Interannual Variability in the Northern Humboldt Current System: Insights from various Upwelling Indices

Coastal upwelling, driven by alongshore wind that pushes surface waters away from the shore, brings nutrient-rich, cold, deep waters to the surface, fueling productivity and biodiversity in marine ecosystems. The Northern Humboldt Current System, as one of the most productive coastal upwelling systems, is renowned for its high fish catches due to persistent upwelling. Since upwelling cannot be directly measured, scientists rely on indices to estimate it through upward velocity, the Ekman transport, and temperature gradients. These indices are derived from winds data, nitrogen concentrations, and sea surface temperature. To identify which indices best correlate with upward velocity and local biogeochemistry in the system, we used a high-resolution regional model: CROCO-BioEBUS. Our results show that wind-based and nitrogen-based indices better correlate with the upward velocity and biogeochemical factors like nitrogen and chlorophyll concentrations.  In contrast, temperature-based indices display different response compared to the wind- and nitrogen-based indices, particularly during extreme events like El Nino. During El Nino, the upwelling intensity increases at the base of the mixed layer depth. Nevertheless, this upwelling intensification did not lead to a major transport of nutrients, as the overall upwelled nitrogen decreased. Discrepancies between the sea-surface temperature-based and wind-based upwelling indices are highlighted during El Nino, suggesting the need to reconsider how upwelling is defined and measured in the Northern Humboldt Current System.

Key words: Upwelling Indices, Northern Humboldt Current System, El Niño