Drifter observations in the summer time Bay of Biscay slope current

•A summer release of 20 drifters on the Bay of Biscay slope•Lagrangian statistics for the Bay of Biscay reveal diffusive system.•On-shelf flow through genesis of small, anticyclonic eddies over complex topography

During the summer of 2012, 20 surface drifters drogued at 50 m depth were deployed on the continental slope to the north of the Bay of Biscay. Initially after release the drifters all crossed the slope, with 14 continuing equatorward, parallel to the slope following an absolute dynamic topography feature and 6 returning to the slope, in an eddy, visible in chlorophyll-a maps. Lagrangian statistics show an anisotropic flow field that becomes less tied to the absolute dynamic topography and increasingly dominated by diffusion and eddy processes. A weaker tie to the absolute dynamic topography allowed for total of 8 of the drifters crossed from the deep water onto the shelf, showing pathways for flow across the slope. A combination of drifter trajectories, absolute dynamic topography and chlorophyll-a concentration maps has been used to show that small anticyclonic eddies, tied to the complex slope topography provide a mechanism for on shelf transport. During the summer, the presence of these eddies can be seen in surface chlorophyll-a maps.