The California Current System (CCS) is one of the major eastern boundary upwelling systems, which are characterized by a seasonal wind regime that upwells deep nutrient-rich water to the surface, favorable to high biological activity at coast. In the long term, the CCS ecosystem reveals still unexplained shifts in marine communities. In this context, this thesis aims at understanding how pluriannual wind variability influences the structure and functioning of the lower trophic levels of the CCS ecosystem. With this end in view, we carry out process studies based on a numerical approach. In a first study, we show that the low frequency variability of coastal upwelling and cross-shore transport is strongly correlated with that of alongshore wind stress and with the North Pacific Gyre Oscillation (NPGO). This latter mode was recently discovered and is known to explain part of the low frequency variability of the upwelling winds and chlorophyll in the CCS. At a finer scale, we show a strong relationship between the seasonal variability of the upwelling winds and the NPGO, which is translated into temporal modulation of the upwelling onset in the CCS. We investigate the influence of such a delay in upwelling onset on a planktonic ecosystem. Nearshore, in an early upwelling scenario, the ecosystem is immediately more productive. Offshore, the ecosystem is also influenced via cross-shore transport processes. The effect on zooplankton species is more pronounced than on phytoplankton species and may impact higher trophic levels.
