Cross-shelf exchanges in the northern Bay of Biscay
Transporting nutrients, sediments, biota and materials, and regulating their delivery and removal rates on the continental shelf, cross-shelf exchanges are important for the carbon and nutrient budgets of the ocean. Cross-shelf exchange remains a complex issue to address, considering the weak intensity of cross-shelf circulation, which is difficult to observe and often ageostrophic. Lack of observations and dedicated shelf-break observatories makes it even more difficult to investigate cross-shelf exchanges. This study investigates observations and highresolution models to determine cross-shelf exchanges in the northern Bay of Biscay. Focusing on the simulations, estimates of cross-shelf exchange are provided, as well as the main mechanisms leading to these exchanges. Wind, tide and eddy induced cross-shelf flows were found to be the major mechanisms. Eddies contribute to on-shelf transport, whereas a quasipersistent bottom boundary flow leads to offshore transport. Cross-shelf bottom flow displays spatio-temporal variability along the shelf-break. An eddy tracking algorithm is employed to investigate the dynamics of eddies. Simulations suggest canyons as eddy-generation hotspots along the shelf break. Eddy-induced cross-shelf transport is often neutralized by on-shelf and off-shelf components of the flow. Yet, this balance is violated in the presence of non-spherical eddies and associated filaments.
Figure: Sea Surface Temperature (top left). Sea Surface Salinity (top right). Cross-shelf volume transport, positive values represent offshore transport (bottom).