A Mesoscale Resolving Model Reveals Pathways of Nutrient Transport Across Southern Ocean Fronts

Understanding the processes that drive nutrient transport across Southern Ocean (SO) fronts is critical for accurately modeling global biogeochemical cycles. In this study, we examine the role of mesoscale processes in cross frontal nitrate transport in the SO. Using a novel decomposition method for tracer advection, we spatially decompose velocity and nitrate concentration fields from global ocean models into mesoscale and larger-scale components. Our analysis reveals two key ways in which mesoscale features influence cross frontal nutrient transport. First, mesoscale flows modify large-scale circulation, leading to differences in large-scale nitrate fluxes in mesoscale resolving models compared to coarser resolution models. Second, mesoscale flows and enhanced nitrate gradients directly drive additional nutrient fluxes that are absent in coarse-resolution models. The importance of these effects varies with region and depth with small-scale velocity features dominating nutrient transport near rough bottom topography, and mesoscale nitrate structures being crucial for capturing transient, eddy driven transport in the upper water column. These findings highlight the significance of mesoscale processes in cross frontal nutrient fluxes, especially at depth, and underscore the need for improved parameterizations of mesoscale processes in lower resolution models to better simulate cross frontal nitrate transport.