends on the effective numerical resolution: laminar for low Re and turbulent for large enough Re. In the forced case, the boundary layers allow the turbulence to reach a statistical steady state without any artificial hypo-viscosity. We discuss the implications for the oceanic mesoscale turbulence.
Yoshikazu Sasai Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan
Impacts of submesoscale dynamics on the phytoplankton field in the western North Pacific
A high-resolution coupled physical-biological model of the North Pacific is used to investigate the impacts of submesoscale (O(10km)) dynamics on the phytoplankton field in the Kuroshio Extension. The focus is on the seasonality of submesoscale dynamics effects on the surface phytoplankton patterns. Satellite ocean color imagery captures the energetic pattern of phytoplankton associated with the mesoscale and submesoscale features. The model presents the similar pattern of submesoscale variability in the surface phytoplankton field in early spring, but in fall, this submesoscale pattern is weak because the mesoscale variability is dominant. In winter, the energetic submesoscale variability plays a strong nitrate injection into the euphotic layer, and the distribution of phytoplankton is patchy with the finer-scale horizontal advection and the response time of growth. In fall, the small-scale variance of nitrate in the upper layer is introduced by the upwelling, and the phyto! plankton is large variance with the horizontal advection scale.
François Schmitt Laboratory of Oceanology and Geosciences, Wimereux, France
Analysis of turbulence intermittency and scaling regimes using Empirical Mode Decomposition and Arbitrary Order Spectral Analysis
with Yongxiang Huang (SIAMM, Shanghai University, China)
We consider here a time series of oceanic turbulence in the coastal waters of the eastern English Channel, recorded using an ADV. The turbulence intermittency is studied using a method we developed in the recent years: arbitrary order Hilbert spectral analysis associated with EMD. This method has been shown to be less influenced by periodic forcing such as tide and daily cycles, and can be used to separate waves and turbulence. We show an application of this methodology to newly acquired ADV time series.
References:
Huang Y., F. G. Schmitt, Z. Lu, Y. Liu, An amplitude-frequency study of turbulent scaling intermittency using Hilbert spectral analysis, EPL 84, 40010, 2008.
Schmitt FG, Y Huang, Z. Lu, Y. Liu, N. Fernandez, Analysis of turbulent fluctuations and their intermittency properties in the surf zone using empirical mode decomposition, Journal of Marine Systems 77, 473-481, 2009.
Huang, Y., F.G. Schmitt, J.-P. Hermand, Y. Gagne, Z. M. Lu, Y.L. Liu, Arbitrary order Hilbert spectral analysis for time series possessing scaling statistics: a comparison study with detrended fluctuation analysis and wavelet leaders, Physical Review E 84, 016208, 2011.
Huang, Yongxiang, François G Schmitt, Application of an empirical mode decomposition based time dependent intrinsic correlation to marine data, Journal of Marine Systems 130, 90-100, 2014.
francois.schmitt@univ-lille1.fr
Alexandre Stegner Laboratoire de Météorologie Dynamique, Ecole Polytechnique, Paris, France astegner@lmd.polytechnique.fr
Meso and sub meso scale dynamics of coastal current along a steep shelf bathymetry.
with G. Roullet, R. Pennel, F. Poulin
The impact of shelf slope on the stability of coastal currents and the nonlinear formation of coastal meanders and eddies are investigated by linear analysis, numerical simulations and laboratory experiments. The simulations were performed using the Regional Oceanic Modeling System (ROMS) and the laboratory experiments were achieved on the UME-LMD rotating platform. High resolution is achieved in both investigations (PIV measurements and numerical grid) in order to quantify accurately the meso and the sub meso scale structures. Our results confirm that the topographic parameter To (ratio between the shelf slope and the isopycnal slope of the current) is the relevant parameter to quantify the shelf impact on the linear and nonlinear dynamics of the surface current. When the evolution of the coastal current is controlled by the baroclinic instability, the increase of To yields a selection of smaller unstable wavelengths and a decrease of the unstable growth rates. For moderat! e To or w hen the current is slightly shifted off the shelf, we find that a secondary nonlinear processes may lead to smaller eddies. We exhibit here a new dynamical sequence, leading to the formation of sub meso scale structures over a steep shelf by splitting of mesoscale eddies. Finally, for finite values of To, a complete stabilization of the surface current can be reached.
