Study of the tidal dynamics in the Southwestern Atlantic Continental Shelf based on data assimilation

Abstract

The Southwestern Atlantic Continental Shelf (SWACS) is a large oceanic region with remarkably barotropic dynamics. Several scientific studies have described how processes, such as tide or surface winds, affect the variability of the sea surface height and currents. However the tidal dynamics has not received attention for at least the last 15 years, in spite of their importance for both local and global dynamics. Since the last works, the amount of available observations and numerical models (physics, resolution, numerics, etc.) have all greatly improved. In this context, data assimilation (DA) becomes an relevant tool to merge both the observations and the model solutions, producing a better representation of the regional processes. Particularly, DA provides, in addition, an objective methodology to calibrate model parameters. Thus, the aim of this work is to perform, for the first time for this outstanding region, the calibration of the numerical model bottom friction coefficient ($c_D$) by means of DA; then, the opportunity of a better simulation is seized to update the description of tidal dynamics. The spatial variability of the derived $c_D$ is consistent with the bathymetry, with a mean value of $2.0\times 10^{-3}$ along the coast and $2.5\times 10^{-3}$ nearby the shelf-break. Results show that the incorporation of a spatially varying $c_D$ improves the representation of the tidal amplitude and phase compared to the case when it is considered homogeneous, and drives in a single model to results of a better quality than previous nested models with much larger resolution. The optimal representation of the regional tide with a single model allowed us to provide a deeper, improved and novel description of the tidal dynamics. It was found that the energy enters the domain not only from the south but also from the north, being the flux to the north two orders of magnitude larger; those two fluxes produce an cyclonic circulation consistent with the behaviour of the SWACS as a semidiurnal tidal resonant canal theoretically proposed by Webb (1976). This explains why the energy flux is, by far, domained by the potential energy and the large amplitudes of the tide. Finally, a remaining and weaker branch exits along the coast; it enters the Río de la Plata Estuary from its southwesternmost tip and travels upstream along the Argentinean coast, reaching the upper estuary strongly attenuated.