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The Senegalese coastal and shelf systems comprises a southern part of the Canary Islands upwelling system. The present study focuses on the study of phytoplankton from meso- to submesoscale during the transition period from the warm West African monsoon season to the cold upwelling season. This period coincides with the return of sardinella from their northward migration to its second most important spawning area resulting in a high retention on the southern coast, as well as possible events of the Senegalese fishermen's skin disease (as it was the case in November 2020). This is a very poorly documented period. The last studies allowing the study of the phytoplankton compartment date from the 1980s. Several data on phytoplankton were collected during the period from 29 November to 02 December 2017 for addressing phytoplankton distribution and dynamics: pigmentary data, microscopic counts, metabarcoding analysis of plankton diversity, single-cell analysis and characterization of optical groups by automated (in vivo) flow cytometry (CytoSen) as well as in vivo characterization of spectral/pigmentary groups by multispectral fluorometry (Fluoroprobe). Environmental data was supplied by CTD RBR concerto and the analysis of several physical and chemical parameters. In particular, FluoroProbe continuous subsurface measurements and profiles made it possible to considerably improve the spatial and temporal resolution of measurements and the dynamics of phytoplankton groups at submesoscale. Moreover, it was possible to follow spatial and temporal changes in the phytoplankton community, particularly at stations sampled twice at few days interval. Many unknown species characterized this period, especially in the nanophytoplankton size range. Distinct communities were found in the upwelling on the coastal fringe and in the old waters offshore, as shown by multispectral analysis. Phytoplankton blooms were observed, some of which being caused by the upwelling of cold water, but intermittently and weakly. In some stations, toxic species were found, such as species belonging to the genus Pseudo-Nitzschia. Keywords: Upwelling, multi-spectral fluorometry, CTD, automated flow cytometry, metabarcoding, microscopy, phytoplankton diversity and dynamics.

Saharan dust represents more than 50% of the total desert dust emitted around the globe and its radiative effect significantly affects the atmospheric circulation at a continental scale. Atmospheric models often fail to represent the dust vertical distribution and the Saharan Air Layer. They underestimate the effects of deep convection on the vertical transport and of the role of scavenging on the confinement of dust aerosols in this layer. Using multi-year simulations performed with a variable-resolution climate model and processed-based analysis, we show that scavenging in deep convection and further re-evaporation of dusty rainfall in the lower troposphere are critical processes for explaining the vertical distribution of desert dust. They play a key role in maintaining a well-defined dust layer with sharp transition at the top of the SAL and in establishing the seasonal cycle of dust distribution.