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Correntes superficiais na zona costeira ao largo de Sines: variabilidade e forçamento
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O objetivo deste estudo era separar e quantificar as contribuições do vento e da maré para a corrente oceânica à superfície num local perto da costa, utilizando dados de corrente de um correntómetro eletromagnético S4 e dados de vento de um anemógrafo. Os dados foram recolhidos ao largo de Sines, sobre a plataforma continental interna, na costa ocidental de Portugal Continental, no Verão de 2012. Durante a maior parte do período de registo verificaram-se as condições oceanográficas típicas de Verão para este local: o vento soprou sobretudo de norte (“nortada”) e a corrente foi essencialmente de norte para sul. Utilizaram-se vários métodos no processamento dos dados. Começou por se realizar a análise harmónica dos dados de corrente, de modo a tentar estimar a fração da corrente que era forçada pela maré. Esta análise calculou uma corrente de maré com um período predominante diurno; a sua componente mais forte foi K1, com uma amplitude de 3.67 cm/s. Compararam-se os resultados desta análise com os resultados de um modelo numérico dinâmico da maré oceânica (TPXO). Os resultados deste modelo apresentaram várias diferenças em relação aos resultados da análise harmónica. A principal diferença foi que, no modelo TPXO, o período predominante foi semidiurno e a componente M2 foi a mais intensa (amplitude de 0.73 cm/s). Realizou-se também a análise por onduletas (wavelets) dos dados de corrente e do vento. Com base nos resultados obtidos, aplicou-se de novo a análise harmónica aos dados de corrente, agora para um intervalo de tempo mais curto em que o efeito expectável do vento sobre a corrente à superfície fosse mais fraco. Concluiu-se que a corrente, em particular a sua componente norte-sul, foi mais influenciada pelo vento do que pela maré. No entanto, chegou-se também à conclusão que os dados de corrente de maré obtidos com a análise harmónica estavam provavelmente contaminados pelo vento, ou seja, não foi possível separar a frequência do ciclo diurno do vento e a frequência da componente diurna principal da maré (K1). Isso poderá ter levado a que os resultados obtidos com a aplicação da análise harmónica tivessem sempre uma componente diurna maior do que aquela que seria esperada com base nos resultados do modelo TPXO. Finalmente, apresentaram-se algumas sugestões para a resolução deste problema em estudos futuros.
The goal of this study was to separate and quantify the contributions of the wind and the tide for the oceanic surface current, at a location near the coast, using current data from an electromagnetic S4 current meter and wind data from an anemograph. The data were collected off the coast of Sines, over the inner continental shelf, in western mainland Portugal, during the summer of 2012. During most of the recorded period, the usual summertime oceanographic conditions in this location were observed: the wind blew mainly from North and the current flowed mainly from North to South. Several analysis methods were used to process the data. First, a harmonic analysis of the current data was carried out, to estimate the fraction of the current that was forced by the tides. This analysis computed a tidal current with a mainly diurnal period and its strongest component was K1, with an amplitude of 3.67 cm/s. These results were compared with the results of a dynamic numerical model of the oceanic tides (TPXO) with somewhat different results. The main difference was that, in the TPXO model, the dominant period was semidiurnal and M2 was the strongest component (amplitude of 0.73 cm/s). A wavelet analysis of the current and wind data was also carried out. Based on the previous results. the harmonic analysis of the current data was done again, now for a shorter period of time in which the expected effect of the wind on the surface current would be weaker. It was concluded that the current, in particular its North to South component, was more influenced by the wind than by the tide. However, it was also concluded that the current data obtained with the harmonic analysis was probably contaminated by the wind, in the sense that it was not possible to separate the frequency of the wind’s diurnal cycle and the frequency of the main diurnal component of the tide (K1). This may have caused the harmonic analysis results to have a bigger diurnal component than expected from the TPXO model results. Finally, some suggestions to solve this problem were presented.
The goal of this study was to separate and quantify the contributions of the wind and the tide for the oceanic surface current, at a location near the coast, using current data from an electromagnetic S4 current meter and wind data from an anemograph. The data were collected off the coast of Sines, over the inner continental shelf, in western mainland Portugal, during the summer of 2012. During most of the recorded period, the usual summertime oceanographic conditions in this location were observed: the wind blew mainly from North and the current flowed mainly from North to South. Several analysis methods were used to process the data. First, a harmonic analysis of the current data was carried out, to estimate the fraction of the current that was forced by the tides. This analysis computed a tidal current with a mainly diurnal period and its strongest component was K1, with an amplitude of 3.67 cm/s. These results were compared with the results of a dynamic numerical model of the oceanic tides (TPXO) with somewhat different results. The main difference was that, in the TPXO model, the dominant period was semidiurnal and M2 was the strongest component (amplitude of 0.73 cm/s). A wavelet analysis of the current and wind data was also carried out. Based on the previous results. the harmonic analysis of the current data was done again, now for a shorter period of time in which the expected effect of the wind on the surface current would be weaker. It was concluded that the current, in particular its North to South component, was more influenced by the wind than by the tide. However, it was also concluded that the current data obtained with the harmonic analysis was probably contaminated by the wind, in the sense that it was not possible to separate the frequency of the wind’s diurnal cycle and the frequency of the main diurnal component of the tide (K1). This may have caused the harmonic analysis results to have a bigger diurnal component than expected from the TPXO model results. Finally, some suggestions to solve this problem were presented.
Descrição
Tese de mestrado, Ciências Geofísicas (Oceanografia) Universidade de Lisboa, Faculdade de Ciências, 2018
Palavras-chave
Oceanografia costeira Correntes de maré Correntes induzidas pelo vento Teses de mestrado - 2018