ENSO related Changes in Offshore Transport and Biological Productivity within the California Current System from 1979 to 2014

In the California Current System (CalCS), much of the observed year-to-year variability in biological production and offshore transport of organic matter and nutrients is associated with El Niño-Southern Oscillation (ENSO). While many studies have elucidated the CalCS ecosystem response to individual ENSO events, more comprehensive work on longer records that capture the complex coastal dynamics is still lacking. In particular, the interannual variability of the offshore transport and its associated impacts on adjacent waters beyond the upwelling front remain unclear.

Here, we assess the response of coastal production and offshore transport of organic matter and nutrients to ENSO within the CalCS between 1979 and 2014 based on a high-resolution modeling study. We use the Regional Oceanic Modeling System coupled to a biogeochemical/ecological model with three plankton functional types on a novel telescopic model setup with substantial grid refinement toward the coastal region. This setup facilitates an investigation of the underlying processes in a manner hitherto not possible, since it accurately represents the coastal mesoscale dynamics but still permits large-scale connectivity between the tropical Pacific, the CalCS, and the North Pacific. We find that during months of prevailing El Niño conditions, both the biological production of organic carbon and the relative fraction of this production that is transported offshore (export efficiency) within the CalCS are substantially reduced by 25% and 10%, respectively, whereas during La Niña events these tendencies reverse. The changes in offshore transport efficiencies are a result of both physical and biological responses to ENSO and differ between El Niño and La Niña. ENSO related modifications within the upwelling region thus potentially have repercussions on ecosystems of adjacent waters, whose metabolism is supported by the offshore transport of coastal water masses with high nutrient and organic matter concentrations.