Interactions of river discharge and tidal modulation in a tropical estuary, NE Brazil

Observations of thermohaline properties and currents were undertaken in the Curimatau River estuary (6 degrees 18'S), Rio Grande do Norte state (RN), Brazil, during consecutive neap-spring tidal cycles in the austral autumn rainy season. Highly asymmetric neap tide along channel velocities (-0.4 to 0.9 m s(-1)) and highly stratified conditions were generated by an increase of the buoyancy energy from the freshwater input (R-iE approximate to 5.6). During the spring-tidal cycle the river discharge decreased and the longitudinal velocity components were higher, less asymmetrical (-0.8 to 1.1 m s(-1)) and semi-diurnal, associated with moderately stratified conditions (R-iE approximate to 0.1) due to the increase of the kinetic tidal energy forcing mechanism. The overall salinity variation from surface to bottom during two tidal cycles was from 20.5 to 36.3 and 29 to 36.7 in the neap and spring tide experiments, respectively; in the last experiment, the tropical water (TW) mass intrusion was enhanced. The net salt transport reversed from down to up estuary during the neap and spring tide experiments, respectively, varied from 6.0 to -2.0 kg m(-1) s(-1), an indication of changes in the main forcing of the estuary dynamics. Evaluation of a classical steady analytical model, in comparison with nearly steady experimental vertical profiles of velocity, shows an agreement classifiable as reasonably fair.