51 / 2024-09-02 15:14:25

Recent Changes in Global Ocean N2O Emissions and Their Attribution to Climate Modes of Variability

N2O,Global-Ocean,Biogeochemistry model

Nitrous oxide (N₂O) is a long-lived potent greenhouse gas with the Global Warming Potential (GWP) ~300 times higher than that of CO₂. So far, the global estimation of oceanic N₂O flux remains highly uncertain due to poorly spatial-temporal coverage of historical measurements (1.8－9.4 Tg N/yr). To improve the global oceanic N₂O emission estimation, better quantify its spatial-temporal variability, and gain insights into the driving mechanisms in the world’s oceans, we employed a global coupled ocean circulation-biogeochemistry model (ECCO-DARWIN). In the model, N₂O production was parameterized as a function of O₂ consumption and depth. Furthermore, the thermal and ventilation air-sea N₂O fluxes were counted separately. We first run the model from 1992 to 2017 and compared the simulation results with the MEMENTO database. The model successfully captured the high subsurface N₂O concentration in the tropical zone of the Atlantic Basin, while displaying low concentrations in the extratropical zone. In the Pacific ocean, the model reproduced the general pattern of high subsurface N₂O from polar to tropical regions. In the Indian ocean, both the model and observations showed high N₂O in the northern part. Overall, global N₂O emission is ~2.43Tg N/yr, and the Eastern Tropical Pacific, Subtropical Indian and Southern Oceans emerged as N₂O hotspots, represent more than 50% of global oceanic emissions. Furthermore, we investigated the dominant modes of N₂O flux variability in 8 major ocean basins. The results indicated that detrended and deseasonalized air-sea N₂O flux shows substantial regional variations, with frequencies aligning with major modes of climate variability (AMO, PDO, ENSO, and SAM). Correlation coefficients between these climatic indices and N₂O flux in the frequency domain corresponded to as high as 0.9, 0.6, and 0.4 for the Tropical Pacific, Indian Ocean and Southern Ocean, respectively. The variability in the Tropical Pacific and Southern Ocean was equally influenced by thermal and ventilation, whereas the Indian Ocean are more driven by the ventilation. Our findings underscore the significant influence of nature variabilities on oceanic N₂O emissions, which may help formulating effective management strategies to mitigate greenhouse gas emissions.