Jean-François Ternon Institut de Recherche pour le Développement, Sète, France
Cruise proposal: Eddy-topography interaction South of Madagascar (MAD-Ridge project)
Jean-François TERNON, IRD, UMR 212 EME, Sète (France)
Steven HERBETTE, LMI ICEMASA, University of Cape Town (South-Africa), University of Brest (France)
The oceanic circulation south of Madagascar is highly turbulent, with a peak of eddy kinetic energy at the mesoscale. Satellite altimetry shows that coherent mesoscale eddies continuously flow over the Madagascar Ridge, a topographic high in the continuation of the island. These eddies are either formed further east, in the Indian Ocean, or generated at the retroflection of the East Madagascar Current.
There are several seamounts on the Madagascar Ridge, peaking at shallow depths below the sea surface, which provide suitable habitats for endemic species. Two of these, the Walter’s Schoals (33°S/44°E, peak at 15m) and a smaller one (27°30’S/46°25’E, peak at 200m) will receive special attention in the next three years regarding their great ecological interest: biodiversity hotpots, fishing areas, and seabird feeding grounds. Seamount ecosystems are particularly vulnerable to the increasing human pressure and recent initiatives aim to promote and design science-based regulations for their conservation. In this context, international sea-cruises are planned in 2014, 2015 and 2016 on these two seamounts. The following MAD-Ridge proposal (2016) will focus on the seamount located closer to Madagascar.
MAD-Ridge objectives consist in studying extensively the links between the physical environment and the marine biology, related to the interactions between mesoscale eddies and the seamount. Two legs are planned. The first one aims at monitoring the environment at the regional scale, by carrying out two transects crossing the seamount with CTD stations every 15 nautical miles. The second leg of the cruise wishes to focus on processes related to flow / topography interactions. CTD stations spaced by 2.5 nautical miles are planned nearby the seamount using a strategy adaptable to the eddy field during the cruise. ADCP moorings will also be deployed for a few months, north and south of the seamount (at depth ~500m). The proposed cruise is multi-disciplinary and involves scientists in physical and chemical
Jacques Verron Laboratoire de Glaciologie et de Géophysique de l’Environnement, CNRS, Grenoble, France
On the joint use of high resolution tracer images and altimetric data for the control of ocean circulations
avec Lucile Gaultier, CNRS / LGGE, Pierre Brasseur, CNRS / LGGE, Jean-Michel Brankart, CNRS / LGGE
Over the past two decades, altimetric satellites observed turbulent features of ocean dynamics at the mesoscale. High resolution sensors of tracers such as the Sea Surface Temperature or the Ocean Color reveal even smaller structures at the submesoscale, which are not seen by altimetry. The role of the submesoscale in the ocean may be very important for the dynamic at larger scales. Therefore, we must benefit from the two types of observations (mesoscale dynamic and submesoscale tracer image) to refine the estimation of the ocean circulation.
The goal of this study is to explore the feasibility of using tracer information at the submesoscales to complement the control of ocean dynamic fields that emerge from altimeter data analysis at larger scales. To do so, an image data assimilation strategy is developed in which a cost-function is built that minimizes the misfits between image of submesoscale flow structure and tracer images. In the present work, we have chosen as an image of submesoscale flow structure the Finite-Size Lyapunov Exponents (FSLE). The choice of FSLE as a proxy for tracers is motivated by d'Ovidio et al (2004), where similar patterns between tracers and FSLE images are evidenced.
A prerequisite to the study is that the relation between the ocean dynamics and FSLE can be inverted, in other words that the submesoscale information transmitted through the intermediate FSLE proxy is effective in controlling the ocean system. This assumption has been successfully tested on several regional pieces of the ocean. Using a strategy similar to the one used in Data Assimilation, the sensitivity of FSLE horizontal patterns to velocity errors is investigated. To do so, a Gaussian velocity error field is created using fifteen years of altimetric data. A cost function is then defined to measure the misfit between the FSLE computed using velocities with errors and the FSLE derived from a 'true' (error free) velocity.
