Award: OCE-2048518

Nitrous oxide (N2O) is a powerful greenhouse gas, ~300 times stronger than CO2, implicated in the destruction of stratospheric ozone, and is currently increasing at a rate of ~0.25% yr-1 from its preindustrial value of 270 ppb (parts per billion) to ~325 ppb at present. The ocean is a significant source of N2O to the atmosphere, however, estimates of the sea-air N2O flux are poorly constrained, ranging from 1.2 to 7.8 Tg N yr-1. Understanding the sources of N2O emissions is critical to predicting future climate impacts. The eastern tropical Pacific (ETP) oxygen deficient zones (ODZs) host some of the highest N2O supersaturations observed in the ocean, resulting in a large air-sea flux. Previous estimates of N2O emissions from the ETP ODZs considered only local escape of N2O to the atmosphere. In focusing on the extremely high N2O supersaturations of the ETP ODZs proper, N2O advected west in zonal tropical currents has been overlooked when determining the contribution of the ETP to marine emissions of N2O. In this work, we used dissolved N2O concentrations from 3 Pacific meridional transects from Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP) cruises (P16N in 2015, P15S in 2016, and P18 in 2016), collected concurrently with other hydrographic measurements (i.e., T, S, transient tracers, O2, nutrients, current velocities) to investigate the transport, production, and efflux of N2O across the tropical Pacific Ocean. Using only observational data, the N2O flux to the atmosphere from the tropical South Pacific was 0.7 +/- 0.4 Tg N2O yr-1 between P18 and P16N and 0.1 +/- 0.06 Tg N2O yr-1 between P16N and P15S. Net N2O zonal flux across the tropical region of P18, south of the equator, was 0.5 Tg N2O yr-1 to the east, i.e., towards the eastern tropical South Pacific ODZ. Net N2O zonal flux across the tropical region of P16N, south of the equator, was 1.3 Tg N2O yr-1 to the west, and 0.01 Tg N2O yr-1 to the west across P15S. Taken together, this suggested that in the central tropical South Pacific (i.e., between P18 and P16N) there was 1.8 Tg N2O yr-1 being transported zonally out of this region after presumably being produced there, with approximately 1/3 of this flux being east towards the ETSP ODZ. This indicated that the central tropical South Pacific was actually a source of N2O to the ODZ, which was a surprising result. In the western tropical South Pacific (i.e., between P16N and P15S) 1.29 Tg N2O yr-1 was presumably lost to the atmosphere, since this is the only significant process that consumes marine N2O outside of anoxic zones. This flux divergence between P16N and P15S is inconsistent with the calculated 0.1 Tg N2O yr-1 air-sea flux. In order to consider temporal variability, output from SODA3 and observations from BGC Argo floats was invoked in conjunction with a Random Forest regression, developed using hydrographic data from all GO-SHIP cruises on which N2O was measured. The hyperparameters for the regression were optimized and the sensitivity of the regression to input parameters, such as using total dissolved oxygen vs. apparent oxygen utilization, or total dissolved N2O vs. the N2O excess above atmospheric saturation was explored. No biases between cruises or ocean basins using this method were found. The Random Forest regression best fit the measured N2O concentrations with dissolved oxygen, temperature, salinity, and nitrate as input variables. Oxygen was the most predictive variable which is consistent with previous work, indicating that nitrification is the dominate process producing N2O in the global ocean. Additionally, this project afforded the opportunity for mentoring two undergraduate summer interns (one in each year of the project), funded through University of Washington Cooperative Institute for Climate, Ocean, and Ecosystem Studies' dedicated summer intern program, which encourages students that are part of traditionally under-represented groups in sciences to apply. Undergraduate interns were introduced to marine biogeochemistry, physical oceanography, and basic numerical modeling, in addition to receiving mentorship and guidance about careers in environmental science from graduate students and other professionals at the University of Washington and NOAA Pacific Marine Environmental Laboratory. Last Modified: 08/29/2023 Submitted by: Bonnie Chang