Identification and characterisation of submesoscale activity over the continental shelf in the Bay of Biscay
Frontal instabilities commonly detected in regions of freshwater influence (ROFIs), are considered as fundamental processes in the generation of submesoscale features (a few kilometres to tens of kilometres spatial scale and temporal variability of the order of a day). Consequently, the present study (part of DYMETER project, IFREMER) aims at exploring the development mechanisms of these structures over the continental shelf in the Bay of Biscay. The project is principally based on the analysis of remotely sensed images (high resolution ocean colour MODIS 800m) of Chlorophyll-α concentration, Sea Surface Temperature and Suspended Particulate Matter in view of identifying and characterizing submesoscale activity developing in the vicinity of the Loire River. A 10-year dataset (2003-2013) was explored and satellite ocean colour images revealed a spatial variability in the surface chlorophyll-α distribution. In this context, several events on daily, seasonal and interannual scales were selected and compared but the difficulty lies in tracking the continuous evolution of submesoscale structures in time and space due to the presence of clouds obscuring remote optical sensors. Hydrodynamic parameters like wind forcing and river runoffs were also studied to examine their impact on the submesoscale dynamics. Singularity exponent analysis was performed on the chlorophyll-α images to highlight distinct frontal structures which revealed in turn, a multitude of submesoscale fronts and filaments widespread in the upper ocean. The probability density function (PDF-skewness performed on singularity exponents) was used to characterize the submesoscale structures. The analyses carried out showed that during winter, frontal structures gain in intensity due to a high river outflow regime and dominant wind influence. As such, the PDF curve is skewed-right (strong frontal gradient) in winter and skewed-left (weak frontal gradient) in summer. Wavenumber spectrum analysis was also used to characterize submesoscale structures. Slope values ranging between -0.2 to -0.4 were noted but results obtained did not display significant differences in frontal spatial scales with time. Further investigations (beyond the scope of this thesis) will need to be undertaken to evaluate interactions between hydrodynamics and biogeochemistry (using satellite observations and coupled physical-biogeochemical models) for targeted events in the river plumes.
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