It is found that a global minimum can be identified in the cost function proving that the inversion of FSLE is feasible. The next step is the inversion of submesoscale tracer information to correct a mesoscale altimetric field using real observation. The ocean dynamical variable to be corrected is the mesoscale altimetric velocity field using a high resolution tracer image. The strategy is similar to the one used to invert FSLE. The cost function measures the misfit between the FSLE derived from the altimetric velocity and the high resolution tracer image. Several test cases have been studied and demonstrating the success of the inversion of submesoscale tracer information to correct a mesoscale altimetric velocity field.
A high resolution physico-biogeochemical coupled model of process and a high resolution realistic model of the Solomon sea have been used to assess the error associated with the inversion. The efficiency of the correction on the oceanic circulation has also been demonstrated using these models.
These results show the feasibility of assimilating tracer submesoscales into ocean models for the control of mesoscale dynamics and larger scales as deduced from altimetry and therefore the benefit of the joint use of tracer image and altimetric data for the control of ocean circulations.
Session 3 : Stirring and Mixing in the Ocean
Andrea Gabrielski University of Hamburg, Hamburg, Germany
Anomalous dispersion of sea ice in the Fram Strait region
The Arctic is experiencing drastic changes in the recent years as sea ice is reacting very sensitively to climate change. The Fram Strait region is the main gateway for ice export from the Arctic into the North Atlantic and it is subject to a strong background flow due to strong winds and the East Greenland current (highest drift velocities in the whole Arctic). Lagrangian statistics are used to study the dispersion of sea ice in the Fram Strait region based on the trajectories of 51 ice buoys deployed in the winters of the years 2002, 2003, 2007, 2008 and 2009. It is shown that sea ice motion follows an anomalous dispersion growing with t^5/4, a scaling law characteristic for the dispersion in 2D turbulence in regions dominated by deformation, i.e. shear and divergence. This result is further supported by a non-Gaussian distribution of the displacements and a power spectrum with a slope close to -2.
Noé Lahaye Laboratoire de Météorologie Dynamique, ENS, Paris, France
Centrifugal, barotropic and baroclinic instabilities of isolated ageostrophic anticyclones in the two-layer rotating shallow-water model and their nonlinear saturation.
Mesoscale and submesoscales eddies are ubiquitous in the ocean, and much effort is dedicated to the analysis of such structures, especially in what concerns their stability.
While quasi-geostrophic (barotropic and baroclinic) instabilities have been well documented for the last decades, we have fully realized relatively recently that other ageostrophic instabilities -- in particular the centrifugal instability -- play a role in the submesoscale dynamics.
The importance of such a mechanism, besides being responsible for coherent vortex structure breakdown, resides in the fact that it produces overturning vertical motions that enhance strong mixing and energy dissipation.
The impact of the vertical structure of the flow (vertical shear and stratification) upon the centrifugal instablity is still an issue to be clarified.
In this paper, we study the instabilities of the isolated anticyclonic vortices in the 2-layer rotating shallow water model at Rossby numbers up to 2, with the main goal to understand the interplay between the classical centrifugal instability and other possible ageostrophic instabilities.
We find that different types of instabilities with low azimuthal wavenumbers exist, and may compete. In a wide range of parameters an asymmetric version of the standard centrifugal instability has larger growth rate than this latter.
The dependence of the instabilities on the parameters of the flow: Rossby and Burger numbers, vertical shear, and the ratios of the layers' thicknesses and densities is investigated. The zones of dominance of each instability are determined in the parameter space.
It is shown that density step tends to inhibit the centrifugal instabilities and may thus play a key role on the issue of the destabilization process in parameters regimes where they compete with the barotropic instability.
Nonlinear saturation of these instabilities is then studied with the help of a high-resolution finite-volume numerical scheme, by using the unstable modes identified from the linear stability analysis as initial conditions.
Differences in nonlinear development of the competing centrifugal and ageostrophic barotropic instabilities are evidenced.
François Schmitt Laboratory of Oceanology and Geosciences, Wimereux France
Particle dynamics in a coastal environment, using simultaneous LISST and ADCP measurements
P.R. Renosh（1), F.G. Schmitt(2), H. Loisel(3), X. Meriaux(3) and A. Sentchev(3)
(1) University of Lille 1, Laboratory of Oceanology and Geosciences, UMR 8187 LOG, 28 Avenue Foch, 62930 Wimereux, France.
