On the magnitude and mechanisms of occurrence of nonlinear tide-surge interactions in the Southwestern Atlantic Continental Shelf

Abstract

This paper aims to characterise the nonlinear tide-surge interactions (NTSIs) and their impact on the Southwestern Atlantic Continental Shelf (SWACS) barotropic dynamics. In addition, the associated atmospheric configuration, the generation mechanisms and their long-term variability were also studied. For it, process oriented numerical simulations with realistic geometry and bathymetry which consider the effect of atmospheric and tidal forcing both individually and jointly are run. Results reveal that NTSIs are significant in the shallower areas (depth less than 50 m) of the SWACS where both the tide and the surge present comparable magnitudes. North of 40°S, where shallow areas are relatively extensive, NTSI can account for between 5 and 40% of the total sea surface height anomaly which, in turn, is related to a reduction of the resulting sea level anomaly. In agreement, the bottom friction term was found to be the main source of NTSI explaining the loss of energy in shallow areas. In turn, the NTSI is modulated by both the tide and the surge: due to the relatively high frequency of the tide with respect to that of the surge, there are moments when the tidal and surge currents are aligned (opposed), increasing (reducing) the magnitude of the bottom friction and, thus, the NTSI. NTSIs in the SWACS can be described in terms of two spatial modes: one is strongly modulated by the surge and occurs when storm surges affect the Río de la Plata (microtidal) estuary; the second one is weaker and affects the northern SWACS coast where tides become larger and is mixed modulated by the tide and the surge. The first mode occurs in association with atmospheric Rossby waves that travel along 45°S, which are efficient in producing the winds that force the resonant response of the estuary forcing a locally generated surge; the second mode is related to atmospheric Rossby waves that travel south of 45°S, which are more efficient in generating surges in the SWACS. Both modes are found to show pseudo-periodicities on the interannual time scale in a band centred at around 2.5 years, which is consistent with the known scales of surge variability. Finally, results highlight the need to use regional tide-surge coupled models to forecast the surge in the SWACS; otherwise the estimation errors could be very large in some coastal and shallow regions due to misrepresentation of the barotropic regional dynamics.