Effects of model physics on hypoxia simulations for the northern Gulf of Mexico: A model intercomparison

Abstract

A large hypoxic zone forms every summer on the Texas-Louisiana Shelf in the northern Gulf of Mexico due to nutrient and freshwater inputs from the Mississippi/Atchafalaya River System. Efforts are underway to reduce the extent of hypoxic conditions through reductions in river nutrient inputs, but the response of hypoxia to such nutrient load reductions is difficult to predict because biological responses are confounded by variability in physical processes. The objective of this study is to identify the major physical model aspects that matter for hypoxia simulation and prediction. In order to do so, we compare three different circulation models (ROMS, FVCOM, and NCOM) implemented for the northern Gulf of Mexico, all coupled to the same simple oxygen model, with observations and against each other. By using a highly simplified oxygen model, we eliminate the potentially confounding effects of a full biogeochemical model and can isolate the effects of physical features. In a systematic assessment, we found that (1) model-tomodel differences in bottom water temperatures result in differences in simulated hypoxia because temperature influences the uptake rate of oxygen by the sediments (an important oxygen sink in this system), (2) vertical stratification does not explain model-to-model differences in hypoxic conditions in a straightforward way, and (3) the thickness of the bottom boundary layer, which sets the thickness of the hypoxic layer in all three models, is key to determining the likelihood of a model to generate hypoxic conditions. These results imply that hypoxic area, the commonly used metric in the northern Gulf which ignores hypoxic layer thickness, is insufficient for assessing a model's ability to accurately simulate hypoxia, and that hypoxic volume needs to be considered as well. Key Points:Model intercomparison of three hypoxia models of the northern Gulf of Mexico is presented Bottom water temperature and bottom boundary layer thickness are important for hypoxia simulation Overall stratification strength does not explain model-to-model differences in hypoxic conditions Supporting Information:Supporting Information S1

Bayesian mechanistic modeling characterizes Gulf of Mexico hypoxia: 1968–2016 and future scenarios

“…A similar apparent underestimation of HA in June and overestimation in September is also observed in other mechanistic model formulations (Fennel et al. ). A likely reason for the more accurate representation of intraseasonal hypoxic patterns is our extensive and robust data set of shelf‐wide BWDO that spans hypoxic conditions from late spring to early autumn, thus enabling a more realistic tuning of model parameters.…”

“…, Fennel et al. ). First, simulation with our model is almost instantaneous and only requires easy‐to‐obtain hydrometeorological and nutrient loading data.…”

Climate Change Projected to Exacerbate Impacts of Coastal Eutrophication in the Northern Gulf of Mexico

“…Physical drivers include surface O 2 saturation, stratification, bottom boundary layer thickness, supply from the open ocean, whereas biological sources and sinks are primary production, water‐column respiration, nitrification and sediment O 2 consumption. In the northern Gulf, hypoxia mainly occurs near the bottom within the bottom boundary layer and therefore the thickness of the bottom boundary layer influences bottom O 2 and hypoxia (Fennel et al, ). Since changes in stratification and bottom boundary layer thickness (e.g., Figure ) are closely related, we did not attempt to distinguish their individual contributions; however, the variations in bottom boundary layer thickness are relatively small and stratification is probably the dominant contributor.…”

“…Daily freshwater fluxes from the Mississippi and Atchafalaya Rivers are prescribed using freshwater transports estimated by the US Army Corps of Engineers at Tarbert Landing and Simmesport, respectively. Further details on setup and validation of the physical model are given in Hetland and DiMarco (), Marta‐Almeida et al (), and Fennel et al ().…”

Impact of Satellite Remote Sensing Data on Simulations of Coastal Circulation and Hypoxia on the Louisiana Continental Shelf

“…The spatially/temporally-varying Kd from MODIS effectively prevented over-penetration of solar radiation and improved the nearshore vertical thermal structure. In a model inter-comparison study, Fennel et al [42] shows the NCOM-LCS applying the Kd from MODIS produced colder bottom water on the Louisiana shelf than other two models that used constant and uniform Kd. The colder bottom water in NCOM-LCS is due to reduced solar penetration in the nearshore murky water which leads to a much lower bias evaluated against the observations than other two models, again demonstrating the importance of applying spatially/temporally varying satellite Kd on the overall model temperature prediction.…”