Accuracy Assessment of Global Internal-Tide Models Using Satellite Altimetry

Altimeter measurements are corrected for several geophysical parameters in order to access ocean signals of interest, like mesoscale or sub-mesoscale variability. The ocean tide is one of the most critical corrections due to the amplitude of the tidal elevations and to the aliasing phenomena of high-frequency signals into the lower-frequency band, but the internal-tide signatures at the ocean surface are not yet corrected globally. Internal tides can have a signature of several centimeters at the surface with wavelengths of about 50–250 km for the first mode and even smaller scales for higher-order modes. The goals of the upcoming Surface Water Ocean Topography (SWOT) mission and other high-resolution ocean measurements make the correction of these small-scale signals a challenge, as the correction of all tidal variability becomes mandatory to access accurate measurements of other oceanic signals. In this context, several scientific teams are working on the development of new internal-tide models, taking advantage of the very long altimeter time series now available, which represent an unprecedented and valuable global ocean database. The internal-tide models presented here focus on the coherent internal-tide signal and they are of three types: empirical models based upon analysis of existing altimeter missions, an assimilative model and a three-dimensional hydrodynamic model. A detailed comparison and validation of these internal-tide models is proposed using existing satellite altimeter databases. The analysis focuses on the four main tidal constituents: M2, K1, O1 and S2. The validation process is based on a statistical analysis of multi-mission altimetry including Jason-2 and Cryosphere Satellite-2 data. The results show a significant altimeter variance reduction when using internal-tide corrections in all ocean regions where internal tides are generating or propagating. A complementary spectral analysis also gives some estimation of the performance of each model as a function of wavelength and some insight into the residual non-stationary part of internal tides in the different regions of interest. This work led to the implementation of a new internal-tide correction (ZARON\u27one) in the next geophysical data records version-F (GDR-F) standards

Altimetry for the future: Building on 25 years of progress

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the ‘‘Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion

Altimetry for the future: building on 25 years of progress

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the “Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion

Etude et modélisation de la réponse haute fréquence de l'océan global aux forçages météorologiques

Sea level changes continuously due to a wide variety of complex processes such as ocean tides resulting from the gravitational attraction of the Moon and Sun, or atmospheric forcing from pressure and wind. Altimetry (T/P and Jason) now provides high-precision information on the height of the water column at a global scale. For the study of the mesoscale circulation, the high frequency ocean signal that are aliased in the lower frequencies must be corrected with independent and accurate models. The ocean tide correction has achieved the required accuracy, while the ocean response to atmospheric forcing is still approximated by the inverse barometer effect (IB). Our first objective is to propose a new high-precision global model forced by realistic meteorological fields, the second objective being to propose a correction of the high-frequency signal that is more efficient than the IB.We use the barotropic, time-stepping hydrodynamic model MOG2D-G, forced by ECMWF pressure and wind fields. Its main characterictic is a spatial finite element discretization, which allows to increase the resolution in shallow areas or steep bathymetry suitable for internal wave generation. The stability of the model is essentially controlled by a sub-time step process.A global simulation was carried out over the period 1992-2002. The model outputs are compared with in situ observations (tide gauges and bottom pressure sensors), and with altimetry data over the period 1993-2001. The correction given by the model reduces the variance by more than 50% at the tide gauges, and by 15% at the T/P crossover points, compared to a static correction of IB. The impact of the model differs according to the location: MOG2D-G very effectively reduces the variance at high latitudes, which are very energetic, while at low latitudes the results are close to the static response. The preponderant effect of wind forcing is clearly apparent. In conclusion, MOG2D-G allows to model the meteorologically forced variability of the global ocean with an unprecedented accuracy.Finally, we show the interest of MOG2D-G global simulations for applications concerning the impact of the barotropic ocean on the Earth's rotation or the calculation of the S1, S2 and Sa radiational tides.Le niveau de la mer change continuellement sous l'effet de processus très variés et complexes comme les marées océaniques résultant de l'attraction gravitationnelle de la Lune et du Soleil, ou les forçages atmosphériques de la pression et du vent. L'altimétrie (T/P et Jason) fournit aujourd'hui une information de haute précision sur la hauteur de la colonne d'eau à l'échelle globale. Pour l'étude de la circulation mésoéchelle, le signal océanique hautes fréquences aliasé dans les basses fréquences doit être corrigé avec des modèles indépendants et précis. La correction de marée océanique a atteint la précision requise, tandis que la réponse de l'océan au forçage atmosphérique est approximée par le baromètre inverse (IB). Notre objectif premier est de proposer un nouveau modèle global haute précision forcé par des champs météorologiques réalistes, le deuxième objectif étant de proposer une correction du signal haute fréquence plus performante que le IB.Nous utilisons le modèle hydrodynamique barotrope, time-stepping, MOG2D-G, forcé par les champs de pression et de vent ECMWF. Sa principale caractéristique est une discrétisation spatiale en éléments finis, qui permet d'augmenter la résolution dans les zones peu profondes ou de fortes pentes de la bathymétrie propices à la génération d'ondes internes. La stabilité du modèle est essentiellement contrôlée par un processus de sous pas de temps.Une simulation globale a été réalisée sur la période 1992-2002. Les sorties du modèles sont comparées aux observations in situ (marégraphes et capteurs de pressions de fond), et aux données altimétriques sur la période 1993-2001. La correction donnée par le modèle permet de réduire la variance de plus de 50% aux marégraphes, et de 15% aux points de croisement T/P, par rapport à une correction statique de IB. L'impact du modèle diffère suivant la localisation: MOG2D-G réduit très efficacement la variance aux hautes latitudes qui sont très énergétiques, tandis qu'aux basses latitudes les résultats sont proches de la réponse statique. L'effet prépondérant du forçage du vent apparaît clairement. En conclusion, MOG2D-G permet de modéliser la variabilité de l'océan global forcée par la météorologie avec une précision sans précédent.Nous montrons enfin l'intérêt des simulations globales MOG2D-G pour des applications concernant l'impact de l'océan barotrope sur la rotation de la Terre ou encore le calcul des marées radiationnelles S1, S2 et Sa

