Phenological Variability of Coastal Upwelling in the California Current System

In the California Current System (CCS), wind-driven coastal upwelling draws nutrient-rich water from depth and fuels rich and diverse biological populations at all trophic levels. Historically, spatiotemporal variability of coastal upwelling, including variability in its amplitude and timing (i.e. phenology), has been described by the Bakun Index, which quantifies the amount of offshore transport of surface waters due to the large-scale geostrophic wind field. Recently, a new set of upwelling indices has been developed for the CCS that improves the representation of the surface wind stress, accounts for the influence of geostrophic currents, and estimates vertical nitrate flux. Using satellite chlorophyll observations in combination with these new upwelling indices, we demonstrate that phenological variability of phytoplankton biomass in the CCS is driven by both event-scale wind forcing and remotely-driven fluctuations in subsurface nutrient concentrations. Understanding the causes and ecosystem consequences of phenological changes in coastal upwelling nutrient availability is critical, as climate models project significant variability in the amplitude and phase of coastal upwelling and in the nutrient content of upwelling source waters under climate change.