The northern Indian Ocean is a hotspot for emissions of the potent greenhouse gas nitrous oxide (N₂O) with vigorous seasonality. Yet, the seasonality of N₂O emissions reconstructed from sparse observations is compounded by the strong interannual variability associated with the Indian Ocean Dipole (IOD), rendering the seasonal variability of N₂O efflux to the atmosphere highly uncertain. Here we examined the impacts of the IOD on the seasonal variability of ocean surface N₂O concentration, which controls N₂O emissions. We use a regional ocean model of the Indian Ocean coupled with biogeochemistry and a new mechanistically-based N₂O cycling module that reproduces observed seasonal and interannual patterns of ocean circulation and biological productivity over the past four decades. Positive IOD phases enhance ocean surface N₂O concentrations in the Bay of Bengal (BoB) from autumn to the following spring, while decreasing N₂O concentrations in coastal regions of the eastern Arabian Sea (EAS) from summer to winter. Conversely, negative IOD phases elevate ocean surface N₂O concentrations in the EAS mainly in autumn, and reduce concentrations along the coastal regions of the eastern BoB during winter and the following spring. These patterns can be attributed to interannual variations in the subsurface production of N₂O and its transport to the surface by upwelling/downwelling coastal Kelvin waves propagating anti-clockwise along the rim of the northern Indian Ocean. Overall, we find that the interannual variability of N₂O in these regions accounts for about 30-60% of its seasonal variability, critical for constraining N₂O emissions in (sub)tropical coastal oceans.

interannual variability,seasonal variability,nitrous oxide,northern Indian Ocean,Indian Ocean Dipole