The spatial and temporal variability observed during the last 25 years at Punta Rasa sandspit is studied in this work. Five images Lansat TM (30 m spatial resolution) corresponding to the years 1987, 1994, 2001, 2007 and 2011 were selected in this study. Each one covers an area of 185 km x 172 km but, for this analysis, a smaller area from latitude 36°19´40" to 36°17´08" S and from longitude 56°44´10" to 56°47´25" S was selected. A remarkable retrogression of the sandspit and a gradual widening of the sector located adjacently southward the sandspit was observed by comparison of the selected images. The hypothesis discussed in this work is that such variability would be associated to a change in the wind wave climate which arose a slight change in the long shore wave energy flux and in sand transport. Finally, it is highlighted that the mean sea level rise and the increasing in frequencies and intensities of storm surges in the Río de la Plata could play an important role in the observed erosion.

En este trabajo se estudia la evolución espacio-temporal de la espiga arenosa de Punta Rasa-Punta Médanos durante los últimos 25 años. Se seleccionaron cinco imágenes Lansat TM con resolución espacial de 30 m, correspondientes a los años 1987, 1994, 2001, 2007 y 2011. Cada imagen cubre un área de aproximadamente 185 km x 172 km pero para este trabajo se recortó un área menor comprendida entre las latitudes 36°19´40" y 36°17´08" S y las longitudes 56°44´10" y 56°47´25" S. De la comparación de las imágenes satelitales seleccionadas se pudo observar la decapitación de la espiga arenosa acompañada del progresivo ensanchamiento del sector ubicado adyacentemente al S del extremo distal de la misma. La hipótesis que se discute en este trabajo es que dicha variabilidad estaría asociada a una alteración local del clima de olas el cual trajo aparejado un leve cambio del flujo de energía de olas y del transporte de arena paralelos a la costa. Finalmente, se destaca que el ascenso del nivel medio del mar y el aumento en la frecuencia e intensidad de la onda de tormenta en el Río de la Plata podrían contribuir al aumento de erosión observado.

Na região do Rio de la Plata, o desempenho das reanálises globais do NCEP/NCAR I, NCEP/DOE II, JRA-25 e ERAInterim implementadas como forçantes atmosféricas do modelo SWAN foram quantitativamente acessados através do viés, erro quadrático médio, coeficiente de determinação e inclinação da reta. Estes índices foram obtidos dos parâmetros de ondas observados e simulados (alturas significativas de ondas, período principal e direção). Embora as estimativas estatísticas não mostrem diferenças evidentes para períodos e direções, algumas diferenças notáveis foram obtidas para altura de ondas simuladas, dependendo do vento utilizado. O menor viés para altura significativa (0.22 m) foi obtido quando o SWAM foi forçado com a ERAInterim, enquanto o NCEP/NCAR I (0.16 m) e NCEP/DOE II (0.19 m) forneceram menor erro quadrático médio. A melhor inclinação da reta entre simulação e observação de altura significativa (0.79) foi obtida usando NCEP/DOE II. No período de estudo, o maior episódio de onda registrado na boca do Río de la Plata foi analisado e discutido. Neste evento de condições atmosféricas energéticas o melhor ajuste foi alcançado utilizando os ventos do NCEP/DOE II como forçante. Conclui-se que a base de dados NCEP/DOE II é forçante atmosférica mais adequada para simular alturas significativas de ondas com o modelo SWAN na região estudada.

The performance of NCEP/NCAR I, NCEP/DOE II, JRA-25 and ERA-Interim global databases, implemented as atmospheric forcings of the SWAN model in the Río de la Plata region, was quantitatively tested by calculating the bias, the mean square root error, the determination coefficient and the slope of the line fitted between observed and simulated wave parameters (significant wave height, mean period and direction). Even though statistical estimators showed no evident differences for wave periods and directions some noticeable differences were observed for simulated significant wave heights depending on the forcing used. The lowest bias (0.22 m) was obtained when the SWAN model was forced by ERA-Interim. With regard to the mean square root errors, the lowest values were obtained when NCEP/NCAR I (0.16 m) and NCEP/DOE II (0.19 m) were used as forcing. In addition, the best slope for simulated heights (0.79) was obtained using NCEP/DOE II. Computed determination coefficients for heights, periods and directions were very similar (0.89-0.93) for all the simulations carried out in this study. Energetic and severe wave events were given special consideration. The most energetic wave episode recorded in the Río de la Plata mouth (24 August, 2005) was analyzed and discussed in particular. It was concluded that during energetic atmospheric conditions the best agreement is achieved by implementing NCEP/DOE II as forcing. In the light of these results it is concluded that NCEP/DOE II is the most suitable atmospheric forcing to simulate wave heights with the SWAN model in the Río de la Plata region.

