Temporal Variability of Cross-shore Fluxes in the California Current Ecosystem

Eastern Boundary Current Upwelling Systems are regions of elevated primary production and play a central role in the global carbon cycle. Because of their strong spatial and temporal variability and the fine-scale activity associated to their dynamics, they are notorious sources of uncertainty in climate-resolving circulation models. In these regions, nutrient upwelling occurs in a narrow region close to the coast, but primary production and carbon export are typically observed across a broader region. This spatial discrepancy suggests the importance of horizontal transport linking coastal and offshore domains. Three primary mechanisms have been identified for cross-shore horizontal transport: Ekman transport, filaments and eddies. However, quantification and discrimination of the dominant processes and offshore extent, seasonality and interannual variability of these fluxes remain poorly described. Here we address these questions focusing on the California Current Ecosystem (CCE). By employing satellite-derived Lagrangian diagnostics of horizontal transport, we characterize the cross-shore fluxes in terms of their spatial extent and advection time over 24 years. These data are assessed by comparison with surface chlorophyll concentration. We find that cross-shore fluxes are driven by a strong seasonally variable mean transport accompanied by pronounced, intermittent mesoscale events. There is considerable interannual variability in the offshore extent of transport, which is partially explained by covariation with ENSO. This long term analysis of cross-shore fluxes in the CCE can be applied to better understand the ecological impact of climate forcing and thus help improve the resolution of upwelling-related processes in climate models.