Etude et modélisation de la réponse haute fréquence de l'océan global aux forçages météorologiques

Sea level changes continuously due to a wide variety of complex processes such as ocean tides resulting from the gravitational attraction of the Moon and Sun, or atmospheric forcing from pressure and wind. Altimetry (T/P and Jason) now provides high-precision information on the height of the water column at a global scale. For the study of the mesoscale circulation, the high frequency ocean signal that are aliased in the lower frequencies must be corrected with independent and accurate models. The ocean tide correction has achieved the required accuracy, while the ocean response to atmospheric forcing is still approximated by the inverse barometer effect (IB). Our first objective is to propose a new high-precision global model forced by realistic meteorological fields, the second objective being to propose a correction of the high-frequency signal that is more efficient than the IB.We use the barotropic, time-stepping hydrodynamic model MOG2D-G, forced by ECMWF pressure and wind fields. Its main characterictic is a spatial finite element discretization, which allows to increase the resolution in shallow areas or steep bathymetry suitable for internal wave generation. The stability of the model is essentially controlled by a sub-time step process.A global simulation was carried out over the period 1992-2002. The model outputs are compared with in situ observations (tide gauges and bottom pressure sensors), and with altimetry data over the period 1993-2001. The correction given by the model reduces the variance by more than 50% at the tide gauges, and by 15% at the T/P crossover points, compared to a static correction of IB. The impact of the model differs according to the location: MOG2D-G very effectively reduces the variance at high latitudes, which are very energetic, while at low latitudes the results are close to the static response. The preponderant effect of wind forcing is clearly apparent. In conclusion, MOG2D-G allows to model the meteorologically forced variability of the global ocean with an unprecedented accuracy.Finally, we show the interest of MOG2D-G global simulations for applications concerning the impact of the barotropic ocean on the Earth's rotation or the calculation of the S1, S2 and Sa radiational tides.Le niveau de la mer change continuellement sous l'effet de processus très variés et complexes comme les marées océaniques résultant de l'attraction gravitationnelle de la Lune et du Soleil, ou les forçages atmosphériques de la pression et du vent. L'altimétrie (T/P et Jason) fournit aujourd'hui une information de haute précision sur la hauteur de la colonne d'eau à l'échelle globale. Pour l'étude de la circulation mésoéchelle, le signal océanique hautes fréquences aliasé dans les basses fréquences doit être corrigé avec des modèles indépendants et précis. La correction de marée océanique a atteint la précision requise, tandis que la réponse de l'océan au forçage atmosphérique est approximée par le baromètre inverse (IB). Notre objectif premier est de proposer un nouveau modèle global haute précision forcé par des champs météorologiques réalistes, le deuxième objectif étant de proposer une correction du signal haute fréquence plus performante que le IB.Nous utilisons le modèle hydrodynamique barotrope, time-stepping, MOG2D-G, forcé par les champs de pression et de vent ECMWF. Sa principale caractéristique est une discrétisation spatiale en éléments finis, qui permet d'augmenter la résolution dans les zones peu profondes ou de fortes pentes de la bathymétrie propices à la génération d'ondes internes. La stabilité du modèle est essentiellement contrôlée par un processus de sous pas de temps.Une simulation globale a été réalisée sur la période 1992-2002. Les sorties du modèles sont comparées aux observations in situ (marégraphes et capteurs de pressions de fond), et aux données altimétriques sur la période 1993-2001. La correction donnée par le modèle permet de réduire la variance de plus de 50% aux marégraphes, et de 15% aux points de croisement T/P, par rapport à une correction statique de IB. L'impact du modèle diffère suivant la localisation: MOG2D-G réduit très efficacement la variance aux hautes latitudes qui sont très énergétiques, tandis qu'aux basses latitudes les résultats sont proches de la réponse statique. L'effet prépondérant du forçage du vent apparaît clairement. En conclusion, MOG2D-G permet de modéliser la variabilité de l'océan global forcée par la météorologie avec une précision sans précédent.Nous montrons enfin l'intérêt des simulations globales MOG2D-G pour des applications concernant l'impact de l'océan barotrope sur la rotation de la Terre ou encore le calcul des marées radiationnelles S1, S2 et Sa

