Variability Of Water Column Denitrification and its Drivers in the Eastern Tropical Pacific Oxygen Deficient Zones

In the Eastern Tropical Pacific (ETP) oxygen-deficient zones (ODZs), organic matter remineralization occurs through water column denitrification (WCD), leading to an estimated annual nitrogen (N) loss of 52.9 Tg. In this study we characterize the processes driving WCD variability in the ETP and examine its implications. This study found that WCD predominantly occurs at the upper boundaries of the ODZs, where enhanced organic matter fluxes, and the shoaling and expansion of the ODZ synergistically intensify WCD. The stable ODZ cores contribute the most to WCD. However, the variable outer edges of the ODZs exhibit the highest WCD rates, despite their smaller volumes and shorter residence times, highlighting the influence of mesoscale variability in controlling WCD dynamics. The seasonal cycle, through wind-induced upwelling, enhances WCD during austral summer in the Southern ETP, whereas intermittent gap winds during boreal winter and spring enhance WCD in the Northern ETP. The El Niño Southern Oscillation drives a ~50% increase in WCD during La Niña episodes, while the Pacific Decadal Oscillation describes 15.7% of the decadal variability in WCD. WCD leads to a 20% increase in N limitation, impacting the euphotic zone, with potential implications for primary productivity and the efficiency of the biological pump.