(2) CNRS, Laboratory of Oceanology and Geosciences, UMR 8187 LOG, 28 Avenue Foch, 62930 Wimereux, France.
(3) ULCO, Laboratory of Oceanology and Geosciences, UMR 8187 LOG, 32 Avenue Foch, 62930 Wimereux, France.
This study describes physical processes (mainly the turbulence and re-suspension of particles due to turbulence) which control the micro scale variability of the bio-optical properties in highly turbid coastal waters. For this, time series analyses of different bio-optical (LISST measurements) and current velocity have been performed from moored stations in coastal waters. The data base gathers high frequency (1 Hz) simultaneous measurements performed during several days in a fixed point in the highly turbid coastal environments of the English Channel. We consider power spectra and intermittency properties, of particles and velocity time series.
Ref： Renosh, P.R., Schmitt F.G., Loisel H., Sentchev A., Mériaux X., High frequency variability of particle size distribution and its dependency on turbulence over the sea bottom during re-suspension processes, Continental Shelf Research 77, 51-60, 2014
Keywords: Coastal Environments, Particle dynamics, Turbulence, Intermittency.
Eunok Yim Laboratoire d’Hydrodynamique, Ecole polytechnique, Palaiseau, France
Stability of pancake vortices in stratified fluids
In stratified and rotating fluids, vortices have a pancake shape with a small thickness relative to their horizontal extent due to the stable density stratification. A famous example is the Mediterranean eddies (Meddies) formed by salty water flowing from Mediterranean sea to Atlantic ocean. Only few results exist on the stability of such pancake vortices (Nguyen et al., 2012; Hua et al., 2013; Negretti & Billant, 2013) while the two and three dimensional instabilities of columnar vortices are well-known. Here, we investigate the linear stability of an axisymmetric pancake vortex in stratified non-rotating fluids as a function of the stratification (Fh=Ω0/N where Ω0 is the maximum angular velocity of the vortex and N the Brunt-Väisälä frequency), aspect ratio (α=Z0/R0 where Z0 and R0 are the thickness and radius of the pancake vortex, respectively), and Reynolds number (Re). The angular velocity of the base state is chosen as Gaussian both in radial and vertical di! rections. Several types of instability are found for the azimuthal wavenumbers m=0,1,2 depending on the aspect ratio(α). These instabilities can be similar to the centrifugal and shear instabilities of columnar vortices but modified by the confinement due to the pancake shape. Other types of instabilities specific to the pancake shape also appear. The domain of existence and the physical mechanism of these instabilities will be discussed.
Session 4-5: Ocean Layering - Seismic Observations
Yannis Cuypers Laboratoire d’Océanographie et du Climat, UPMC, Paris, France
Observations of internal tides modulation by near-inertial internal waves during the Cirene experiment
The Cirene experiment was conducted in the southwestern tropical Indian Ocean in early 2007. High-resolution measurements during that cruise reveal energetic, semi-diurnal tides, generated at the nearby southwestern Indian Ocean Ridge. The passage of a developing cyclone during the cruise was also associated with energetic near-inertial internal wave (NIW) packets [Cuypers et al. 2013]. Here, we report a clear modulation of the short-scale semi-diurnal baroclinic tide amplitude by the longer-scale NIW down to 350 m, the maximum propagation depth of the NIW packet. Numerical integration of ray-tracing equations indicate that this amplitude modulation is associated with the focussing of internal tides rays by the time-varying NIWs current signal. Although the refraction of a small scale wave by NIW was previously described in theoretical and numerical studies [e.g. Broutman and Young , 1986, Vanderhoff et al 2008], it has to the best of our knowledge never been observed befo! ! re. The f ocussing of internal tides by NIWs results in a larger vertical shear, that could produce “bursts” of vertical mixing.
