| Resumo: |
For the first time, the analysis of historical salinity data and new current series is combined with high resolution numerical simulations, with the aim of understanding the processes that occur in the active and important Río de la Plata estuary (South America). With that objective, the first relatively long ADCP current series collected in two points of the estuary are analyzed. Only 25% of the currents variance is accounted by the tide. Approximately another 25% is associated with internal wave activity at the sampling locations, where a strong stratification exists. These waves are forced by land-sea breeze and tide and are very frequent during spring and summer. The remaining 50% of the energy is wind driven and occurs in synoptic to intra-seasonal time scales. The estuary responds to wind variability in a time scale of approximately 6 hours. Response displays an essentially barotropic structure to winds with a dominant component perpendicular to the estuary axis and with a strong baroclinic structure, with inversion in current direction between upper and lower layers, to winds with a dominant component along the estuary axis. Although this kind of response is characteristic of semi-enclose basins, it is not a typical response in estuaries, and it is observed in the Río de la Plata due to its huge width. To evaluate the implications of the wind forced circulation on stratification, Estuary, Coastal and Ocean Model (ECOM) together with CTD data from synoptic campaigns were used. It is observed that, even though the salt wedge structure is a consequence of the intense discharge, its existence is favoured by winds that prevail in the region. The stratification can only be completely destroyed by strong or persistent south-easterly winds. Nevertheless, the structure is reconstructed in a relatively short period of time after wind relaxation, and that kind of events are not frequent in the region. This has a strong impact on biology because the species housed by the estuary require specific salinity conditions for their successful reproduction. Additionally, the response of the salinity field to wind forcing was characterized. High surface salinity, consistent with upwelling along the Uruguayan coast is observed for north-easterly winds, which are typical of the warm season. The present conceptual scheme involving the seasonal signal as the main feature of the salinity filed variability is modified by the results of this Thesis. This signal result of a larger frequency of occurrence of some wind direction along the different seasons, and conditions classically though as characteristic of ‘winter’ or ‘summer’ can take place during any season with high variability. The fresh water plume would impact the continental shelf in the form of alternating pulses toward the northeast or to the southwest in a time scale around three to four days. The main forcings of estuarine variability in different time scales were identified by means of a long term numerical simulation using Hamburg, Shelf Ocean Model (HamSOM). The first mode of the sea surface height variability on inter-annual time scale is forced simultaneously by runoff and wind. Results show clear evidence that both forcings are associated to ENSO cycles. The seasonal scale variability accounts by a very low percentage of variance and it is a combination of an annual and a semi-annual signal forced by wind and runoff, respectively. Approximately 90% of the variance is accounted by sub-annual scale variability, essentially wind driven. Extreme flood events in the estuary increase their frequency of occurrence and intensity over the time. Finally, the model hierarchy applied for barotropic circulation shows to be a robust tool for forecasting and for the study of estuarine climate variability in response to climate change.
Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias de la Atmósfera y los Océanos
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