Evolution of upper layer temperature in the Bay of Biscay during the last 40 years

The temperature evolution over the past 40 years in the Bay of Biscay (North-East Atlantic) is investigated from an in situ data analysis, completed with a satellite SST (Sea Surface Temperature) analysis over the last 20 years. The in situ dataset is an interannual version of the BoByClim climatology, covering the Bay of Biscay area with a 10-km horizontal resolution and a 5-m vertical step. The satellite dataset is the European COastral Sea Operational Observing and Forcast System Program (ECOOP) daily analysis, covering the IBIROOS (Iberian-Biscay-Irish Sea) area with a 4-km resolution. The study area (43°N-50°N/12°W-1°W) extends over the intersection of the domains covered by these two analyses. In the 0–200 m layer of this domain, a heat budget has been computed from an ocean circulation model over the period 1965-2004. Heat is essentially imported from the west by the North Atlantic Drift, then exported southward and northward, or transferred downward into deeper layers. The annual average of air-sea flux is weak, with a zero isoline crossing the domain from the northwest to the southeast. Ocean and atmosphere, forming a coupled system, have comparable contributions and may both result in a heat sink or source. At the interannual scale, variability is mainly controlled by the air-sea flux, with a secondary contribution from oceanic transport. A strong warming of $\sim$0.2 °C/decade for the period 1965-2004 is apparent from the surface down to the 200-m depth. The decadal evolution is consistent with several analyses over the North Atlantic. Meanwhile, warming is approximately twice faster than in the whole basin. This trend is not steady, as a cooling period occurred until the early 1970's, and the temperature increase has been significantly stronger during the last 20 years ($\sim$0.3 °C/decade between 0 and 100 m, from 1986 to 2005). The trend and the interannual variability are maximal over the northern part of the shelf. The typical duration of interannual anomalies is two years and their penetration depth is ~100 m, although they occasionally exceed 200 m. The interseasonal anomalies are also stronger within the mixed layer (root-mean-square, RMS = 0.6 °C at the surface) than below (RMS = 0.3 °C at 100 m). Their geographic distribution is radically different between the surface and the deep levels. A seasonal dependence of the warming rate is evident in the upper layer, as the temperature trend is stronger in summer than in winter (by a factor of 2 at the surface). Some of the potential implications for the ecosystem are discussed, particularly in terms of species distribution and dynamics of species exploited by fisheries

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

Interaction between an eddy and a zonal jet Part I. One-and-a-half-layer model.

International audienceThe interaction between a stable zonal jet and a vortex is studied numerically with two one-and-a-half layer models, one with quasi-geostrophic dynamics, the other with shallow-water equations. In both models, simulations on the f-plane evidence three regimes occuring with increasing vortex strength: (regime 1) weak vortices do not cross the jet and steadily drift along it; (regime 2) stronger vortices cross the jet, tear an opposite-sign meander from the jet with which they pair as a dipole; the trajectory of this dipole depends on the strength of the initial vortex; since most dipoles are asymmetric, they veer back towards the jet axis where they are split apart in the ambient shear; (regime 3) even stronger vortices cross the jet and tear a vorticity filament without dipole formation. The influence of various physical parameters on jet–vortex interaction is studied. In particular, β-effect is not sufficient to drive all vortices through the jet. Numerical simulations show that jet crossing occurs when the maximum velocity of the vortex is larger than, and opposite to, that of the jet. This allows the mathematical derivation of an analytical criterion for jet crossing in both models, which relates the potential vorticity anomalies of the jet and vortex, the vortex and internal deformation radii. In the shallow-water model, an asymmetry is observed between anticyclones north of the jet and cyclones south of it. The role of a spatially varying deformation radius and of vortex cyclostrophy on this asymmetry is discussed

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

Investigation of 2-D and 3-D characteristic-based open boundary conditions for regional ocean models

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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

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