Observations of cross-shelf transport due to internal wave pumping on the Bay of Biscay shelf

Abstract

Coastal cross-shelf transport drives the redistribution of sediment, nutrients and pollutants on continental shelves. Here, the cross-shelf volume flux is quantified from in situ measurements at two coastal sites on the Bay of Biscay (BoB) shelf. At both sites, a semidiurnal internal tide propagates onshore, and mode 1 nonlinear internal wave packets are observed. The Eulerian and Stokes drift contributions to the subtidal cross-shelf transport are estimated along density layers from ADCP, temperature sensors, and CTD measurements at 62 m water depth on the Landes plateau or SE-BoB (44° N) and 47 m water depth on the Armorican shelf or N-BoB (47° N). The Stokes drift transport has possible contributions from the internal tides, nonlinear internal waves and the surface waves and tide. At both sites, the vertical profile of the month-long averaged Stokes drift volume flux matches the shape of the semi-analytical Stokes drift volume flux due to a linear mode-1 internal tide. We demonstrate that nonlinear internal wave events can also contribute to the Stokes drift volume flux. We thereby attribute the Stokes drift volume flux at the study sites to internal wave pumping (IWP). At both sites, the IWP is responsible for a near-seabed onshore volume flux during stratified conditions and spring internal tides, that is equivalent to a wind-driven upwelling event generated by a 4 ms-1 wind. At N-BoB (47° N), IWP is the main contributor to the total cross-shelf volume flux under stratified conditions and a spring internal tide. At SE-BoB (44° N), IWP augments the near-seabed onshore volume flux during upwelling events and maintains a near-seabed onshore volume flux even during downwelling events.