Etude et modélisation de la réponse haute fréquence de l'océan global aux forçages météorologiques

Sea level changes continuously due to a wide variety of complex processes such as ocean tides resulting from the gravitational attraction of the Moon and Sun, or atmospheric forcing from pressure and wind. Altimetry (T/P and Jason) now provides high-precision information on the height of the water column at a global scale. For the study of the mesoscale circulation, the high frequency ocean signal that are aliased in the lower frequencies must be corrected with independent and accurate models. The ocean tide correction has achieved the required accuracy, while the ocean response to atmospheric forcing is still approximated by the inverse barometer effect (IB). Our first objective is to propose a new high-precision global model forced by realistic meteorological fields, the second objective being to propose a correction of the high-frequency signal that is more efficient than the IB.We use the barotropic, time-stepping hydrodynamic model MOG2D-G, forced by ECMWF pressure and wind fields. Its main characterictic is a spatial finite element discretization, which allows to increase the resolution in shallow areas or steep bathymetry suitable for internal wave generation. The stability of the model is essentially controlled by a sub-time step process.A global simulation was carried out over the period 1992-2002. The model outputs are compared with in situ observations (tide gauges and bottom pressure sensors), and with altimetry data over the period 1993-2001. The correction given by the model reduces the variance by more than 50% at the tide gauges, and by 15% at the T/P crossover points, compared to a static correction of IB. The impact of the model differs according to the location: MOG2D-G very effectively reduces the variance at high latitudes, which are very energetic, while at low latitudes the results are close to the static response. The preponderant effect of wind forcing is clearly apparent. In conclusion, MOG2D-G allows to model the meteorologically forced variability of the global ocean with an unprecedented accuracy.Finally, we show the interest of MOG2D-G global simulations for applications concerning the impact of the barotropic ocean on the Earth's rotation or the calculation of the S1, S2 and Sa radiational tides.Le niveau de la mer change continuellement sous l'effet de processus très variés et complexes comme les marées océaniques résultant de l'attraction gravitationnelle de la Lune et du Soleil, ou les forçages atmosphériques de la pression et du vent. L'altimétrie (T/P et Jason) fournit aujourd'hui une information de haute précision sur la hauteur de la colonne d'eau à l'échelle globale. Pour l'étude de la circulation mésoéchelle, le signal océanique hautes fréquences aliasé dans les basses fréquences doit être corrigé avec des modèles indépendants et précis. La correction de marée océanique a atteint la précision requise, tandis que la réponse de l'océan au forçage atmosphérique est approximée par le baromètre inverse (IB). Notre objectif premier est de proposer un nouveau modèle global haute précision forcé par des champs météorologiques réalistes, le deuxième objectif étant de proposer une correction du signal haute fréquence plus performante que le IB.Nous utilisons le modèle hydrodynamique barotrope, time-stepping, MOG2D-G, forcé par les champs de pression et de vent ECMWF. Sa principale caractéristique est une discrétisation spatiale en éléments finis, qui permet d'augmenter la résolution dans les zones peu profondes ou de fortes pentes de la bathymétrie propices à la génération d'ondes internes. La stabilité du modèle est essentiellement contrôlée par un processus de sous pas de temps.Une simulation globale a été réalisée sur la période 1992-2002. Les sorties du modèles sont comparées aux observations in situ (marégraphes et capteurs de pressions de fond), et aux données altimétriques sur la période 1993-2001. La correction donnée par le modèle permet de réduire la variance de plus de 50% aux marégraphes, et de 15% aux points de croisement T/P, par rapport à une correction statique de IB. L'impact du modèle diffère suivant la localisation: MOG2D-G réduit très efficacement la variance aux hautes latitudes qui sont très énergétiques, tandis qu'aux basses latitudes les résultats sont proches de la réponse statique. L'effet prépondérant du forçage du vent apparaît clairement. En conclusion, MOG2D-G permet de modéliser la variabilité de l'océan global forcée par la météorologie avec une précision sans précédent.Nous montrons enfin l'intérêt des simulations globales MOG2D-G pour des applications concernant l'impact de l'océan barotrope sur la rotation de la Terre ou encore le calcul des marées radiationnelles S1, S2 et Sa

