Ramonville-Saint-Agne, France
Ramonville-Saint-Agne, France

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Escudier R.,CSIC - Mediterranean Institute for Advanced Studies | Escudier R.,CNRS Laboratory for Glaciology and Environmental Geophysics | Bouffard J.,Aix - Marseille University | Bouffard J.,CNRS Mediterranean Institute of Oceanography (MIO) | And 3 more authors.
Geophysical Research Letters | Year: 2013

We present an innovative approach to the generation of remotely sensed high-resolution sea surface topography that improves coastal and mesoscale dynamic characterization. This new method is applied for the period 2002-2010 in the northwestern Mediterranean Sea, an area marked by a small Rossby radius. The spectral content of the new mapped data is closer to that of the along-track signal and displays higher levels of energy in the mesoscale bandwidth with the probability distribution of the new velocity fields 30% closer to drifter estimations. The fields yield levels of eddy kinetic energy 25% higher than standard altimetry products, especially over regions regularly impacted by mesoscale instabilities. Moreover, qualitative and quantitative comparisons with drifters, glider, and satellite sea surface temperature observations further confirm that the new altimetry product provides, in many cases, a better representation of mesoscale features (more than 25% improvement in correlation with glider data during an experiment). © 2013 American Geophysical Union. All Rights Reserved.

Guinehut S.,CLS Space Oceanography Division | Dhomps A.-L.,LEGOS | Larnicol G.,CLS Space Oceanography Division | Le Traon P.-Y.,French Research Institute for Exploitation of the Sea
Ocean Science | Year: 2012

This paper describes an observation-based approach that efficiently combines the main components of the global ocean observing system using statistical methods. Accurate but sparse in situ temperature and salinity profiles (mainly from Argo for the last 10 yr) are merged with the lower accuracy but high-resolution synthetic data derived from satellite altimeter and sea surface temperature observations to provide global 3-D temperature and salinity fields at high temporal and spatial resolution. The first step of the method consists in deriving synthetic temperature fields from altimeter and sea surface temperature observations, and salinity fields from altimeter observations, through multiple/simple linear regression methods. The second step of the method consists in combining the synthetic fields with in situ temperature and salinity profiles using an optimal interpolation method. Results show the revolutionary nature of the Argo observing system. Argo observations now allow a global description of the statistical relationships that exist between surface and subsurface fields needed for step 1 of the method, and can constrain the large-scale temperature and mainly salinity fields during step 2 of the method. Compared to the use of climatological estimates, results indicate that up to 50% of the variance of the temperature fields can be reconstructed from altimeter and sea surface temperature observations and a statistical method. For salinity, only about 20 to 30% of the signal can be reconstructed from altimeter observations, making the in situ observing system essential for salinity estimates. The in situ observations (step 2 of the method) further reduce the differences between the gridded products and the observations by up to 20% for the temperature field in the mixed layer, and the main contribution is for salinity and the near surface layer with an improvement up to 30%. Compared to estimates derived using in situ observations only, the merged fields provide a better reconstruction of the high resolution temperature and salinity fields. This also holds for the large-scale and low-frequency fields thanks to a better reduction of the aliasing due to the mesoscale variability. Contribution of the merged fields is then illustrated to describe qualitatively the temperature variability patterns for the period from 1993 to 2009. © 2012 Author(s).

Rio M.-H.,CLS Space Oceanography Division | Pascual A.,CSIC - Mediterranean Institute for Advanced Studies | Poulain P.-M.,National Institute of Oceanography and Applied Geophysics - OGS | Menna M.,National Institute of Oceanography and Applied Geophysics - OGS | And 2 more authors.
Ocean Science | Year: 2014

The accurate knowledge of the ocean's mean dynamic topography (MDT) is a crucial issue for a number of oceanographic applications and, in some areas of the Mediterranean Sea, important limitations have been found pointing to the need of an upgrade. We present a new MDT that was computed for the Mediterranean Sea. It profits from improvements made possible by the use of extended data sets and refined processing. The updated data set spans the 1993-2012 period and consists of drifter velocities, altimetry data, hydrological profiles and model data. The methodology is similar to the previous MDT by Rio et al. (2007). However, in Rio et al. (2007) no hydrological profiles had been taken into account. This required the development of dedicated processing. A number of sensitivity studies have been carried out to obtain the most accurate MDT as possible. The main results from these sensitivity studies are the following: moderate impact to the choice of correlation scales but almost negligible sensitivity to the choice of the first guess (model solution). A systematic external validation to independent data has been made to evaluate the performance of the new MDT. Compared to previous versions, SMDT-MED-2014 (Synthetic Mean Dynamic Topography of the MEDiterranean sea) features shorter-scale structures, which results in an altimeter velocity variance closer to the observed velocity variance and, at the same time, gives better Taylor skills.

