Small-Scale Variability of Bottom Oxygen in the Northern Gulf of Mexico

While the occurrence of seasonal bottom hypoxia in the Northern Gulf of Mexico is an extensively studied subject, most research effort has been put on understanding and quantifying hypoxic extent, and little is known about internal variability and short-term shifts. We use a realistic hydrodynamic model with a simple oxygen parametrization to demonstrate that hypoxia development in the far-field of the Mississippi plume is subject to physical processes with spatial scales ranging from O(10) to O(100 km) and temporal scales from the near-inertial period to seasonality. We use a budget decomposition of the oxygen equation to explore patterns. The mean spatial structure of the local rate of change reveals features shaped as oxygen loss rings enclosing oxygenation cores. This structure is mainly a balance between net advection (horizontal and vertical) and sediment oxygen demand. In terms of temporal variability, we found a large near-inertial signal in advection, following a convergence-divergence pattern, at all spatial scales, and a strong subinertial signal at smaller scales only. Variability in advection increases with decreasing temporal and spatial scales, consistently with a field rich in instabilities introducing small-scale, strong anomalies. Through a Reynolds decomposition of the budget, we separate anomalies from the main flow and identify the total vertical perturbation flux as the primary counterbalance to sediment oxygen demand during periods when the hypoxic extent is maintained or destroyed. Vertical flux anomalies manifest as bottom water intrusions into the mid water column, which have been captured by high-resolution observations.

Automated summary

The occurrence of seasonal bottom hypoxia in the Northern Gulf of Mexico has been extensively studied, but little is known about internal variability and short-term shifts. This research uses a hydrodynamic model with oxygen parametrization to demonstrate the influence of physical processes with spatial and temporal scales on hypoxia development in the far-field of the Mississippi plume. Variability in advection is found to increase with decreasing temporal and spatial scales, introducing small-scale, strong anomalies in the field. Through a budget decomposition and Reynolds decomposition, the study identifies vertical flux anomalies as the primary counterbalance to sediment oxygen demand during periods of maintaining or destroying hypoxic extent.