Extreme events in the Eastern South Atlantic Ocean enhance regional coastal N2O emissions

Coastal areas within the Eastern South Atlantic Ocean are known hotspots for production and emissions of climate-relevant trace gases. Local circulation, the occurrence of upward tracer transport events through e.g. coastal upwelling and coastally-trapped waves, and a pronounced oxygen minimum zone are crucial in setting the overall emissions of nitrous oxide (N2O) towards the atmosphere. While previous studies quantified the magnitude of cross- and along-shelf gradients of N2O in the region, its main formation pathways, and its seasonal variability, to date it is unclear at what extent extreme events might affect N2O dynamics. Given the projected increase in frequency and severity of events such as storms and oceanic heat waves, which might temporarily, yet significantly modify environmental conditions under which N2O is produced and transported across the sediment-water-air interfaces, it is therefore critical to assess the role of such events on its distribution and emissions. In this study we combine physical, chemical and microbial observations gathered during two major expeditions in 2018 and 2023 to present evidence of a hitherto unseen enhancement in air-sea fluxes of N2O in association with storm events and mesoscale activity. We show that during periods of sustained winds off Walvis Bay at 23°S, water column mixing down to 100 m depth can lead to a two-fold increase in air-sea N2O fluxes driven by the transport of enriched, near-bottom waters towards the surface, which surpasses by far values observed during typical upwelling events. Observations across a mesoscale cyclonic eddy off Angola centered at 16⎼17°S (a rare feature which is thought to occur in average 2 times per year in the region), show that both extreme warming-driven outgassing at the sea surface and enhanced upward transport of N2O-enriched waters at the eddy core play a role in enhancing the overall emissions from waters otherwise thought to be mostly representative of open ocean conditions (i.e. in near equilibrium with the atmosphere). In this contribution, we discuss the main mechanisms by which these extreme events resulted in enhanced N2O air-sea fluxes and how they might impact current marine N2O emission estimates, which due to the lack of targeted observations, do not capture this source of variability.