Spatial variability of the barotropic M2 constituent tidal current over the Rias Baixas Galician shelf (NW Spain)

Using current meters and systematically repeated vessel mounted ADCP data legs, we describe the role of the semidiumal and diurnal constituents in the tidal currents and investigate the mesoscale variability of the M-2 tidal currents field along three cross-shelf legs over the Galician shelf We found that the estimated values of the semi-major axes of the main semidiumal tidal constituents are closely related to those obtained in previous research and that the tidal currents are predominantly semidiumal. Amplitudes of the M-2 and S-2 semi-major axes constituents are comparable, and important fortnightly variability should thus be expected in the amplitude of the semidiumal tidal currents. Vertical profiles of the semi-major axes of the main tidal constituents reveal that, in the absence of stratification, and far from the bottom frictional effects, the tidal currents are mostly barotropic. Over the slope, the tidal ellipses tilt from a north-south orientation toward the northeast-southwest direction and, as we go further into the shelf, their amplitude is increased, a feature that is reasonably explained by the transition from the dynamics of a Poincare wave offshore to the dynamics of a northward alongshore propagating Kelvin wave over the shelf. The Ria de Vigo exerts a notable influence over the tidal currents, acting as an extension of the shelf and introducing a meridional variation in the orientation of the ellipses from the alongshore direction in the southern straight coast toward the northeast-southwest direction at the latitude of the Rias. Finally, the spatial fitting methods we tried have proved to be successful in VMADCP data detiding in this region. (C) 2007 Elsevier B.V. All rights reserved.