Section: New Results

Surface mixing and biological activity in the Northwest African upwelling

Participants: A. El Aouni, K. Daoudi, H. Yahia.

The aim of this work is to study the horizontal stirring and mixing in different upwelling areas of the Northwest African margin using attracting/repelling Lagrangian coherent structures (LCS) obtained as subsets of hyperstreamline of the Cauchy-Green strain tensor, whose normal repulsion rate is larger than tangential stretch over backward/forward time interval, and their link to the chlorophyll fronts concentrations, based on 10 years satellite data. The temporal variability of surface stirring is compared to the fronts chlorophyll concentration. Two of the four studied areas exhibit negative correlation between mixing and the chlorophyll concentration. The other two regions show similar seasonal variations, nearly coincident maxima and minima, leading to a global positive correlation. These results are compared to other works that make use of Finite Size Lyapunov Exponent (FSLE) whose output is a plot of scalar distributions. Furthermore, we compare the chlorophyll concentrations with both compressing and stretching lines. Results show different regions with different properties. The surface mixing and chlorophyll concentrations correlation is governed by stretching lines in two regions, by compressing lines in one region with, while no different is shown between attracting and repelling LCS in the northenrn region of the studied area.

Publication: [20].