Marine N₂O emissions during a Younger Dryas-like event: the role of meridional overturning, tropical thermocline ventilation, and biological productivity

Past variations in atmospheric nitrous oxide (N₂O) allow important insight into abrupt climate events. Here, we investigate marine N₂O emissions by forcing the Bern3D Earth System Model of Intermediate Complexity with freshwater into the North Atlantic. The model simulates a decrease in marine N₂O emissions of about 0.8 TgN yr⁻¹ followed by a recovery, in reasonable agreement regarding timing and magnitude with isotope-based reconstructions of marine emissions for the Younger Dryas Northern Hemisphere cold event. In the model the freshwater forcing causes a transient near-collapse of the Atlantic Meridional Overturning Circulation (AMOC) leading to a fast adjustment in thermocline ventilation and an increase in O₂ in tropical eastern boundary systems and in the tropical Indian Ocean. In turn, net production by nitrification and denitrification and N₂O emissions decrease in these regions. The decrease in organic matter export, mainly in the North Atlantic where ventilation and nutrient supply is suppressed, explains the remaining emission reduction. Modeled global marine N₂O production and emission changes are delayed, initially by up to 300 years, relative to the AMOC decrease, but by less than 50 years at peak decline. The N₂O perturbation is recovering only slowly and the lag between the recovery in AMOC and the recovery in N₂O emissions and atmospheric concentrations exceeds 400 years. Thus, our results suggest a century-scale lag between ocean circulation and marine N₂O emissions, and a tight coupling between changes in AMOC and tropical thermocline ventilation. (letter)