Surface water circulation patterns in the southeastern Bay of Biscay: New evidences from HF radar data

Abstract

High frequency (HF) radar stations have been working operationally in the southeastern part of the Bay of Biscay since 2009. The (2) systems provide hourly surface currents, with 5km spatial resolution and a radial coverage lying close to 180km. The detailed and quantitative description of the spatial patterns observed by the HF radar offers new evidence on the main ocean processes, at different time scales, affecting a study area where surface currents show marked temporal and spatial variability. A clear seasonality in terms of sea surface currents and along-slope circulation is observed, with cyclonic and anticyclonic patterns during the winter and summer months, respectively. From the analysis of low-pass filtered currents, a key component of this seasonal variability is associated with the surface signature of the slope current (Iberian Poleward Current (IPC)). Clearly intensified over the upper part of the slope, this current circulates eastward off the Spanish coast and northward over the French shelves in winter.Examination of the HF radar current fields reveals the presence of mesoscale structures over the area. At higher frequencies, an EOF (empirical orthogonal function) analysis of the inertial band-pass filtered data is used to study the complex spatial and temporal patterns associated with these processes and to evaluate quantitatively the relative contribution of the high frequency to the total variability, in space and time. Overall, inertial currents represent between 10 and 40% of the total variability; their contribution is significantly greater in summer and over the deeper part of the slope. Tides contribute much less than the total kinetic energy (KE), although their contribution over the shelf can be higher than that of the inertial oscillations, during winter.