Extreme 2016 El Niño heatwave weakened carbon export and respiration in the Equatorial Pacific.

The 2016 El Niño was the strongest warm-phase of the El Niño-Southern Oscillation recorded to date over the 21st century, leading to the development of severe ocean surface temperature anomalies - marine heatwaves - across the equatorial Pacific. Here we analyze biogeochemical model output and machine learning-based reconstructions of primarily Argo oxygen and backscattering measurements to demonstrate the impact of the 2016 El Niño in weakening oceanic carbon export - the transfer of biogenic carbon from the surface ocean to depth. We show that central equatorial Pacific anomalies in modeled carbon export flux and reconstructed optical particle backscatter and ecosystem respiration display interannual variations linked to the El Niño-Southern Oscillation, with an acute reduction of 50 % in carbon export during the 2016 El Niño. This plunge in export production is attributed to a large decrease in chlorophyll biomass observed from space, and modeled ecological changes in phytoplankton community composition.