Decoupling of N2O Production and Emissions in the Northern Indian Ocean

Abstract

The northern Indian Ocean is a hotspot of nitrous oxide ($N_2$O) emission to the atmosphere. Yet, the direct link between production and emission of $N_2$O in this region is still poorly constrained, in particular the relative contributions of denitrification, nitrification and ocean transport to the $N_2$O efflux. Here, we implemented a mechanistically based $N_2$O cycling module into a regional ocean model of the Indian Ocean to examine how the biological production and transport of $N_2$O control the spatial variation of $N_2$O emissions in the basin. The model captures the upper ocean physical and biogeochemical dynamics of the northern Indian Ocean, including vertical and horizontal $N_2$O distribution observed in situ and regionally integrated $N_2$O emissions of 286 $\pm$ 152 Gg N ${\text{yr}}^{-1}$ (annual mean $\pm$ seasonal range) in the lower range of the observation-based reconstruction (391 $\pm$ 237 Gg N ${\text{yr}}^{-1}$). $N_2$O emissions are primarily fueled by nitrification in or right below the surface mixed layer ($\sim$57%, including 26% in the mixed layer and 31% right below), followed by denitrification in the oxygen minimum zones ($\sim$30%) and $N_2$O produced elsewhere and transported into the region ($\sim$13%). Overall, $\sim$74% of the emitted $N_2$O is produced in subsurface and transported to the surface in regions of coastal upwelling, winter convection or turbulent mixing. This spatial decoupling between $N_2$O production and emissions underscores the need to consider not only changes in environmental factors critical to $N_2$O production (oxygen, primary productivity etc.) but also shifts in ocean circulation that control emissions when evaluating future changes in global oceanic $N_2$O emissions.