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The open ocean is a major source of nitrous oxide (N2O), an atmospheric trace gas attributable to global warming and ozone depletion. Intense sea-to-air N2O fluxes occur in major oceanic upwelling regions such as the eastern tropical South Pacific (ETSP). The ETSP is influenced by the El Nino-Southern Oscillation that leads to inter-annual variations in physical, chemical, and biological properties in the water column. In October 2015, a strong El Nino event was developing in the ETSP; we conduct field observations to investigate (1) the N2O production pathways and associated biogeochemical properties and (2) the effects of El Nino on water column N2O distributions and fluxes using data from previous non-El Nino years. Analysis of N2O natural abundance isotopomers suggested that nitrification and partial denitrification (nitrate and nitrite reduction to N2O) were occurring in the near-surface waters; indicating that both pathways contributed to N2O effluxes. Higher-than-normal sea surface temperatures were associated with a deepening of the oxycline and the oxygen minimum layer. Within the shelf region, surface N2O supersaturation was nearly an order of magnitude lower than that of non-El Nino years. Therefore, a significant reduction of N2O efflux (75 %-95 %) in the ETSP occurred during the 2015 El Nino. At both offshore and coastal stations, the N2O concentration profiles during El Nino showed moderate N2O concentration gradients, and the peak N2O concentrations occurred at deeper depths during El Nino years; this was likely the result of suppressed upwelling retaining N2O in subsurface waters. At multiple stations, water-column inventories of N2O within the top 1000m were up to 160% higher than those measured in nonEl Nino years, indicating that subsurface N2O during El Nino could be a reservoir for intense N2O effluxes when normal upwelling is resumed after El Nino.