Influence of the Remote Forcing and Local Winds on the Barotropic Hydrodynamics of an Elongated Coastal Lagoon

摘要

The main objective of this study is to investigate the effects of local winds and sea-level variations at tidal and subtidal frequencies on the water levels, depth-averaged velocities, and volume fluxes of an elongated and shallow coastal lagoon using field measurements and a barotropic two-dimensional numerical model. Tides are mainly diurnal and attenuate similar to 85% from the mouth to the lagoon head, indicating a barotropic propagation dominated by friction. On the other hand, the low-frequency water-level fluctuations propagate with little attenuation (similar to 17%), dominating the water-level variability at the lagoon head. It is considered important to elucidate whether this low-frequency variability is related to local winds or to remote effects. Modeling results show that local winds can increase water levels up to similar to 1.9 cm in the lagoon head (5% of tide level), whereas remote sea-level variability accounts for up to similar to 26.6 cm (320% of astronomical tidal level), clearly dominating lagoon water levels. On the other hand, diurnal sea breezes play a minor role in water levels inside the lagoon and do not reinforce diurnal tides in order to generate an important set-up. The cumulative water volume is highly correlated with sea level and shows that low-frequency sea-level fluctuations are especially efficient at accumulating or exporting water from the lagoon during periods of ascending or descending sea level, respectively. This implies that water exchange between the lagoon and the coastal ocean is dominated not only by diurnal tides but also by the low-frequency sea-level fluctuations of astronomical and meteorological origin (i.e. wind forcing through Ekman transport).