The Texas-Louisiana shelf in the Northern Gulf of Mexico faces numerous coastal challenges, including the formation of a summer dead zone due to excess nutrients from the Mississippi/Atchafalaya River and suppressed vertical mixing. The freshwater plume creates density fronts, which are potential pathways for water exchange between surface and bottom waters. This study investigates the hypothesis that slantwise vertical motions at these fronts, driven by the summer land-sea breeze, bypass the stratification barrier and ventilate the hypoxic bottom waters. Understanding this process is crucial for managing coastal vulnerabilities exacerbated by human activities and climate change, impacting ecosystems, fisheries, and ocean heat content, especially in a hurricane-prone region. The diurnal land-sea breeze, near-resonant with the local inertial frequency, generates intense currents that interact with the fronts, potentially facilitating vertical exchange.

Previous research has established the existence of the summer dead zone on the Texas-Louisiana shelf and the role of stratification in its formation (Rabalais et al., 2002; Bianchi et al., 2010). Studies have also highlighted the seasonal variation of wind-driven diurnal current cycling on the shelf (DiMarco et al., 2000) and the characteristics of tidal currents (DiMarco & Reid, 1998). The formation and characteristics of submesoscale frontal eddies in this region have also been documented (Kobashi & Hetland, 2020). Prior work has explored processes controlling mid-water column oxygen minima (Zhang et al., 2015). However, the specific mechanism of rapid vertical exchange driven by the interaction of land-sea breezes and plume fronts has not been fully investigated.

The study integrated observational data from the SUNRISE (Submesoscales Under Near-Resonant Inertial Shear Experiment) field campaign in June 2021 with high-resolution numerical simulations. The field campaign used R/Vs Pelican and Walton Smith to conduct repeated, high-frequency profiling transects across plume fronts, measuring hydrographic data (temperature, salinity) with Vertical Microstructure Profilers (VMPs) and velocities using ADCPs. The numerical simulations used a triple-nested ROMS/CROCO model (TXLA) with varying horizontal resolutions (650 m to 100 m) to simulate the region. The model incorporated surface forcing from ERA interim, river discharge data, and a k-ε turbulence closure model. Dissolved oxygen was simulated as a tracer, including benthic respiration but excluding photosynthesis. A reduced physics model and a slab mixed layer model were also used to isolate key dynamic processes. Lagrangian particle tracking was employed to analyze water parcel trajectories and exchange processes.

High-resolution observations during SUNRISE 2021 revealed strong frontal convergence and divergence, with downward transport of warm surface waters (subduction) and upwelling of cooler waters. Numerical simulations demonstrated that the diurnal land-sea breeze resonantly excites near-inertial oscillations, interacting with the fronts to create strong convergence/divergence. The surface convergence induces downward motions that advect surface water along isopycnals (subduction), while divergence on the lighter side of the front drives upwelling. Lagrangian analysis confirmed rapid vertical transport of both surface and bottom waters, with subduction of warm surface water and upwelling of bottom water with low oxygen, traversing approximately 10 meters in a few hours. The potential vorticity (PV) field showed similar patterns to temperature and oxygen, highlighting the conservative nature of these tracers, with non-conservative effects (friction and diabatic processes) becoming important at the surface. The rate of oxygenation during upwelling pulses was estimated at 0.82 mg L⁻¹ day⁻¹, comparable to deoxygenation rates. The inertially modulated convergence and frontogenesis were attributed to periodic mixing of the geostrophic flow at the front, caused by diurnally varying winds. A reduced physics model captured the key wave-mean flow interactions driving the convergence.

The findings demonstrate that the summer land-sea breeze plays a significant role in the rapid vertical exchange at density fronts in the Northern Gulf of Mexico. This slantwise vertical exchange acts as a ventilation pathway for bottom waters, bypassing the stratification barrier of the Mississippi/Atchafalaya River plume. The upwelling of oxygenated surface water and the downwelling of hypoxic bottom water is key in the ventilation process. This process could significantly impact the dynamics of the region's dead zone. The rapid vertical transport also affects the distribution of buoyant materials like microplastics, oil droplets, and phytoplankton, which tend to aggregate at fronts. The subduction associated with frontal convergence is comparable to that observed in other high-volume river outflows, despite the smaller density differences in this study.

This study reveals a crucial mechanism for vertical exchange in a highly stratified coastal environment. The rapid, slantwise vertical motions at plume fronts, driven by the interaction of diurnal winds and near-inertial oscillations, effectively ventilate the bottom waters and potentially mitigate hypoxia. Future research should investigate the seasonal variability of this process and its broader implications for the ecosystem health and the fate of buoyant pollutants within the Gulf of Mexico.

The simulations were conducted for a specific year (2010) with similar wind conditions to the 2021 field campaign. Generalizing these findings to other years and conditions requires further investigation. The oxygen model simplified some processes, such as photosynthesis, which could influence the oxygen dynamics. The study focused on a specific region of the shelf, and the spatial extent of this exchange mechanism needs further examination.