Claudia G. Simionato

The salinity front of the Río de la Plata ‐ A numerical case study for winter and summer conditions

A 3-D baroclinic model has been applied to investigate the location of the salinity front of the Rio de la Plata. River run-off, tides and mean winds for summer and winter conditions respectively were considered in order to find out the most important factors for the observed seasonal variability of this location. The results show that wind forcing is responsible for the observed variability in summer. In winter, wind forcing is not important, but the river discharge. The resulting surface salinity distributions show a surprisingly good agreement with the present knowledge from observations.

Surface Wind Variability on Seasonal and Interannual Scales Over Rio de la Plata Area

Abstract Previous works show that wind forcing is the main source of circulation seasonal variability in the Río de la Plata estuary, located on the southeastern coast of South America. Wind forcing exceeds by far the role of fresh water discharges. However, due to a lack of enough observations, the features and causes of surface wind variability are not well understood yet. Therefore this paper presents a comprehensive study of surface wind variability over the Río de la Plata estuary using the National Center for Environmental Prediction-National Center for Atmospheric Research (NCEP/NCAR) reanalysis data between 1948 and 1997. It is expected that this study contributes to better understand, model and thus predict the estuary circulation. An onshore to offshore rotation characterizes the seasonal variations of the surface winds from summer to winter. A linear trend analysis shows a displacement of the summer–winter seasonal features to earlier months. On inter-annual time-scales, the first leading pattern describes east–west changes of surface winds that seems to be forced by the quasibiennial tropospheric oscillation excited in the western tropical Pacific and previously identified by many authors. The conditions over the South Atlantic and in particular the Río de la Plata are influenced by such oscillation through an atmospheric Rossby wave train propagating out of the tropics. This result is very important for its implication on the predictability levels in the region. The second leading mode is associated with anticyclonic/cyclonic wind rotations off the estuary on interannual times scales which are related with changes in both atmospheric and oceanic surface conditions at Southern Hemispher high-latitudes.

A Set of 3-D Nested Models for Tidal Propagation from the Argentinean Continental Shelf to the Río de la Plata Estuary—Part I. M2

Abstract As a contribution to the UNDP/GEF project ‘Environmental Protection of the Río de la Plata and its Maritime Front’, the three-dimensional primitive equation Hamburg Shelf Ocean Model is being implemented for forecasting purposes. As a first step a study of the tidal propagation was done. Data for the estuary were gained through a set of three one-way nested models. Simulations were started with a large-scale model covering the Argentinean and Uruguayan and part of the Brazilian continental shelves. This model provides boundary conditions to a smaller scale model of the Río de la Plata and adjacent continental shelf, which in turn is used to force a small-scale high-resolution model of the Río de la Plata estuary. Model sensitivity to different boundary conditions and to model parameters was investigated. Solutions are not sensitive to the two different boundary conditions tested, derived from global data assimilating models. It results also not sensitive to lateral diffusion but to bottom friction. M2 tidal wave propagates northwards as a Kelvin wave, with amplitudes reaching almost 4 m in Southern Patagonia and a few centimeters at the Río de la Plata estuary. Simulation results for the M2 component propagation were validated using all tidal gauge data available and several currents observations, resulting in a very good agreement. These simulations have permitted, therefore, the construction of more reliable model derived cotidal, corange and tidal currents charts. The nonlinear transfer of energy from semidiurnal to higher order harmonics was mapped. It can reach very high values at some locations of the Patagonian coast. Tidal energy dissipation derived from the simulations shows that it constitutes an important amount of the globally estimated one.