Berta Biescas Institute of Marine Sciences, Barcelona, Spain
Spectral analysis from inverted acoustic reflectivity data in the Gulf of Cadiz, NE Atlantic Ocean
with Claire Ménesguen, Jhon F. Mojica and Valentí Sallarès
Seismic oceanography (SO) detects the thermohaline structure of the ocean from acoustic reflectivity data, which have lateral resolution on the order of 10-100 m. Bach Lien Hua achieved several advances in this field relating the high lateral resolution observations of SO with numerical models of eddies. Spectral analysis from Fourier transforms and structure functions are calculated from inverted acoustic reflectivity data, following the work proposed by Ménesguen et al 2009. The inverted data correspond to the profile GOLR01 acquired in the GO Survey, April-June 2007 in the Gulf of Cadiz, NE Atlantic Ocean. The inverted method consists in the summation of high frequency ( > 10 Hz) acoustic impedance recovery from the reflectivity data and low frequency ( < 10 Hz) XBT impedance. Temperature and salinity data are then recover from impedance profiles using the GSW equations and an empirical TS relation of the area. The spectral analysis results from the inverted data ! are compa red with the results from the direct reflectivity and the main differences are discussed.
Ménesguen, C, Hua, B.L., Papenberg, C. Klaeschen, D., Geli, L. And Hobbs, R. 2009. Effect of bandwidth on seismic imaging of rotating stratified turbulence surrounding an anticyclonic eddy from field data and numerical simulations” Geophysical research Letters, Vo 36.
Thomas Meunier Laboratoire de Physique des Oceans, UBO, Brest, France
Tracer stirring around a meddy: The formation of layering
The dynamics of layering surrounding meddy-like vortex lenses is investigated from a tracer stirring point of view using a tracer advection model as well as Primitive Equation (PE) and Quasi Geostrophic (QG) models. Recent in situ data inside a meddy as well as high resolution PE simulations reveal the formation of highly density-compensated layers in temperature and salinity at the periphery of the vortex core, suggesting a predominant role of stirring in the layering process. Passive tracer experiments using a tangent-linear QG model confirm this essential aspect and show in more details the transformation of horizontal variability into vertically stacked layers. The time evolution of this process is quantified and a simple scaling is proposed and shown to describe precisely the thinning down of the layers as a function of the initial tracer column's horizontal width and the vertical shear of the azimuthal velocity. Non-linear ! QG simula tions are performed and analysed for comparison with the work of Hua et. al. [2013]. A step-by-step interpretation of these results on the evolution of layering is proposed in the context of tracer stirring.
Session 6: Scientific breakthroughs in Ocean Atmosphere Interactions
Peter Hitchcock DAMTP, University of Cambridge, Cambridge, UK
Stratospheric sudden warmings, wind forcing of the North Atlantic, and the suppression of planetary waves
The polar vortex in the Arctic stratosphere is observed to breakdown roughly two out of every three winters. The evolution of the stratospheric flow in some cases is highly predictable in the aftermath of these events, and has been shown to produce a southward shift in the Atlantic jet. These events are thus of considerable interest for seasonal forecasting. Simulations with coupled atmosphere-ocean models have shown an apparent further influence on the Atlantic meridional overturning circulation.
Carefully controlled experiments with a comprehensive stratosphere-resolving model in an AMIP configuration are presented which unambiguously establish the downward influence of the stratospheric circulation, and suggest a significant influence on the wind stresses in the North Atlantic. Planetary-scale Rossby waves, which are suppressed in the stratosphere following the breakdown of the polar vortex, play a key role in this downward influence. Results from a simple theoretical model of these waves are shown to explain some features of this suppression.
Patrick Hyder Met Office, Exeter, UK
Resolution-dependence of the representation of surface flux characteristics in a series of ocean-only and coupled NEMO simulations
with John Siddorn (Met Office), Andrew Coward (NOC), Joel Hirschi (NOC), David Munday (Oxford University), and Chun-lei Liu (Reading University).
We analyse a series of consistent 1, 1/4 and 1/12o ocean only NEMO-LIM simulations in conjunction with coupled 1 and 1/4o degree NEMO-CICE simulations. As expected, sea surface height variance, eddy kinetic energy, surface current speed and mean absolute vertical velocities all increase with increasing ocean resolution in the simulations.
We compare the models both with CORE II fluxes and with observationally estimated monthly mean fluxes derived from top of atmosphere net fluxes minus ERA Interim total energy divergences (by University of Reading). Mean fluxes in the boundary currents appear to improve as the ocean resolution is increased. As expected, high frequency (< 2 month period) flux variability increases with ocean resolution in eddying regions in all simulations However, there appears to be increased variability at inter-annual periods in eddying regions in the 1/4 and 1/12o degree eddying ocean-only runs which is not evident the 1o parameterised eddy runs. These regions correspond to regions where ocean variability is known to be dominated by intrinsic ocean variability. This increased interannual variability at 1/4o resolution compared to 1o is also evident in the coupled runs, which suggest that this intrinsic ocean variability may not be damped in a coupled system. We also examine fluxe! s composi ted by NAO states across the forced ocean-only models which also appear to show ocean resolution-dependent differences.