Mendoza C.,Institute Ciencias Matematicas | Mancho A.M.,Institute Ciencias Matematicas | Rio M.-H.,CLS Space Oceanography Division
Nonlinear Processes in Geophysics | Year: 2010

In this article we explore the utility of dynamical systems tools for visualizing transport in oceanic flows described by data sets measured from satellites. In particular we have found the geometrical skeleton of some transport processes in the Kuroshio region. To this end we have computed the special hyperbolic trajectories, and identified them as distinguished hyperbolic trajectories, that act as organizing centres of the flow. We have computed their stable and unstable manifolds, and they reveal that the turnstile mechanism is at work during several spring months in the year 2003 across the Kuroshio current. We have found that near the hyperbolic trajectories takes place a filamentous transport front-cross the current that mixes waters from both sides. © 2010 Author(s).

Buongiorno Nardelli B.,CNR Institute of atmospheric Sciences and Climate | Buongiorno Nardelli B.,CNR Institute for Coastal Marine Environment | Guinehut S.,CLS Space Oceanography Division | Pascual A.,CSIC - Mediterranean Institute for Advanced Studies | And 3 more authors.
Ocean Science | Year: 2012

The MyOcean R&D project MESCLA (MEsoSCaLe dynamical Analysis through combined model, satellite and in situ data) was devoted to the high resolution 3-D retrieval of tracer and velocity fields in the oceans, based on the combination of in situ and satellite observations and quasi-geostrophic dynamical models. The retrieval techniques were also tested and compared with the output of a primitive equation model, with particular attention to the accuracy of the vertical velocity field as estimated through the Q vector formulation of the omega equation. The project focused on a test case, covering the region where the Gulf Stream separates from the US East Coast. This work demonstrated that innovative methods for the high resolution mapping of 3-D mesoscale dynamics from observations can be used to build the next generations of operational observation-based products. © 2012 Author(s).

Cabanes C.,French National Center for Scientific Research | Cabanes C.,Institut Universitaire de France | Grouazel A.,French National Center for Scientific Research | Von Schuckmann K.,French National Center for Scientific Research | And 12 more authors.
Ocean Science | Year: 2013

The French program Coriolis, as part of the French operational oceanographic system, produces the COriolis dataset for Re-Analysis (CORA) on a yearly basis. This dataset contains in-situ temperature and salinity profiles from different data types. The latest release CORA3 covers the period 1990 to 2010. Several tests have been developed to ensure a homogeneous quality control of the dataset and to meet the requirements of the physical ocean reanalysis activities (assimilation and validation). Improved tests include some simple tests based on comparison with climatology and a model background check based on a global ocean reanalysis. Visual quality control is performed on all suspicious temperature and salinity profiles identified by the tests, and quality flags are modified in the dataset if necessary. In addition, improved diagnostic tools have been developed - including global ocean indicators - which give information on the quality of the CORA3 dataset and its potential applications. CORA3 is available on request through the MyOcean Service Desk (http://www.myocean.eu/). © Author(s) 2013. CC Attribution 3.0 License.

Bouffard J.,CSIC - Mediterranean Institute for Advanced Studies | Bouffard J.,University of Toulon | Renault L.,CSIC - Mediterranean Institute for Advanced Studies | Ruiz S.,CSIC - Mediterranean Institute for Advanced Studies | And 3 more authors.
Progress in Oceanography | Year: 2012

The study of mesoscale and submesoscale [hereafter (sub)mesoscale] hydrodynamic features is essential for understanding thermal and biogeochemical exchanges between coastal areas and the open ocean. In this context, a glider mission was conducted in August 2008, closely co-located and almost simultaneously launched with a JASON 2 altimetric pass, to fully characterize the currents associated with regional (sub)mesoscale processes regularly observed to the north of Mallorca (Mediterranean Sea). A synoptic view from satellite remote-sensing fields, before and during the glider mission, provided a descriptive picture of the main surface dynamics at the Balearic Basin scale. To quantify the absolute surface geostrophic currents, the coastal altimetry-derived current computation was improved and cross-compared with its equivalent derived from glider measurements. Model simulations were then validated both qualitatively and statistically with the multi-sensor observations. The combined use of modeling and multi-sensor observational data reveals the baroclinic structure of the Balearic Current and the Northern Current and a small-scale anticyclonic eddy observed northeast of the Mallorca coast (current ∼ 15 cm/s, <30 km in extent and >180 m deep). This mesoscale structure, partially intercepted by the glider and along-track altimetric measurements, is marked by relatively strong salinity gradients and not, as is more typical, temperature gradients. Finally, the use of the validated model simulation also shows that the geostrophic component of this small-scale eddy is controlled by sub-surface salinity gradients. We hypothesize that this structure contains recently modified Atlantic water arriving from the strait of Ibiza due to a northerly wind, which strengthens the northward geostrophic circulation. © 2012 Elsevier Ltd.

