Author(s): Manel Grifoll; Alan Cuthbertson; Jarle Berntsen; Maria Chiara De Falco; Claudia Adduce

Linked Author(s): manel grifoll, Maria Chiara De Falco

Keywords: Exchange flows; Numerical simulations; Earth rotation; Saline intrusion; Trapezoidal channel

Abstract: Restricted exchange flows occur in many natural aquatic environments (e.g. estuaries, sea straits, fjords, ocean basins) when horizontal density differences or pressure gradients are generated between adjacent, connected water masses due to variations in salinity and temperature (i.e. baroclinic forcing) or tides, freshwater inflows and wind-driven currents (i.e. barotropic forcing), respectively. The hydraulic control, lateral distribution and mixing of exchange flows between adjacent water masses also depends on topographical constraints (e.g. seafloor bathymetry, channel shape and roughness), and Coriolis forces due to the Earth’s rotation when the channel is relatively wide in comparison to the Rossby radius of deformation. This paper presents new laboratory-scale numerical experiments of uni and bi-directional exchange flows generated within an idealised trapezoidal channel topography. These simulations utilise the Bergen Ocean Model (BOM), a three-dimensional general ocean circulation model, in both non-rotating and rotating frames of reference. The results from the numerical simulations are validated against large-scale experimental data obtained in the LEGI Coriolis rotating platform in Grenoble (De Falco et al., 2021), with BOM-simulated velocity and density fields compared directly with particle image velocimetry (PIV) measurements and micro-conductivity density probe data from the equivalent laboratory experiments. The BOM simulations reproduce well the main dynamic properties of the large-scale exchange flows through the trapezoidal channel, with the lower layer saline intrusion flux shown to reduce (i.e. due to partial blockage) as the upper freshwater flow is increased. This saline intrusion flux can become fully blocked in the BOM simulations when the net-barotropic forcing imposed by upper freshwater layer is sufficiently high (outside the parametric range considered in the laboratory experiments). This blockage effect was also demonstrated in a previous BOM study of exchange flows over a submerged sill (Cuthbertson et al., 2021). For the rotating exchange flows, the geostrophic adjustment of the lower saline intrusion layer tends to increase the overall flux blockage compared to equivalent non-rotating exchange flows. This effect is attributed to the development of Ekman boundary layer dynamics and associated secondary circulations generated in the cross-sectional plane of the trapezoidal channel, and is shown to be a function of the non-dimensional Burger number. These numerical simulations thus provide improved parametric understanding of the separate and combined obstructing effects on exchange flows from (i) the trapezoidal channel topography, (ii) net-barotropic forcing in the upper freshwater layer, and (iii) the Coriolis forces due to Earth’s rotation. In this regard, they are relevant to many exchange flows generated in wide estuaries, sea straits and deep-ocean channels. De Falco, Adduce, Cuthbertson, Laanearu, Malcangio, Kaur, Negretti, