A numerical study of the mixing and stratification alterations in estuaries due to climate change using the potential energy anomaly

Abstract

The competition between mixing and stratification in estuaries determines the quality of their waters, living conditions, and uses. These processes occur due to the interaction between tidal and fluvial contributions, which significantly vary depending on the estuarine characteristics. For the study of mixing and stratification alterations in estuaries due to climate change, a new methodology is proposed based on high-resolution 3D hydrodynamic modeling to compute the Potential Energy Anomaly (PEA). Regarding the model scenarios, first, a base case is analyzed with the realistic forcings of the year 2020. Subsequently, the forecasts of anomalies due to climate change for sea conditions (level, temperature, and salinity), atmosphere conditions (precipitation, air temperature, relative humidity, and solar irradiance), and river conditions (flow and temperature) are projected for the year 2020. The selected scenarios to analyze hydrodynamic changes are RCP 4.5 and 8.5 for the years 2050 and 2100. The proposed methodology has been applied to the Suances estuary. Independently of the climate change scenario, the stratification intensity increases and decreases upstream and downstream of the estuary, respectively. These results indicate that unlike the 2020 base scenario, in which the stratification zone has been mainly centered between km 4 and 8, for the new climate change scenarios, the stratification zone will be displaced between km 2 and 8, attenuating its intensity from km 4 onwards. The Suances estuary presents and will present under the considered scenarios a high spatiotemporal variability of the mixing and stratification processes. On the one hand, sea level rise will pull the stratification zones back inland from the estuary. On the other hand, climate change will generate lower precipitations and higher temperatures, decreasing runoff events. This phenomenon will decrease the freshwater input to the estuary and increase the tidal excursion along the estuary, producing a displacement of the river/estuarine front upstream of the areas.

A Dynamic Estuarine Classification of the Vertical Structure Based on the Water Column Density Slope and the Potential Energy Anomaly

“…Salinity has often been used as a classification parameter for stratification [20]. However, in this study we have opted for the use of density, since it incorporates changes in salinity and temperature, and therefore can more accurately assess changes in estuarine hydrodynamics due to climate change, in which it has been seen that the increase in temperature will be more significant than an increase in salinity in some estuaries [26]. Thus, a new classification will be proposed by considering a visible physical effect: the slope of the density profiles and their relationship to the potential energy anomaly (φ) values.…”

“…Based on a 3D high-resolution numerical model developed by [26], the specific objectives of this work are: (a) proposing an estuarine classification based on the slope of the density profiles; (b) relating the proposed classification to φ; (c) analyzing the spatiotemporal evolution of the vertical structure in the Suances estuary for the year 2020 and (d) projecting different climate change scenarios in order to explore the projected changes in the vertical structure of this estuary compared with the year 2020.…”

“…Note also that the high resolution of the mesh grid and bathymetry allowed the full definition of dykes bordering the main channel (see Figure 1). For more information on SE, the reader is referred to [10,26,[34][35][36].…”

“…The reason for choosing this model is its worldwide use that has confirmed its ability to simulate 3D hydrodynamics in estuaries [3,5,44,45]. The 3D hydrodynamic model results obtained by [26] using the interface of the suite of Delft3D-FLOW software in its three-dimension option [37][38][39] has been used to carry out this study. This numerical model is based on finite difference that solves the shallow water equations in three dimensions [40,41].…”

“…For more details about the model set up (time step, numerical scheme, turbulence closure model, etc. ), calibration, validation and climate change projections of forcings, readers are referred to [26].…”