Tidal frequency dynamics of a southern Brazil coastal lagoon: choking and short period forced oscillations

Abstract

The Patos Lagoon is a choked, microtidal coastal lagoon situated in southern Brazil between 30uS and 32uS. The response of the lagoon to tidal oscillations is studied through data analysis and numerical modeling experiments. Two types of high frequency oscillations are observed in the tidal frequency band: mixed tides, predominantly diurnal; and forced oscillations having a period of 24 h occurring in the inner lagoon. In the southern portion of the lagoon, tides are selectively filtered by the entrance channel. The main diurnal constituent O1 is linearly attenuated as it progresses landwards. In the inner parts of the lagoon, 24-h oscillations are mainly forced by the combined effect of diurnal tides and sea breeze action. They are tied with a natural period of oscillation of 24 h. Results also indicate that these are not inertial frequency oscillations, despite the lagoon being placed in a critical 30uS area. The interaction between astronomical tides and meteorological effects produces a complicated picture for tidal forecasting derived from data collected inside the lagoon.