Séminaire

The California Current System (CCS) is an eastern boundary upwellingsystem characterized by high biological production along the coastcontrasted with oligotrophic offshore waters, creating a cross-shoregradients of biological and physical properties. This cross-shoregradients are affected by intense mesoscale eddy activity thatmodulates and exports biological production from the coastal upwellingsystem, entraining and redistributing nearshore nutrients andassociated planktonic organisms. High planktonic biomass can be foundin these eddies months after detaching from the coast. The mechanismsdriving these patterns, and their ecological impacts in EBUSs arestill in debate. First, to characterize and quantify the ability of mesoscaleeddies to affect the local and regional planktonic ecosystem of theCCS, we analyzed a 10-year-long physical-biological model simulation(ROMS-NEMURO, at 5 km horizontal resolution), using eddy detection andtracking to isolate the dynamics of cyclonic and anticyclonic eddies.Using a composite analysis of thousands of detected eddies, we foundthat as they propagate westward across the shelf, cyclonic eddiesefficiently transport coastal planktonic organisms, and maintainlocally elevated production for up to one year (800 km offshore).Anticyclonic eddies, on the other hand, have a limited impact on localproduction over their ~6 month lifetime as they propagate 400 kmoffshore. At any given time ~8% of the model domain was covered byeddy cores. Though the eddies cover a small area, they explain ~50 and20% of the transport of nitrate and plankton, respectively. Then, to elucidate the mechanisms that influence the dynamics ofecosystems trapped in eddies, and the relative contribution ofhorizontal and vertical advection in determining local production, weanalyze one single cyclonic eddy from our model simulation, usingLagrangian particle-tracking analyses. The eddy formed in a coastalupwelling system, and sustained enhanced biological production withinits core for several months. Coastal waters trapped in the eddyenabled it to leave the upwelling region with high concentrations ofplankton and nutrients. We highlight the role of this eddy in alteringlocal planktonic ecosystem dynamics, and contrast those dynamics withthe coastal upwelling source waters, and the waters surrounding theeddy. We conclude that cyclonic eddies play a key role for biologicalproduction in the CCS: they contribute both to the redistribution ofthe coastal upwelling ecosystem and are local spots of new production.Together, these processes impact cross-shore gradients of importantbiological properties.

Related Publications:Chenillat, F., V. Combes, and P. J. S. Franks, 2016. Biogeochemicaleddy properties in the California Current System. Geophysical ResearchLetters http://onlinelibrary.wiley.com/doi/10.1002/2016GL068945/full Chenillat, F., P.J.S. Franks, P. Rivière, X. Capet, N. Grima and B.Blanke, 2015. Mesoscale activity in the Southern California CurrentSystem. Biological dynamics of a coastal eddy. Journal of GeophysicalResearch: Oceans. http://dx.doi.org/10.1002/2015JC010826