Cross-shore transport varaibility in the California Current: Ekman upwelling vs. eddy regime Combes V., F. Chenillat, P. Riviere, E. Di Lorenzo, M. Ohman and S. J. Bograd We investigate the low-frequency variability in the transport (cross-shore and alongshore) of coastal water masses in the Central and Southern California Current System (CCS) over the period 1950-2008 using the Regional Ocean Modeling System (ROMS). Our approach consists of an ensemble of passive tracers released in the numerical model to characterize the effects of linear (Ekman upwelling) and non-linear (eddy activity) circulation regimes on the statistic of low frequency advection of coastal waters. By looking at the passive tracer concentration distribution, we find that the low-frequency upwelling and the surface offshore transport of the upwelled nutrient-rich coastal water are strongly correlated with the alongshore wind stress, and is coherent between the central and southern CCS. However, the offshore transport of the surface coastal water is not anymore coherent between those two regions, and has been found to be associated with mesoscale eddy activity, where both surface and subsurface waters propagate offshore, trapped in the eddy core. Our results also show that the poleward California Under-Current, at about 200m depth, affects the alongshore transport and provides rich waters to the Central California dominant upwelling cell. Therefore the passive tracer experiments, performed in this study, could provide a dynamical framework to understand the dynamics of the upwelling and offshore transport of nutrient rich coastal water and to interpret how it responds to atmospheric forcing. This also could reinforce our interpretation (and therefore predictions) in the changes in vertical and offshore advection of other important biogeochemical quantities, essential in understanding the Californian ecosystem variability.