Senegal-Mauritania wave and hydrodynamic hindcast models now available

MetOcean Solutions recently completed the development of high-resolution wave and hydrodynamic hindcast models offshore Senegal and Mauritania, West Africa.

“Senegal and Mauritania coastal areas are influenced by oceanographic processes, including tides, coastal upwelling/downwelling, eddies, internal waves, and highly-energetic wave conditions,” says Senior Oceanographer Dr Séverin Thiébaut. “Our challenge was to ensure the models would adequately replicate those multi-scale processes and both ambient and extreme metocean conditions.”

The Simulating WAves Nearshore (SWAN) model was used to resolve the wave climate and the Regional Ocean Modeling System (ROMS) was applied to simulate the hydrodynamic circulation. The technique implemented is known as ‘dynamical downscaling’, using information from large scale global models to drive regional/nearshore models at much higher resolution. All models were carefully calibrated with measured data from several current meters and wave buoys that were made available.

The SWAN model simulates the growth, refraction and decay of each frequency-direction component of the complete sea state, providing a realistic description of the wave field as it changes in time and space.

“In order to reliably replicate the regional and nearshore wave climate, the SWAN nests were defined with increasing resolutions of 5 km, 1 km and 100 m,” explains Séverin. “This approach allows the model to resolve fine-scale features near the coast while still accounting for remote influences to the area from far-field generated swell.”

Full spectral boundaries were prescribe from the MetOcean Solutions’ global wave model to the 5-km SWAN domain. The latter was used to force the boundaries of the 1-km SWAN domain, which in turn was applied to the boundaries of the high-resolution 100-m SWAN domain (Figure 1). All model nests were simulated in series over 39 years (1979 to 2017).

 Figure 1: Snapshot of modelled significant wave height from the 5-km resolution SWAN parent nest off the Senegal/Mauritania coasts, delimited by the outer rectangle on (a). Extents of the 1-km resolution child nest are represented by the outer and inner rectangles on (a) and (b), respectively. Extents of the 100-m resolution child nest are represented by the inner rectangle on (b).

Figure 1: Snapshot of modelled significant wave height from the 5-km resolution SWAN parent nest off the Senegal/Mauritania coasts, delimited by the outer rectangle on (a). Extents of the 1-km resolution child nest are represented by the outer and inner rectangles on (a) and (b), respectively. Extents of the 100-m resolution child nest are represented by the inner rectangle on (b).

This ROMS model is an open source state of the art ocean model which has been used widely in the scientific community and industry for a range of ocean basin, regional and coastal scales. ROMS has a curvilinear horizontal coordinate system and solves the hydrostatic, primitive equations subject to a free-surface condition. Its terrain-following vertical coordinate system results in accurate modelling of areas of variable bathymetry, allowing the vertical resolution to be inversely proportional to the local depth. Two ROMS nests were defined with horizontal resolutions of approximately 6 km and 2 km for the regional and local model grid domains, respectively, as shown in Figure 2.

 Figure 2: ROMS (a) regional (6 km) and (b) local (2km) computational model grids. The red lines illustrate the transect corresponding to the vertical sigma grid structures provided in the following figure. Note the bathymetry is represented with distinct colorbars in (a) and (b).

Figure 2: ROMS (a) regional (6 km) and (b) local (2km) computational model grids. The red lines illustrate the transect corresponding to the vertical sigma grid structures provided in the following figure. Note the bathymetry is represented with distinct colorbars in (a) and (b).

The terrain-following grid configuration consisted of 30 and 23 vertical levels with increased resolution at surface and near-bottom to better represent the boundary layers (Figure 3). The model was produced over 25 years (1993 to 2017), delivering 3-dimensional current, water temperature and salinity and sea surface elevation data.

 Figure 3: Representation of the 30 vertical sigma levels of the regional grid domain over a cross-shelf transect along the latitude of 16.064०N.

Figure 3: Representation of the 30 vertical sigma levels of the regional grid domain over a cross-shelf transect along the latitude of 16.064०N.

Hindcast datasets offer key baseline information for project scoping, offshore and coastal design, project planning and environmental impact assessments.

For further information about MetOcean Solutions hindcast datasets, please contact hindcast@metocean.co.nz.