Gravity Waves Modelling Model Outputs Analysis Coastal Altimetry 3D Circulations Sediment transport, Ecology Data Assimilation    

  The Coastal Oceanography group POC was formed in 2001 and is constituted of scientists from LEGOS and LA. POC's primary objective is the study of ocean circulation at regional and coastal scales. The approach is essentially based on numerical modelling, by means of the in-house models Symphonie and T-UGO. Substantial effort is focused on model development, such as in the investigation of several grid-types (C-grid, hybrid coordinates and Unstructured grids), as well as the application of different hypotheses (hydrostatic, non-hydrostatic).

   To date, the POC models T-UGO and Symphonie have been used to study gravity waves, wind-induced currents, river plumes, along-slope circulations, dense water formation. Several POC-members are involved in biogeochemical and sediment transport modelling. This field is particularly challenging due to the complexity of ecological processes and their strong dependence on physical conditions. This requires close collaboration with the POC ocean physics modellers, notably in coupling pelagic models with the POC ocean circulation models. Furthermore, POC closely collaborates with external groups involved in observational studies, which is particularly important for model validation and physical and/or ecological interpretation.

  Data assimilation is also an important research topic. In the first few years of POC's existence data assimilation was mainly used for optimisation of the barotropic tide model. At the present time, 3D regional applications are conducted. Since it is based on EOF analysis of ensembles of runs, this branch requires substantial computing resources provided by a cluster of linux-pc's. Satellite altimetry data is an important potential source of coastal observational data, but several methodological problems specific to the coastal zone are to be overcome. Therefore, POC develops new methods for satellite altimetry data treatment.

Modelling:

  The 3D circulation models are based on finite difference and finite volume models developed by the group. The vertical coordinates, lateral boundary conditions and the formulation of air-sea fluxes are specific points of detailed examination. Variational tools devoted to the initialisation of models from observations and/or OGCM output are an originality of our group.
The modelling strategy is based on grid-nesting from the regional scale that is forced by OGCM data up to the chosen scale. The resolution ratio of successive grid meshes varies between 2 and 5.

Model Outputs Analysis:

  The model output is analysed using various tools, involving from wavelet to EOF-analysis, and combinations thereof. In practice, our approach is based on the analysis of ensembles (or series) of simulations. In order to provide meaningful statistics, an ensemble should contain on the order of 100 runs, for which a linux cluster is used.


Coastal Altimetry:

   Satellite altimetry data is generally used the framework of offshore circulation studies. Indeed, in coastal zones the use of altimetry data is subject to technical constraints involving continental influence (making radiometric corrections tricky), the resolution of geophysical corrections (high-frequency sea level variations associated with tides and atmospheric pressure forcing), uncertainties in mean sea level and, finally, the spatial and temporal resolution, which is low with respect to the scale of variability of coastal ocean processes. However, on the one hand post-processing of altimetry data for coastal research purposes has seen recent improvements and on the other, new satellite projects will provide an improved spatial description of the sea surface.

Data Assimilation:

  Data assimilation is required for short-term forecasting but also to get realistic simulations at annual or multi-annual time-scales. It is a necessary condition to accurately quantify the budgets of carbon and associated elements in the coastal zone. We develop ensemble methods which allow for empirical characterisation of model errors, and their association with errors in forcing terms. The application of these methods to 2D and 3D models is well-adapted to the complexity of coastal physics.

Operational Oceanography:

  In collaboration with the engineering company Noveltis, the POC has developed an operational system which provides weekly predictions of the ocean circulation in the North-western Mediterranean.		 Gravity Waves:
  
  We feel it necessary to simultaneously take into account high frequency (external gravity waves induced by tide and atmospheric pressure) as well as low frequency dynamics (circulations induced by wind and density gradients) in order to fully represent processes such as turbulent mixing and the generation of internal gravity waves. Thus, we develop a large-scale barotropic modelling approach which provides boundary conditions to coastal models, which allows the representation in the coastal zone of gravity waves formed far way.
An important effort is furthermore devoted to the accurate representation of sea level variations in the Mediterranean and on the European Atlantic shelf with the T-UGO-2D model. The non-linear tide is the object of particular attention. Finally, the generation of internal waves by the barotropic tide is studied in the Bay of Biscay using the Symphonie 3D model. A similar study starts in the Southwest Pacific with the finite volume model T-UGO-3D.

3D Circulations:

  Exchanges between the coastal zone and open sea, wind-induced circulation on the continental shelves, dilution of fluvial waters, interactions with topography, formation and cascading of dense water are some of the various applications of our circulation models. The regions under study notably include the Northwest Mediterranean, the Gulf of Thermaikos in the Aegean Sea, the Bay of Biscay and the Gulf of Lions.


Sediment transport, Ecology:

  The horizontal circulation and vertical motion are key to understanding transport and transformation of continental input and the cycle of major constituents in the coastal zone. The transport of sediment from rivers to the open sea is one of our research topics, for which associated models are developed. We also collaborate with colleagues from other groups to couple biogeochemical models with physics.
POC (Pôle Océan & Couplages) - Observatoire Midi-Pyrénées - 14 Avenue Edouard Belin - 31400 Toulouse - France