Compensatory Mechanisms Reduce ENSO-driven Nitrous Oxide Emission Variability in the Eastern Tropical Pacific

Nitrous oxide (N₂O) is a potent greenhouse gas, with the Eastern Tropical Pacific (ETP) being a hotspot of N₂O emissions due to high N₂O production in the oxygen minimum zones (OMZs). However, N₂O emissions in this region remain poorly constrained due to (i) temporal variability, which is hypothesized to be largely driven by the El Niño-Southern Oscillation (ENSO), and (ii) limited process understanding. To address these shortcomings and improve the quantification of N₂O emissions and ENSO-driven variability in the ETP, we run a regional ocean model on a telescopic grid (~4km), spanning the entire Pacific Ocean, from 1979 to 2019. The model includes a biogeochemical model and a novel nitrogen module (NitrOMZ), which explicitly resolves the N₂O production via incomplete denitrification and ammonium oxidation and accounts for the different oxygen inhibition thresholds of these biological N₂O production pathways. We find that 1 Tg N of N₂O is emitted annually in the ETP, and that N₂O emissions deviate up to ±0.18 Tg N y^-1 from the mean during ENSO events across the entire ETP, with La Niña increasing N₂O emissions and El Niño decreasing them. Most of the ENSO-driven N₂O emission anomalies can be attributed to variability in incomplete denitrification in the oxyclines of the oxygen minimum zones. Compensatory effects among gross N₂O production, consumption, and transport reduce both the total N₂O emissions and their interannual variability by an order of magnitude. Our results alleviate previously raised concerns that La Niña events may substantially amplify N₂O emissions. Such compensatory mechanisms might also reduce N₂O emissions in other OMZs and mitigate the impact of climate change on N₂O emissions, provided that compensatory mechanisms remain effective in the future.