Etude et modélisation de la réponse haute fréquence de l'océan global aux forçages météorologiques

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Background stratification impacts on internal tide generation and abyssal propagation in the western equatorial Atlantic and the Bay of Biscay

International audienceAbstract. The forthcoming SWOT altimetric missions aim to resolve the mesoscale with an unprecedented spatial resolution and swath. However, high-frequency processes, such as tides, are undersampled in time and aliased onto lower frequencies, so they need to be corrected properly. Unlike barotropic tides, internal tides (ITs) are not completely stationary and have significant temporal variability due to their interactions with the ocean circulation and the stratification variability. Stratification changes impact both the generation and the propagation of ITs. The present study proposes a methodology to quantify the impacts of background stratification using a clustering method for the classification of a broad range of stratification and idealized modeling of ITs in the frequency domain. The methodology is successfully tested in the western equatorial Atlantic and in the Bay of Biscay. For the western equatorial Atlantic, a single pycnocline is observed and only the two first vertical modes of ITs have significant amplitudes. With no variation in the stratification intensity, the variation in the depth of this single pycnocline linearly impacts the elevation amplitude, energy fluxes and surface wavelength of the two modes. In the Bay of Biscay, there is a permanent deep pycnocline and secondary seasonal pycnoclines near the surface. No proxy have been found to describe the changes in ITs, so a seasonal climatology is explored. The seasonality of the stratification strongly affects the elevation amplitudes as well as the energy fluxes of modes 1, 2 and 3. The distribution of the modes vary with the background stratification, changing the horizontal scales of the ITs

Simultaneous estimation of ocean mesoscale and coherent internal tide sea surface height signatures from the global altimetry record

International audienceThis study proposes an approach to estimate the ocean sea surface height signature of coherent internal tides from a 25-year along-track altimetry record, with a single inversion over time, resolving both internal tide contributions and mesoscale eddy variability. The inversion is performed on a reduced-order basis of topography and practically achieved with a conjugate gradient. The particularity of this approach is to mitigate the potential aliasing effects between mesoscales and internal tide estimation from the uneven altimetry sampling (observing the sum of these components) by accounting for their statistics simultaneously, while other methods generally use a prior mesoscale. The four major tidal components are considered (M2, K1, S2, O1) over the period 1992-2017 on a global configuration. From the solution, we use altimetry data after 2017 for independent validation in order to evaluate the performance of the simultaneous inversion and compare it with an existing model