Northern boundary current variability and mesoscale dynamics: a long-term HF RADAR monitoring in the North-Western Mediterranean Sea

Coastal waters are globally challenging areas to monitor not only because of the high resolution needed to resolve the scales at stake but also because most satellites are not yet suited to near-coast observations. We present here the analysis of an 8-year-long high-frequency RADAR (HFR) dataset off the coasts of Var region (France) in the North-Western Mediterranean Sea. Successive interruptions and errors intrinsic to the nature of the HFR remote-sensing technology have been compensated by the DINEOF statistical filling method. This unprecedented dataset enables the observation of many oceanic processes ranging from the interannual variability of large-scale structures to the identification of (sub)mesoscale features. Comparing and coupling this analysis with outputs from a regional ocean circulation model (GLAZUR), we also put emphasis on the extent of the possibilities of the HFR for the description of coastal surface circulation and its value as part of integrated observation systems. The data showed the diversity of the surface circulation in the region, mainly marked by the Northern boundary Current (NC), but undergoing a great variety of spatial and temporal fluctuations, interannually, seasonally, and at higher frequencies. Strong gusts of wind or regional upstream circulation can cause the undulations of the NC, modifying its shape and its strength, and fostering the emergence and zonal displacement of mesoscale to submesoscale eddies. Using an eddy tracking algorithm, we show that the occurrence of mesoscale eddies off this region display a strong inter-annual variability that is linked to the spatio-temporal variability of the NC’s characteristics. The NC system thus plays an intermittent role as a transport carrier or barrier of heat, energy, or matter, which has important consequences for neighbouring coastal areas.