Intraseasonal Variability in Northern Gulf of Mexico Hypoxia: Impacts of Baroclinic Instability, Rough Topography, and Exposure Duration

Abstract

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. The general objective of this work is to gain a better understanding of the different scales of temporal and spatial variability found in the bottom hypoxia that develops every summer over the Texas-Louisiana shelf. Abundant submesoscale fronts over the shelf shape the main variability in oceanographic patterns at the surface, but evidence suggests they also interact with the bottom boundary layer. After setting up the problem, the first analytic part of this study show that hypoxia development in the far-field of the Mississippi plume is subject to physical processes with spatial scales ranging from O(10km) to O(100km), and temporal scales from the near-inertial period to seasonality. It is shown that variability in oxygen advection increases with decreasing temporal and spatial scales, consistently with a field rich in instabilities introducing small-scale, strong anomalies in the shelf. Through a Reynolds decomposition of the budget, we separate anomalies from the main flow and identify the vertical turbulent flux _T as the primary counterbalance to sediment oxygen demand during periods when the hypoxic extent is maintained or destroyed. Next, the spatial structure of hypoxia is explored analyzing time of exposure and variability utilizing 24 years of simulations. Several generalizations are made about of the distribution of hypoxia over the far field of the shelf, and its relationship to the intrusion low oxygen water from the bottom into the interior. Finally, a set of idealized simulations is used to further prove the modulation effect of instabilities in the oxygenation of bottom waters, linking apparent oxygen utilization to intrusions and ventilation induced by the instabilities.