A slight southward shift of the South Atlantic semi-permanent high pressure system and an intensification of the eastern wind at the río de la Plata have been documented by several authors. A mild wind wave height increase has been detected at the Río de la Plata and the adjacent continental shelf, from 1970 to the present, as a consequence of the aforementioned climate variability. The most noticeable increase was observed between the 80’and 90’decades, in good agreement with a general increment of erosion at Samborombón Bay and neighboring areas. The annual mean wave height was investigated using numerical simulations. The highest differences in wind wave heights (0.20 m, 9%) were observed at 34° S - 48° W, approximately. The increase was a little lower (7%) at the continental shelf and the Río de la Plata. The annual mean of the simulated wave height showed significant positive trends at the Río de la Plata mouth, the Uruguayan coast and Mar del Plata. The wind wave energy density (annual mean, averaged at the computational domain) also presented a significant positive trend and a relatively high inter-annual variability. From the analysis concludes that overall the shoreline has retreated about 50 m in the last 49 years at Samborombón Bay.

El corrimiento del anticiclón semipermanente del Atlántico Sur hacia el S y una intensificación de los vientos del E en el río de la Plata ha sido documentada por diversos autores. Como consecuencia de esta variabilidad climática se ha detectado un leve incremento en las alturas de las olas desde 1970 al presente en el río de la Plata y océano adyacente. El incremento más notable se observó entre las décadas del 80’ y 90’ coincidentemente con el incremento de erosión en la bahía Samborombón. La altura de ola media anual se investigó mediante simulaciones numéricas. Las mayores diferencias en alturas (0.20 m, 9%) se observaron en 34° S - 48° O, aproximadamente. En la plataforma continental y en el río de la Plata el incremento fue algo menor (7%). Las medias anuales de las alturas de ola simuladas mostraron tendencias positivas significativas en Mar del Plata, la boca del río de la Plata y la costa Uruguaya. La densidad de energía de olas (media anual, espacialmente promediada) también presentó una tendencia positiva significativa y una relativamente alta variabilidad interanual. Los análisis se concluye que en general la línea de costa en la bahía Samborombón ha retrocedido unos 50 m en los últimos 49 años.

Laboratory experiments are crucial elements in the learning-teaching process of scientific courses. In this work some examples of didactic experiments with fluids under rotation are shown. They were developed with new equipments acquired in the frame of the Laboratory of Geophysical Fluids at the Department of Atmospheric and Oceanic Sciences of the Faculty Natural and Exact Sciences of the University of Buenos Aires. These experiments illustrate and help to understand the complex physical processes which lie behind theoretical results. Experiments motivate the curiosity of the students, make them ask questions and search for answers. The experiments are also useful to promote our sciences among high school students, a critical issue given the low number of undergraduate students.

El desarrollo de experiencias de laboratorio es un elemento crucial en el proceso de enseñanza-aprendizaje de las ciencias. En este trabajo se ilustran algunas experiencias básicas de la mecánica de fluidos geofísicos que resultaron muy útiles en dicho proceso. Las experiencias, que se realizaron en el marco del Taller de Experimentación en Fluidos Geofísicos del Departamento de Ciencias de la Atmósfera y los Océanos de la Facultad de Ciencias Exactas y Naturales de la Universidad de Buenos Aires, ayudan a comprender los procesos físicos que subyacen por detrás de resultados teóricos poco intuitivos, haciendo que el estudiante reavive su curiosidad, se plantee preguntas y se motive a encontrar respuestas. Los experimentos son muy valiosos, además, para la divulgación y extensión de las carreras de meteorología y oceanografía a la hora de despertar vocaciones, lo cual es un aspecto crítico para nuestras ciencias.