Cross-shelf exchanges in the northern Bay of Biscay

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

Interannual evolution of (sub)mesoscale dynamics in the Bay of Biscay

International audienceIn the north-east Atlantic Ocean, the Bay of Biscay is an intersection between a coastal constrained dynamics (wide continental shelf and shelf break regions) and an eastern boundary circulation system. In this framework, the eddy kinetic energy is 1 order of magnitude lower than in western boundary systems. To explore this coastal complex system, a high-resolution (1 km, 100 vertical sigma layers) model experiment including tidal dynamics over a period of 10 years (2001-2010) has been implemented. The ability of the numerical environment to reproduce main patterns over interannual scales is demonstrated. Based on this experiment, the features of the (sub)mesoscale processes are described in the deep part of the region (i.e. abyssal plain and continental slope). A system with the development of mixed layer instabilities at the end of winter is highlighted. Beyond confirming an observed behaviour of seasonal (sub)mesoscale activity in other regions, the simulated period allows exploring the interannual variability of these structures. A relationship between the winter maximum of mixed layer depth and the intensity of (sub)mesoscale related activity (vertical velocity, relative vorticity) is revealed and can be explained by large-scale atmospheric forcings (e.g. the cold winter in 2005). The first submesoscale-permitting exploration of this 3-D coastal system shows the importance of (sub)mesoscale activity in this region with its evolution implying a potentially significant impact on vertical and horizontal mixing

How momentum advection schemes influence current-topography interactions at eddy permitting resolution

International audienceRecent studies have shown that the use of an enstrophy-and-energy-conserving momentum advection scheme substantially reduces widespread biases of mean currents in the global 1/4° DRAKKAR model. This paper investigates the origin of these improvements. A series of sensitivity simulations with different momentum advection schemes is performed with the North Atlantic 1/4° DRAKKAR model. Three second order momentum advection schemes conserving, respectively, enstrophy (ens), energy (efx) and both quantities (een) are tested and their impact on the model solution are compared. The mean kinetic energy vertical profile is found to change up to 10% depending on the chosen scheme. This sensitivity is maximum in bottom layers. The analysis of the vorticity tendency due to horizontal momentum advection reveals that the three schemes differ mostly in bottom layers as well. The average magnitude of this term is enhanced with the efx scheme and reduced with the een scheme. These differences are found to be consistent with the instantaneous tendency of each scheme. In addition, we show that the differences between the schemes are related to the grid-scale irregularity of the velocity field. Both the grid scale irregularity and the differences between the schemes are found to be enhanced in bottom layers. We conclude that the model solution depends crucially on the ability of the momentum advection scheme to handle under-resolved flows close to the bottom topography. This work emphasizes the critical influence of topography in eddy-active regions on mean circulation features such as the position of the North-Atlantic current or the Gulf Stream separation

Wintertime Submesoscale River Plumes in the Bay of Biscay

International audienceRecent observations over the continental shelf in the Bay of Biscay (in the Eastern Atlantic along French and Spanish coasts) has revealed a complex small scale (O(1km)) activity with high frequency variations. A dataset of 11 years' (2003 to 2013) Sea Surface Temperature (SST) remotely sensed by MODIS sensor onboard Aqua and Terra satellites is studied to detail the spatial and seasonal distributions of fronts in this region. Spatial and temporal patterns revealed by this analysis are used to identify the driving mechanisms of these fronts and they are found to be in agreement with the previously well-known tidal or shelf break fronts in the region. Furthermore, these observations have brought to attention one particular group of fronts that occur in mid-shelf during winter, similar examples of which have previously been studied in other regions (e.g. Mid-Atlantic Bight). These observed SST fronts in our region are shown to be the temperature signature of density fronts occurring along the river plume edges, where the main driver of the density difference is actually the increased freshwater input to the shelf. A realistic high-resolution (1 km) hydrodynamic model is applied to the region to investigate the dynamics of such fronts. A scale decomposition that distinguishes the large, meso-, and submesoscale components of model results is carried out. Results show that, along the river plume front, submesoscale patterns prevail. They also possess a certain spatial variability such as filamentation in certain parts of the front, which can be indicative of baroclinic instability. Temporal evolution of this submesoscale variability and some of the forcings that can be responsible for it (e.g. surface cooling, wind stress, topography, or background circulation), together with the role of baroclinic instability are explored

