Assessment of the Impact of the Variability of Freshwater Input and Land-Sea Breeze on the Mississippi Sound Using a Modeling Approach

The Mississippi Sound located on the northeastern Gulf of Mexico is greatly influenced by freshwater inflow and diurnal wind breeze. The Mississippi Sound shows the greatest variation in salinity from January to June due to the dynamics of high river water inflow and shelf water mixing. Besides the river water inflow, Bonnet Carré Spillway contributes a high influx of freshwater into the Mississippi Sound through its opening when it is operated to avoid flooding in New Orleans. After becoming operational in 1931, Bonnet Carré Spillway has been opened in 3 years consecutively (2018,2019, & 2020) for the first time and 2019 is the only year in which Bonnet Carré Spillway was opened twice in a calendar year which caused about 38.1 km3 freshwater input into the Mississippi Sound. In addition to freshwater inflow, diurnal wind breeze acts as a primary source of hydrographic variability in the Mississippi Sound. As 2019 was a major year for Bonnet Carré Spillway opening impacts, the main objective of this study is to hindcast the combined effects of freshwater variability and diurnal land-sea breeze on salinity using observed and hypothetical numerical model scenarios. The main difference between these simulations is using two types of wind forcing in the COAWST modeling framework. Hourly NOAA High-Resolution Rapid Refresh (HRRR) output which captures the diurnal landsea breeze is used as atmospheric forcing for a simulation that represents the real scenario in 2019 and 24-hour low-pass filtered HRRR forcing which smooths out the wind in the diurnal frequency range is used for the hypothetical scenario. These twin experiments show the distinctive variation in salinity because of the effects of diurnal land-sea breeze on freshwater inflow and shelf water mixing. As the diurnal land-sea breeze is strong during late spring, summer, and the beginning of the fall season, salinity variability has been analyzed from March to October 2019 to understand the role of these diurnal effects. It is observed that the hypothetical simulation using filtered wind forcing overestimates the salinity in the month of June to October in the Mississippi Sound. It could be because using filtered wind forcing can shorten the flushing time and affect the shelf-water inflow in the Mississippi Sound. Salinity variation between these two simulations is evident in the central and eastern Mississippi Sound. For quantitative analysis, the sound has been divided into four north-south transects. In terms of surface salinity, the western and central-western transects are mostly affected by the Bonnet Carré Spillway opening and diurnal wind forcing does not play a dominating role here. But significant differences have been found in the central-eastern and eastern transect in the month of August. Filtered wind simulation is 2.4 ppt and 5 ppt saltier than the HRRR wind simulation along the central-eastern and eastern transect respectively during this month. The bottom salinity is less affected than the surface salinity observed in these twin experiments which is expected. As diurnal land-sea breeze plays a key role in the dynamics of freshwater and shelf water mixing in the Mississippi Sound, it can be concluded that high-resolution wind forcing data should be used for more accurate simulations of salinity in the Mississippi Sound.