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Andrade et al.: Sea level oscillations in shallow waters 245 Tidal and subtidal oscillations in a shallow water system in southern Brazil Sea level oscillations in time scales between hours and days have a great vertical amplitude regarding the low lying coastal gradient of the beaches in the Rio Grande do Sul coast. However, the mechanism of oscillations is poorly understood since the scarcity of observational data makes it impossible to determine the forces that control sea level oscillations. Therefore, hourly sea level and wind time series with a time period of 650 days were examined. It has been found that the mean tidal and subtidal amplitudes were very similar to each other and that a considerable portion of the energy from sea level oscillations was due to astronomical forces. A new perspective was introduced when high and low frequencies were compared, leading to the comprehension that astronomical tides should be considered in coastal studies in southern Brazil. The sea level time series analyzed in this study showed that the maximum amplitude of the high spring tide was 0.53m, and that the subtidal rise caused by the wind reached up to 0.66m. In general, when large tidal and subtidal amplitudes are added, it can generate extreme events of sea level rise on the coast, which constitute a direct threat to coastal communities and habitats.

Abstract. The Southern Brazilian Continental Shelf (SBCS) is an area with great ecological and economic importance to Brazil. In this region can be observed the recurrent passage of frontal systems and extra-tropical cyclones, which are more frequent during the winter months of the southern hemisphere. These systems act on the ocean surface layers as direct driving forces, which may change the thermohaline structure of the water column and induce sea level perturbations. This study used the coupled ocean-atmosphere regional model (COAWST) to evaluate the effect of the passage of a frontal system associated with an extra-tropical cyclone. The ROMS oceanic model was configured with two nested grids, in order to solve the hydrodynamic processes at different scales. The parent (20–40° S/40–60° W) and child (25–29.3° S/46.3–50° W) grid comprise the coastal region, with a horizontal resolution of 1/9° and 1/27°, respectively, with 32 vertical levels. The initial conditions are the Global Analysis Forecast from CMEMS and forcing files used the Climate Forecast System v.2 (CFSv2) data, from NCEP. This event took place on the continental shelf of the State of Santa Catarina, in September of 2016. The model results were compared to remote sensing data and to the tide gauges from the City of Imbituba (State of Santa Catarina, Brazil). The comparison showed a correlation higher than 78 % between sea level rise data and the model results. The filtering of sea level data made it possible to identify the meteorological component in the model results. The comparison between the tidal-gauge and the model output presented values under 25 cm. The model was capable of representing the sea level anomalies propagation associated with the passage of the atmospheric frontal system. The model output showed the presence of a sea level anomaly propagating northward along the continental shelf at 480 km day−1, probably associated with the presence of a coastal-trapped wave.

Estuaries are, by definition, very complex coastal systems. The connection with the sea and all the effects of this feature highlight the importance of studying and ac-

Os fenômenos da ressurgência e subsidência costeira em regiões com pequenas profundidades ainda são pouco compreendidos no litoral brasileiro. O presente trabalho visa obter um melhor entendimento das condições oceanográficas em águas rasas, tais como as forçantes das correntes costeiras e das oscilações do nível do mar. Um perfilador acústico de ondas e correntes, AWAC, da marca Nortek, foi fundeado a 11 m de profundidade ao largo da praia de Tramandaí, programado para registrar uma série contínua de dados de ondas e correntes, com início em 17 de dezembro de 2013 e término em 13 de março de 2014 (período do verão). Os resultados indicaram que a circulação tem inversões de curto período no sentido das correntes, ora para S, ora para N. Também foi possível verificar velocidades máximas de 0,6 e 0,9 m.s-1 no fundo e em superfície, respectivamente. Ademais, um fluxo resultante para S, de aproximadamente 0,018 m.s-1 foi calculado com base no perfil médio da coluna de água. Além disso, foram encontrados indicativos de ressurgência e subsidência costeira, através das análises da direção do vento e das correntes, das variações na temperatura da água e também do nível médio do mar.