Tidal amplification and distortion in Guinea-Bissau, West Africa

Understanding tidal behavior is of paramount importance in Guinea-Bissau, a vulnerable low-lying coastal zone with the highest tidal range of any West African country. Our aim is to improve tide prediction by analyzing its characteristics and mechanisms of amplification and distortion. To this end, we use a high-resolution coastal ocean model (CROCO) validated by tidal altimetry and new tide gauge data. We present and analyze the cotidal charts of the main constituents. The amplification occurs first in coastal waters due to resonance over the broad shelf and strengthens in the estuaries, especially in the Geba Channel where a tidal range of 7 m can be reached. This is due, in part, to the funneling of tidal energy, which travels from the south through the Bijagos Archipelago. There is also a resonance of a quarter wavelength (mode 0) for semi-diurnal tides. In smaller estuaries (e.g., Buba), there is no funneling, so the tides can only be amplified by resonance with reflected waves. A strong tidal asymmetry is also demonstrated due to the generation of overtides that are resonant with mode 1 at Geba and mode 0 at Buba. The asymmetry and Stokes drift associated with the progressive waves in the Geba Channel (leading to tidal setup) make it a complex local system with ebb dominance at the mouth, perhaps contributing to the continuous accretion in the Bijagos archipelago. Other smaller estuaries characterized by standing waves are more classically flood-dominated. All this affects the residence time of water along the coasts and estuaries of Guinea Bissau.