Detecting hydrological landscapes over the Bay of Biscay continental shelf in spring

A complex set of hydrodynamical structures co-exist over the Bay of Biscay continental shelf. These structures, which can spread over a limited spatial range (10s of km) and last for periods ranging from days to months, influence the rates and nature of biological production. They need to be accurately described before the mechanisms linking hydrodynamics to production can be unraveled. Such description generally requires not only 3D observations of hydrological fields (temperature, salinity, density), but also a description of how these fields vary with time. This is hardly achievable with current means of field observation but can be mimicked by hydrodynamic model simulations. Using simulations from a 3D hydrodynamic model, we developed a methodology that allows for the description of hydrological landscapes, i.e. a description of the 3D fields and of their changes with time. The method was applied to the structures present over the Bay of Biscay shelf in spring. Eight hydrological landscapes were identified: high estuary, low estuary, coastal, river plume, near coastal, central shelf, open shelf and northwestern shelf. With this technique, we show that structures that have been qualitatively described in the literature can be identified objectively and that some new structures can be detected. All 8 hydrological landscapes are found every year, but their spatial extent can vary greatly from year to year. This variation for all landscapes except for the northwestern shelf can be summarised by a single index, which is strongly related to the runoff from the main rivers along the French coast.