Impact of river discharge on the seasonal variability of salinity in the Mississippi delta region: A modeling approach

Abstract:

Freshwater discharge input owing to rivers in the northern Gulf of Mexico (nGOM) exhibits strong seasonal variability. This temporal variability in freshwater inflow is characterized by a peak (low) discharge during spring (fall) season. The Mississippi river, which is one of the ten largest rivers in the world, contributes more than half of the total freshwater supply to the nGOM region. The large influx of low-salinity waters plays a crucial role in regulating surface stratification and has important implications on the physical, biological, and biogeochemical processes in this region. A three-dimensional regional ocean circulation model with a high spatial resolution (~400 m) and forced with hourly surface meteorological parameters is used to study the impact of riverine freshwater discharge on the salinity variability and stratification over this region. Twin experiments of the model, where freshwater forcing is varied, have been conducted to identify the effect of river discharges on the upper ocean salinity and stratification. The model simulated physical oceanographic parameters are statistically validated with both remote sensing and in situ observations over the study area. The latter consists of a comprehensive gathering of hydrographic field observations carried out as a part of the CONsortium for oil exposure pathways in the Coastal River-Dominated Ecosystem (CONCORDE) research program. Domain averaged surface salinity values between the twin experiments over different seasons are studied. A region of interest is defined in the Mississippi delta region where the freshwater plumes were most likely to traverse under normal conditions. Variation in modeled seasonal variability of upper-ocean salinity, and the accompanying impact on water column stratification is studied. The advective pathways of low saline waters and the regions with maximum salinity differences are identified. These differences can have significant implications on the salinity tolerance of different marine species and hence the biogeochemical variability and biological processes in this region. As the variability in the upper ocean stratification in response to the varying river input are studied, these results are also important for the scenarios leading to the formation of oil mineral aggregates.

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