Meridional and Cross-Shelf Variability of N2O and CH4 in the Eastern-South Atlantic

Upward transport and/or mixing of trace gas-enriched subsurface waters fosters the exchange of nitrous oxide (N2O) and methane (CH4) with the atmosphere in the Eastern-South Atlantic (ESA). To date, it is, however, unclear whether this source is maintained by local production or advection of trace gas-enriched water masses. The meridional and zonal variability of N2O and CH4 in the ESA were investigated to identify the contributions of the major regional water masses to the overall budget of N2O and CH4. The maximal sea surface N2O and CH4 concentrations and the main ESA upwelling cells co-occurred with a strong negative correlation with the sea surface temperature (SST) (p < 0.05). The dominance of the central water masses in the winter and spring seasons and the interplay between shelf topography and wind regime are suggested to determine enhanced gas transfer toward the sea-air interface or "capping" at midwater depth. These parameters are supposed to be critical in the local budget of N2O and CH4 in the ESA. Our findings also show that the shape of N2O and CH4 gradients is very similar both meridionally and zonally; however, the extent of the differences between the high-end and low-end members of the concentrations/saturations range is different. This suggests a more pronounced effect of local sources on CH4 than N2O distribution, in particular in the Walvis Bay area. With respect to N2O, however, low-oxygen waters from the poleward undercurrent impinge in the shelf close to Cape Frio and often result in N2O concentrations significantly higher than off Luderitz (p < 0.05). Plain Language Summary The Eastern-South Atlantic (ESA) with some well-documented local upwelling cells is one of the most productive upwelling regions which create hotspots for climate active trace gas production including nitrous oxide (N2O) and methane (CH4). N2O is mainly produced via microbially driven nitrification and denitrification in less oxygenated waters, whereas CH4 is primarily produced by microbial anaerobic respiration in sediments, and both trace gases are exchanged with the atmosphere in air-sea interface. To what extent the gas distribution and its exchange with the atmosphere is related to water mass advection or local production is still uncertain. Here, we aim to address the contribution of dominant regional water masses to regional trace gas distribution and its atmospheric exchange. We suggest that while there is a more pronounced effect of local sources on CH4 than N2O distribution, in particular in the Walvis Bay area, N2O distribution is more governed by low-oxygen waters from the poleward undercurrent impinging on the shelf close to Cape Frio, deriving significantly higher N2O concentrations than off Luderitz (p < 0.05).