Wind induced variability in the Northern Current (North-Western Mediterranean Sea) as depicted by a multi-platform observing system

International audienceThe variability and evolution of the Northern Current (NC) in the area off Toulon is studied for two weeks in December 2011 using data from a glider, a HF radar network, vessel surveys, a meteo station, and an atmospheric model. The NC variability is dominated by a synoptic response to wind events, even though a seasonal trend is also observed, transitioning from late summer to fall-winter conditions. With weak winds the current is mostly zonal and in geostrophic balance even at the surface, with a zonal transport associated to the NC of ≈ 1 Sv. Strong westerly wind events (longer than 2-3 days) induce 5 an interplay between the direct wind induced ageostrophic response and the geostrophic component: upwelling is observed, with offshore surface transport, surface cooling, flattening of the isopycnals and reduced zonal geostrophic transport (0.5-0.7 Sv). The sea surface response to wind events, as observed by the HF radar, shows total currents rotated at ≈ −55 • to −90 • to the right of the wind. Performing a decomposition between geostrophic and ageostrophic components of the surface currents, the wind driven ageostrophic component is found to rotate of ≈ −25 • to −30 • to the right of the wind. The ageostrophic 10 component magnitude corresponds to ≈ 2% of the wind speed