Abstract
While oceanic circulation in the Gulf of Lion (GoL) has often been studied in calm weather or with northerly winds (Tramontane or Mistral) through observations and numerical circulation models, few studies have focused on southeasterly storm events. Yet, correct representation of the circulation during storms is crucial if the suspension of sediments is to be correctly modelled throughout the Gulf. The purpose of this paper is to describe the hydrodynamics in the Gulf of Aigues-Mortes (NW of the GoL) during the storm of 18 February 2007 by using a set of data from an ADCP station placed at a depth of 65m on the sea bed off the coast at Sète, supplemented by the ocean circulation model SYMPHONIE. This storm was characterized by a moderate south-easterly wind (15m.s−1) and waves of up to 5m of significant height at its apex. At the ADCP, strong currents of up to 0.8m.s−1 near the surface and 0.5m.s−1 near the bottom were recorded, parallel to the coast, flowing towards the south-west. The simulated currents were widely underestimated, even taking the effect of waves into account in the model. It was suspected that the representation of the wind in the atmospheric model was an underestimation. A new simulation was therefore run with an arbitrarily chosen stronger wind and its results were in much better agreement with the measurements. A simplified theoretical analysis successfully isolated the wind-induced processes, responsible for the strong currents measured during the apex and the strong vertical shear that occurred at the beginning of the storm. These processes were: 1/ the barotropic geostrophic current induced by a wind parallel to the coast and 2/ the Ekman spiral. The duration of the storm (about 36 h at the apex) explains the continuous increase of the current as predicted by the theory. The frictionally induced Ekman transport explains the current shear in the surface layer in the rising stage of the storm, and the addition of high waves and strong wind at the apex is more in favour of strong vertical mixing in the surface layer.
Highlights
The large-scale oceanic circulation over the continental shelf of the Gulf of Lion (NW Mediterranean Sea, Fig. 1) is induced by the regional component of the cyclonic general circulation of the western Mediterranean basin, namely the Northern Current
In order to calculate the geostrophic current through Eq 8, we considered the wind averaged over the region extending from the coast to the BESSete station
A sensitivity test was carried out to determine if this underestimation could be induced by the presence of high waves
Summary
The large-scale oceanic circulation over the continental shelf of the Gulf of Lion (NW Mediterranean Sea, Fig. 1) is induced by the regional component of the cyclonic general circulation of the western Mediterranean basin, namely the Northern Current. Strong atmospheric fluxes typical of this region (Hauser et al, 2003) play an important role in inducing local actions, e.g. upwellings (Millot, 1990) or eddy structures, that can interact with the Northern Current (Estournel et al, 2003; Hu et al, 2009; Rubio et al, 2009) This hydrodynamic behaviour is highly variable and sometimes difficult to represent with a simple model. The studies by Leredde et al (2007), carried out in the northern sector of the shelf, i.e. in the Gulf of AiguesMortes (GAM, Fig. 1), demonstrated that the SYMPHONIE model faithfully reproduced the observations, which were made mainly with a hull-mounted ADCP during the 9-day HYGAM cruises. Our knowledge of the currents and bottom stresses in large areas of the shelf is limited
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