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

Abstract

Abstract Upward transport and/or mixing of trace gas‐enriched subsurface waters fosters the exchange of nitrous oxide (N 2 O) and methane (CH 4 ) 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 N 2 O and CH 4 in the ESA were investigated to identify the contributions of the major regional water masses to the overall budget of N 2 O and CH 4 . The maximal sea surface N 2 O and CH 4 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 N 2 O and CH 4 in the ESA. Our findings also show that the shape of N 2 O and CH 4 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 CH 4 than N 2 O distribution, in particular in the Walvis Bay area. With respect to N 2 O, however, low‐oxygen waters from the poleward undercurrent impinge in the shelf close to Cape Frio and often result in N 2 O concentrations significantly higher than off Lüderitz ( p < 0.05).