International audienceThe winter of 2012 experienced peculiar atmospheric conditions that triggered a massive formation of dense water on the continental shelf and in the deep basin of the Gulf of Lions. Multiplatforms observations enabled a synoptic view of dense water formation and spreading at basin scale. Five months after its formation, the dense water of coastal origin created a distinct bottom layer up to a few hundreds of meters thick over the central part of the NW Mediterranean basin, which was overlaid by a layer of newly formed deep water produced by open-sea convection. These new observations highlight the role of intense episodes of both dense shelf water cascading and open-sea convection to the progressive modification of the NW Mediterranean deep waters
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.
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