Spatial structure of tidal and residual currents as observed over the shelf break in the Bay of Biscay

Abstract

Theoretical and laboratory models show that internal-wave energy in continuously stratified fluids propagates in the vertical plane, at an angle set by the wave, buoyancy and Coriolis frequencies. Repeated Acoustic Doppler Current Profiler observations on three transects, crossing the shelf edge, now directly reveal this beam-wise propagation of internal tides in the Bay of Biscay. This confirms previous suggestions based on observations sampled more sparsely in space. The present observation is made by bin-wise harmonic analysis of horizontal currents, leading to the spatial resolution of barotropic and baroclinic semi-diurnal tidal and (time-averaged) residual flows. The observed barotropic tide has a cross-slope mass flux that is roughly constant. Its fast along-slope phase variations can only in part be explained by the spring-neap tidal cycle. The observed baroclinic tide compares favourably to that produced by a 2D numerical model. The observations reveal details of the internal tidal beam, including its spatial amplitude distribution, presence of amphidromes and direction of phase propagation. The cross-isobath structure of the along-slope barotropic mean flow shows a localized maximum near the shelf break. Over two transects it agrees in sign and magnitude with a theoretical tidally rectified flow. The baroclinic, cross-isobath mean flow shows a strong near-bottom downwelling flow, compensated by an on-shelf-directed flow in the upper part. The along-shelf mean flow displays subsurface intensification attributed here to frictional modification of a tidally rectified flow that is bottom-trapped due to stratification.