Competition Between Atmospheric and Tidally Attenuated Forcing in an Elongated Coastal Lagoon

Water level measurements at four sub-basins of a highly dissipative coastal lagoon were used to study variability at tidal and subtidal scales and to determine whether atmospheric forcing influenced variations in water level at tidal scales. The amplitude of the main astronomic constituents, which were diurnal, attenuated quickly to <5% by the second sub-basin. The attenuation was represented well by an analytical solution that described a damped wave. The semidiurnal constituent with a period of 12 h (S2), however, decreased to 50% of the lagoon’s entrance amplitude and then increased to 60% by the fourth sub-basin. Reproduction of this S2 behavior by the analytical model required unrealistically low signal attenuation, much lower than for the other semidiurnal constituent (12.42 h, M2). This behavior suggested that the S2 amplification was non-gravitational and related to semidiurnal atmospheric forcing, which was dominated by barometric pressure. Such suggestion was confirmed by coherence and wavelet coherence analysis and by the fact that the largest overtide was related to the interaction between S2, instead of M2, and the diurnal constituent. This study documents that a highly dissipative coastal lagoon becomes forced semidiurnally by barometric pressure toward its head. Subtidal water levels were studied with empirical orthogonal functions, revealing 2 modes that explained ~85% of the variance. Mode 1 was related to northerly winds (‘Nortes’) and mode 2 was linked to the Trade winds that dominate the region.