Spatial and seasonal distributions of frontal activity over the French continental shelf in the Bay of Biscay

Abstract

The frontal activity in coastal regions remains a research field where a large number of open questions needs to be addressed to quantify the potential impact of these processes on dependent systems (e.g. biogeochemical activity). Spatial and seasonal distributions of Sea Surface Temperature (SST) fronts (∼1-100 km) in the vicinity of main French rivers, Gironde and Loire, are explored over the continental shelf North of 45 °N in the Bay of Biscay. A high resolution (1 km spatial and daily temporal resolutions) dataset of 11 years' (2003 to 2013) remotely sensed SST by MODIS sensor onboard Aqua and Terra satellites has been investigated and compared with coastal numerical model experiments. The detection and characterization fronts with fluctuating amplitudes is achieved through the Singularity Analysis (i.e. the process of calculating the degree of regularity or irregularity of a function at each point in a domain). Seasonality of frontal activity in the Bay of Biscay is then described based on the long-term satellite SST archive and coastal operational model simulations. The identified hot spots of higher frontal occurrences correspond on one hand to previously observed features (e.g. tidal fronts) but also reveal new features. These are investigated to identify fine-scale dynamical drivers. In winter, density fronts are prominent in a coastal strip where freshwater influence is important. In spring, this strip diminishes as plumes detach from the coast, while tidal fronts become apparent in other regions. In summer, tidal fronts in Ushant region and internal wave activity along the shelf break dominate. In autumn, coastal density fronts due to freshwater inputs reappear as these inputs increase, and reduced stratification causes a weakening of the Ushant and shelf break fronts. Additional information and an effort to dynamically interpret these fronts, based on a systematic investigation of the whole seasonal cycle and including modeling insights from coastal operational oceanography, complement the description of frontal activity in the Bay of Biscay. Highlights ►Seasonality of frontal activity over the Bay of Biscay shelf, in a statistically significant sense, has been documented. Previous studies, which mostly report in situ observations, have a limited temporal continuity, whereas our 1 km resolution satellite dataset not only covers an 11 year period with a daily temporal resolution but also gives us a synoptic view spatially. ►The method of front detection in this study is a novel approach that may have so many potential applications in turbulent systems like the oceans. This method has been shown to serve as a valuable tool in image processing and in our study we show that it is an efficient measure of front detection compared with the classical gradient based methods and it can be used not only to detect but quantify the frontal activity both temporally and spatially.

Reducing the arbitrary: fuzzy detection of microbial ecotones and ecosystems – focus on the pelagic environment

“…Our study revealed the existence of another ecotone between the coastal NW and the STW, with the persistence of a coastal microbial biome. The occurrence of coastal front associated with freshwater inputs have already been reported [ 103 ] and linked with distinct plankton communities [ 104 ]. Additionally, we demonstrate its influence on microbial communities, which has implications for primary productivity in coastal waters [ 28 , 105 ].…”

Non-Linear Processes in the Gironde River Plume (North-East Atlantic): Instabilities and Mixing

“…Ayouche et al (2020) showed that in idealized simulations referring to the Gironde river plume, downwelling favorable winds pushed the plume toward the coast and this might be the case in our simulations during episodic wind regimes. The river plume can also be detected from sea surface temperature in winter (Costoya et al, 2016;Yelekçi et al, 2017) when the strong river discharge strengthens the stratification; the mixed layer is then more sensitive to air-sea fluxes. Fronts in winter remain confined over the inner shelf.…”

“…The residual circulation over the shelf is due to wind and tidal forcings, leading to density gradients. On the continental slope, nonlinear processes favor the generation of mesoscale [scales ∼ O (10-100 km)] and submesoscale [scales ∼ O (1-10 km)] structures (Charria et al, 2017;Yelekçi et al, 2017). The open ocean circulation is marked by long-living mesoscale eddies, mainly generated along the continental slope.…”

“…Tidal and buoyant fronts are ubiquitous in the Bay of Biscay. Buoyant fronts are observed over the continental shelf due to the interaction between the Loire and Gironde river plumes and the open ocean (Yelekçi et al, 2017). This frontal activity is intense in winter due to the river influence over the inner-shelf.…”

The Impact of Tides on the Bay of Biscay Dynamics

“…There have been several studies investigating the location and the dynamics of frontal activity in the Northwest European shelf [28][29][30][31][32]. The authors of [33] are the first to study the physical mechanisms related to the generation of tidal fronts in coastal regions, proposing a criterion for the occurrence of fronts.…”