The current Met Office seasonal forecast system, which uses a 1/4o resolution ocean model, has considerably improved predictive skill for the winter mean NAO compared to a system employing a 1o resolution ocean model. Notwithstanding this, it also produces NAO signals with much smaller amplitude than the observed year-to-year fluctuations in the NAO. Planned coupled experiments with a 1/12o ocean model should help to determine whether further increasing ocean resolution might help to resolve this issue.
Milan Klöwer GEOMAR Helmholtz Centre for Ocean Research, Kiel, Germany
The Atlantic Meridional Overturning Circulation (AMOC) is an important driver of climate variability, both regionally and globally and on a large range of time scales from decadal to centennial and even longer. Direct measurements of the AMOC strength have only very recently become available. A novel method based on a global climate model is used to reconstruct decadal AMOC variability during 1900-2010 from the history of the North Atlantic Oscillation (NAO), the most important mode of atmospheric variability in the Atlantic sector. The reconstructed AMOC variability is consistent with that of the observed North Atlantic sea surface temperatures (SSTs). A large decadal predictability potential of North Atlantic SST is suggested, solely arising from the past history of the NAO. The Atlantic Multidecadal Oscillation (AMO), the leading mode of decadal North Atlantic SST variability, which is closely linked to societally important quantiti! es such a s Atlantic hurricane activity and Sahel rainfall, is forecast until 2031 using the reconstructed AMOC up to 2010. The present warm phase of the AMO is predicted to continue until the end of the next decade, but with a negative tendency after 2015.
Xavier Perrot Laboratoire de Météorologie Dynamique, ENS, Paris, France
Atmospheric response to meso-scale sea surface temperature (SST) anomalies
Recent studies showed that strong SST fronts, on the scale of the western boundary currents, strongly affect the planetary boundary layer (PBL) but also all the troposphere. This renewed the interest of air-sea interactions at oceanic meso-scales. Mainly two mechanisms are proposed in the litterature, the first one (due to Wallace et al 1989) is based on the destabilization of the PBL above SST anomalies, the second one (Lindzen and Nigam 1987) is based on the pressure anomalies linked to the atmosphere temperature adjustment to the SST. Those two mechanisms predict different response of the PBL to the SST. We did numerical studies with a meso-scale atmospheric model (WRF) forced by a constant SST to analyse the PBL wind response to prescribed SST anomalies. In the case of weak winds we show that the pressure adjustment process is accurate. On the contrary for stronger winds, depending on the direction of the wind versus the SST gradient, the vertical momentum mixing mechani! sm could be predominant. We present some interpretations of these results.
Nicolas Rascle Laboratoire d’océanographie Spatiale, Ifremer, Plouzané, France
Images of sea surface roughness are routinely obtained by Synthetic Aperture Radars (SAR) or by optical radiometers viewing areas in and around the sun glitter. Under low to moderate wind conditions, they provide spectacular observations of meso and submesoscale oceanic features at high resolution (100m).
We discuss here the physical content of those observations. Interacting with the surface wind waves, particular deformation properties of surface currents are responsible for those manifestations. We limit our discussion to the wave mean square slope (mss) variability, ignoring other sources of surface roughness variations. We show that vortical currents and currents with shear in the wind direction shall not be expressed in surface roughness images. Only divergent currents or currents with no divergence but strained in the wind direction can exhibit surface roughness signature. More specifically, non divergent currents might be traced with a $45^o$-sensitivity to the wind direction. We then show that the use of 2 orthogonal sensor look directions can help us to retrieve those 2 components of the surface current gradients, the divergence and the strain.
That information on the current deformation comes from surface roughness images. It can then be combined with simultaneous images of sea surface temperature to investigate, from an observational point of view, the link between surface tracer gradient and background current deformation, a problematic which has been recurrently investigated by Lien Hua and her collaborators.