Spatial and Seasonal Distributions of Frontal Activity over the Continental Shelf in the Bay of Biscay Focus on Density Fronts in Winter

International audienceAIMS: Describing spatial and seasonal distributions of frontal activity over the continental shelf in the Bay of Biscay.Investigating the wintertime mid-shelf fronts in vicinity of the river plumes.Exploring the physical dynamics and the existence of baroclinic instabilities in such fronts from a scale decomposition of the vertical buoyancy flux

Wintertime Submesoscale River Plumes in the Bay of Biscay

International audienceRecent observations over the continental shelf in the Bay of Biscay (in the Eastern Atlantic along French and Spanish coasts) has revealed a complex small scale (O(1km)) activity with high frequency variations. A dataset of 11 years' (2003 to 2013) Sea Surface Temperature (SST) remotely sensed by MODIS sensor onboard Aqua and Terra satellites is studied to detail the spatial and seasonal distributions of fronts in this region. Spatial and temporal patterns revealed by this analysis are used to identify the driving mechanisms of these fronts and they are found to be in agreement with the previously well-known tidal or shelf break fronts in the region. Furthermore, these observations have brought to attention one particular group of fronts that occur in mid-shelf during winter, similar examples of which have previously been studied in other regions (e.g. Mid-Atlantic Bight). These observed SST fronts in our region are shown to be the temperature signature of density fronts occurring along the river plume edges, where the main driver of the density difference is actually the increased freshwater input to the shelf. A realistic high-resolution (1 km) hydrodynamic model is applied to the region to investigate the dynamics of such fronts. A scale decomposition that distinguishes the large, meso-, and submesoscale components of model results is carried out. Results show that, along the river plume front, submesoscale patterns prevail. They also possess a certain spatial variability such as filamentation in certain parts of the front, which can be indicative of baroclinic instability. Temporal evolution of this submesoscale variability and some of the forcings that can be responsible for it (e.g. surface cooling, wind stress, topography, or background circulation), together with the role of baroclinic instability are explored

Multi-scale coastal surface temperature in the Bay of Biscay and the English Channel

International audienceThe Bay of Biscay and the English Channel, in the Northeastern Atlantic, are considered as a natural laboratory to explore the coastal dynamics at different spatial and temporal scales. In those regions, the coastal circulation is constrained by a complex topography (e.g. varying width of the continental shelf, canyons), river runoffs, strong tides and a seasonally contrasted wind-driven circulation. Based on different numerical model experiments (from 400m to 4km spatial resolution, from 40 to 100 sigma vertical layers using 3D primitive equation ocean models), different features of the Bay of Biscay and English Channel circulation are assessed and explored. Both spatial (submesoscale and mesoscale) and temporal (from hourly to monthly) scales are considered. Modelled spatial scales, with a specific focus on the variability of fine scale features (e.g. fronts, filaments, eddies), are compared with remotely sensed observations (i.e. Sea Surface Temperature). Different methodologies as singularity and Lyapunov exponents allow describing fine scales features and are applied on both modelled and observed datasets. For temporal scales, in situ high frequency surface temperature measurements from coastal moorings (from COAST-HF observing network) provide a reference for the temporal variability to be modelled. Exploring differences in the temporal scales (from an Empirical Mode Decomposition) advises on the efficiency of our coastal modelling approach. This result overview in the Bay of Biscay and the English Channel aims illustrating the input of coastal modelling activities in understanding multi-scale interactions (spatial and temporal)