Response of the subtropical oxygen minimum zone off Chile to El Niño events in a regional biogeochemical model: remote vs local forcings

The Humboldt eastern boundary upwelling system, due to its proximity to the equator is highly sensitive to equatorial Pacific disturbances, particularly those associated with the El Niño Southern Oscillation (ENSO). This has consequences for the so-called oxygen minimum zone, an extensive area of low oxygen waters at intermediate depths that impacts marine biota. During warm El Nino events, the OMZ tends to shrink in its upper margin along both Peru and central Chile but the magnitude of the change is sensitive to the characteristics of El Niño events. Here, we focus on the subtropical OMZ off Chile (18°-38°S) and document OMZ volume and pattern changes during extreme El Niño events (1982/83 and 1997/1998) based on a regional coupled model simulation. This is contrasted to changes that occurred in 1972/73 that correspond to the occurrence of a moderate El Niño event in the tropical Pacific. The results indicated that the volume of the subtropical OMZ off Chile decreased on average by 27-48% during these El Niño events, which was associated with a coastal oxycline deepening that peaked during the development phase of the events. However, we find that the magnitude of the change varies a lot between events with in particular the 1972/73 El Niño exhibiting the largest changes in volume. The model analyses reveal that the OMZ volume reduction resulted from a combined effect of changes in the poleward transport oxygen-poor waters by the Peru-Chile undercurrent and the contribution of Ekman pumping (negative wind stress curl) and mesoscale eddy fluxes.