Seasonal variability in the Agulhas Retroflection Region

The objective of this article is to present evidence for the existence of seasonal variability in sea surface height (SSH) anomaly in the Agulhas Retroflection region. TOPEX/POSEIDON altimeter data are used to estimate seasonal changes in the mesoscale SSH variability. There is a seasonal oscillation of SSH variability characterized by a maximum during the austral summer and a minimum during the austral winter. The amplitude of this seasonal change is approximately 30% of its mean value. During the winter season the spatial distribution of SSH variability resembles that of the annual mean variability, with relative maxima centered at approximately 18°E, 27°E and 38°E. During the summer there is an additional maximum which extends from approximately 20°E to 25°E and from 40° to 42°S. Analysis of longitude-time diagrams reveals that at low latitudes planetary waves propagate freely throughout the basin. Along the latitude of the Agulhas Retroflection region, the East Madagascar Ridge hampers the westward propagation of planetary waves. It is speculated that the difference between summer and winter patterns is caused by an inertially driven bifurcation of the Agulhas Current.

Modeling Ocean Tides and Their Energetics in the North Patagonia Gulfs of Argentina

Abstract During the last decade, global tidal models have spectacularly improved. However, they still have difficulties in resolving tides over continental shelves and near coastlines. This study of tidal propagation from the continental shelf to the North Patagonia Gulfs of Argentina applies a set of three nested high resolution models based on the Hamburg shelf ocean model (HamSOM), where particular attention was paid to the bathymetry and the coast line. The study is complemented by the use of all the tidal gauge and tidal current observations available. Simulations display good agreement with observations, permitting the construction of higher resolution and more reliable cotidal and corange charts. The tidal regime in the area is essentially semidiurnal and dominated by M2. This constituent propagates northward as a Kelvin wave and reaches the gulfs from the south. In their interior an important amplification is observed. Tidal currents are large at the mouths of the gulfs, and weaken toward their interior. The nonlinear transfer of energy from the semidiurnal to higher order harmonics was analyzed. This can be very important in the interior of the gulfs, particularly in Nuevo Gulf and in the northwestern San Matías Gulf, close to San Antonio. Energy flux and energy dissipation by bottom friction has been computed and indicate that this region dissipates 17% of the total energy dissipated on the Patagonian Shelf which, in turn, is one of the most dissipative areas of the world ocean. The Simpson-Hunter parameter computed from the simulations shows that in the mouth of the gulfs, particularly in San Matías and east of the Península de Valdés, the tides are energetic enough to overcome stratification and produce tidal fronts. The locations where tidal fronts are located are highly consistent with results from sea surface temperature and primary productivity data analyzed by other authors.

The Río de la Plata Estuary Response to Wind Variability in Synoptic Timescale: Salinity Fields and Salt Wedge Structure

ABSTRACT Meccia, V.L.; Simionato, C.G., and Guerrero, R.A., 2013. The Río de la Plata estuary response to wind variability in synoptic timescale: salinity fields and salt wedge structure. The Río de la Plata estuary response to wind variability in a synoptic timescale is studied with the Estuarine, Coastal and Ocean Model forced by tides, runoff, and idealized winds. Ancillary conductivity, temperature, and depth data from synoptic oceanographic cruises are used to validate the conclusions derived from the simulations. Results show that the estuarys response to wind variability can be explained in terms of four characteristic patterns associated with winds that blow with a dominant component either along or across the estuary axis. Cross-estuary winds from the SW produce a northward displacement of the surface salinity front toward the Uruguayan coast and cause downwelling between Montevideo and Punta del Este. Reciprocally, under NE winds, the surface salinity front migrates to the SW and upwelling between Montevideo and Punta del Este occurs. Along-estuary winds with a downstream component produce a net outflow of continental surface waters and inflow of bottom shelf waters, resulting in an intensification of the stratification. In opposition, winds with an upstream component produce a weakening of the vertical structure. Stratification can be destroyed by strong or persistent SE winds, which are not frequent in the region, and it recovers in a relatively short period (10–15 days) after wind relaxation. Consequently, the salt wedge structure is observed in the Río de la Plata during most of the year. This implies that significant vertical mixing events producing exchanges of water and properties between the surface and the bottom waters occur only a few times a year. As a consequence, the Río de la Plata shows strong retentive features, favoring biota by retaining eggs and larvae—and favoring pollutant accumulation.