Abstract

The El Niño (EN) event of 2023 exhibited a unique evolution, starting with an extremely warm coastal episode off Peru followed by a moderate basin-scale event. We addressed the associated oceanographic and biological conditions (chlorophyll and zooplankton biomass) in the Humboldt Archipelago. It is part of the Coquimbo upwelling system (29°–30°S), within the Humboldt Current System. Eight (8) campaigns between November 2022 and December 2023, over a deep canyon surrounding the archipelago, provided hydrographic profiles and samples for determinations of chlorophyll (Chl) and particulate organic carbon (POC) concentrations and large- and small-sized mesozooplankton biomass. Oceanographic variability over the period was analyzed through reanalysis products and satellite observations, including data of sea level, surface wind, sea surface temperature (SST) and sea surface chlorophyll, geostrophic currents, and mixed layer depth (MLD). Upwelling was promoted by high-frequency variability in winds and deeper MLD associated with the basin-scale EN. The EN also fosters the arrival of a Kelvin wave in June and July, leading to positive anomalies in SST and sea level, elevated oxygen levels, and increased pH in the upper 100 m. The lowest Chl concentration was recorded after the warming event, while POC concentration and mesozooplankton biomass exhibited temporal and vertical stability. However, a significant surface increase for both size fractions was observed during the spring. Zooplankton biomass was correlated to food resources and transport, suggesting stronger regulation by local drivers. Current findings are discussed in the context of recent studies that have documented the local circulation patterns in this region. © 2025. American Geophysical Union. All Rights Reserved.