Cross-shelf exchanges in the northern Bay of Biscay

Abstract

Abstract The spatio-temporal distribution of cross-shelf exchanges in the northern Bay of Biscay from 2007 to 2010 were investigated using a high-resolution three-dimensional model as well as sea-surface temperature and chlorophyll-a concentration satellite observations. Our results show that the net yearly mean transport was upslope each year, with 2010 showing the highest value (0.93 Sv upslope). Bottom fluxes showed peak values near Chapel Bank, with mean values of 0.1 m3 s−1 m−2 and maximum values of 0.2 m3 s−1 m−2. Our model demonstrated that cross-shelf exchanges can be divided into three vertical layers. At the surface, cross-shelf transports are driven by wind forcing (Ekman transport accounts for about 60% of the total cross-shelf transport) and mesoscale activity (eddy advection accounts for about 30% of transport). In the absence of mesoscale activity, Ekman transport at the surface is typically balanced out by a downslope flux at the bottom boundary layer. Exchanges at mid-depths are regulated by mesoscale activity and tides. A ubiquitous feature appeared at the bottom boundary with a cross-shelf flow in the downslope direction. Numerical simulations suggest bottom fluxes of 0.1 m3 s−1 m−2, in agreement with previous in situ observations. We discuss the impact of winds, tides and mesoscale eddies on cross-shelf exchanges using different examples. The eddy census was obtained using an eddy-tracking algorithm. The shelf break was shown to be an important area of eddy presence due to slope current instabilities. The impact of eddies on surface and mid-depth transport is illustrated with a shelf-break eddy as an example, from its generation to dissipation and its contribution to cross-shelf exchanges. Results suggest that the largest magnitudes of downslope transports occur in the presence of both strong winds and intense eddy activity. Our detailed and quantitative exploration of cross-shelf transports in the northern Bay of Biscay highlights the relative contribution of intermittent processes (e.g. wind-driven events, eddies, frictional bottom layer) and confirms the complex links between the coastal and open ocean over shelf breaks.