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

Abstract

The variability and evolution of the Northern Current (NC) in the area off Toulon is studied for 2 weeks in December 2011 using data from a glider, a high-frequency (HF) radar network, vessel surveys, a weather station, and an atmospheric model. The NC variability is dominated by a synoptic response to wind events, even though the dataset also evidences early stages of transition 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 with the NC of ≈ 1 Sv. Strong westerly wind events (longer than 2–3 days) induce 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 by ≈ −25 to −30° to the right of the wind. The ageostrophic component magnitude corresponds to ≈ 2 % of the wind speed.