Tidal and Low-Frequency Currents Near the Shelf-Break: Northeastern Coast of Brazil

Abstract

Time series of a 40-day (13 March-23 April 1979) wind and current observational experiment are analyzed. The wind data originated from the Fortaleza Airport Weather Station (Ceará, Brazil), and the current meter data originated concurrently from moorings deployed on the shelf break, 40 km offshore. The meters were placed at 8, 16 and 36 m. Strong tidal currents were observed, with a range of 30 to 40 cm s−1 during the equinoctial spring tide period. The analytical model of Battisti and Clarke was applied to the prediction of the diurnal and semidiurnal tidal currents at the mooring site. Good overall agreement with the data was obtained. The wind record spectral and harmonic analysis indicated the presence of a relatively strong diurnal breeze (amplitude of 2.2 m s−1) and a less energetic semidiurnal breeze (amplitude of 0.9 m s−1). A low frequency coastal current flowing northwestward with a near surface mean speed of 16 cm s−1 was also observed. Alongshelf wind and current are highly coherent in the frequency range between 0.01 and 0.02 cycles per hour, indicating that this low-frequency current is locally wind-driven. This result was also confirmed by an analysis of the low-passed wind and current mean and eddy kinetic energies.