Feng H.,University of New Hampshire | Yao S.,University of New Hampshire | Li L.,University of New Hampshire | Tran N.,CLS Space Oceanography Division | And 2 more authors.
IEEE Geoscience and Remote Sensing Letters | Year: 2010

This letter presents a new nonparametric approach, based on spline (SP) regression, for estimating the satellite altimeter sea-state bias (SSB) correction. Model evaluation is performed with models derived from a local linear kernel (LK) smoothing, the method which is currently used to build operational altimeter SSB models. The key reasons for introducing this alternative approach for the SSB application are simplicity in accurate model generation, ease in model replication among altimeter research teams, reduced computational requirements, and its suitability for higher dimensional SSB estimation. It is shown that the SP- and LK-based SSB solutions are effectively equivalent within the data-dense portion, with an offset below 0.1 mm and a rms difference of 1.9 mm for the 2-D (wave height and wind speed) model. Small differences at the 15-mm level do exist in the case of low data density, particularly at low wind speed and high sea state. Overall, the SP model appears to more closely follow the bin-averaged SSB estimates. © 2010 IEEE.

Tran N.,CLS Space Oceanography Division | Labroue S.,CLS Space Oceanography Division | Philipps S.,CLS Space Oceanography Division | Bronner E.,French National Center for Space Studies | Picot N.,French National Center for Space Studies
Marine Geodesy | Year: 2010

We present a Jason-2 sea state bias (SSB) model from the first year of GDR products along with updates of Jason-1, TOPEX-A, and TOPEX-B solutions. Our re-evaluation of the Jason-1 SSB focused on data from the new version (C) of the GDR while for TOPEX, the updated solutions benefited from the availability of both a set of new precise orbits that are consistent with Jason-2 and an updated set of state-of-the-art geophysical corrections. Therefore, derived from homogeneous sea surface height (SSH) data to minimize geographically correlated SSH differences between missions, these empirical SSB models contribute to the consistency between Jason-2 and Jason-1 SSH data and between TOPEX and Jason-1 data during their respective tandem phase and thus to the stability of the whole altimetry system over the past 17-year period. © 2010, Taylor & Francis Group, LLC.

Dibarboure G.,CLS Space Oceanography Division | Labroue S.,CLS Space Oceanography Division | Ablain M.,CLS Space Oceanography Division | Fjortoft R.,French National Center for Space Studies | And 3 more authors.
IEEE Transactions on Geoscience and Remote Sensing | Year: 2012

This paper gives an overview of an empirical cross-calibration technique developed for the Surface Water Ocean Topography mission (SWOT). The method is here used to detect and to mitigate two spatially coherent errors in SWOT topography data: the baseline roll error whose signature is linear across track, and the baseline length error whose signature is quadratic across track. Assuming that topography data are corrupted by coherent error signatures that we can model, we extract the signatures, and we empirically use the error estimates to correct SWOT data. The cross-calibration is tackled with a two-step scheme. The first step is to get local estimates over cross-calibration zones, and the second step is to perform a global interpolation of local error estimates and to mitigate the error everywhere. Three methods are used to get local error estimates: 1) we remove a static first guess reference such as a digital elevation model, 2) we exploit overlapping diamonds between SWOT swaths, and 3) we exploit overlapping segments with traditional pulse-limited altimetry sensors. Then, the along-track propagation is performed taking the local estimates as an input, and an optimal interpolator (1-D objective analysis) constrained with a priori statistical knowledge of the problem. The rationale of this paper is to assume that SWOT's scientific requirements are met on all errors but the ones being cross-calibrated. In other words, the algorithms presented in this paper are not needed at this stage of the mission definition, and they are able to deal with higher error levels (e.g., if hardware constraints are relaxed and replaced by additional ground processing). Even in our most pessimistic theoretical scenarios of baseline roll and baseline length errors (up to 70 cm RMS of uncorrected topography error), the cross-calibration algorithm reduces coherent errors to less than 2 cm (outer edges of the swath). Residual errors are subcentimetric for very low-frequency errors (e.g., orbital revolution). Sensitivity tests highlight the benefits of using additional pulse-limited altimeters and optimal inversion schemes when the problem is more difficult to solve (e.g., wavelengths of less than 1000 km), but also to provide a geographically homogeneous correction that cannot be obtained with SWOT's sampling alone. © 2012 IEEE.

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