Vertical structure and variability of currents on the southern Brazilian inner shelf at 32°S

Abstract

This study investigates the impact of winds, tides, and river discharges on the coastal current and salinity variability of Southern Brazil’s inner shelf. Data on wind speed, current, surface salinity, and temperature were collected for analysis from a metocean buoy of the Brazilian Coastal Monitoring System (SiMCosta). The observed current time series can be interpreted as a sum of highly variable flows correlated with local wind stress, and a residual mean current flowing southward along the coast at a few centimeters per second. Tidal currents were predominantly diurnal, albeit negligible, representing approximately 1.7% of the current variance in the region. We observed a prevalence of northwest winds and southeastward currents on the inner shelf, as well as an intermittent flow in both the alongshore and cross-shore directions due to meteorological frontal system passages. The power spectrum of both current components presented similar frequency patterns, indicating the prevalence of high-energy events in periods of 3 to 10 days over the entire time series. The alongshore current is highly correlated (r=0.73, p<0.05) with alongshore wind and a delay of 3 hours. In the low- (period >40 h) and high-frequency (period <40 h), the temporal lags were of 5 and 3 hours, respectively, with correlations of 0.79 (p<0.05) and 0.60 (p<0.05). The wavelet analysis has shown that high-energy events in alongshore wind stress are more common between August and October and less often between February and March, with similar surface currents and salinity patterns. A decrease in surface salinity during the winter season was observed due to the high level of Patos Lagoon’s outflow. Mean daily salinity correlated negatively with Patos Lagoon’s outflow, however part of this variability is associated with the intrusion of Patos Lagoon’s plume and the passage of frontal systems.

Keywords:
Costal circulation; direct current measurements; tidal currents; south Brazilian inner shelf