Spatial and seasonal distributions of frontal activity over the French continental shelf in the Bay of Biscay. (15th July 2017) - Yelekçi, Özge
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' (2003–2013) 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 using a 1 km resolution satellite dataset, and 2.5 km numerical simulations. The method of front detection in this study, Singularity Exponents, is a novel approach that may have so many potential applications in turbulent systems like the oceans. We show that it is an efficient measure of front detection compared withto the classical gradient based methods and it can be used not only to detect but to quantify the frontal activity both temporally and spatially. Three major types of fronts are identified; fresh water fronts at wintertime plume edges, tidal fronts along the coast, and internal tidal wave induced fronts along the shelf break during thermally stratified months.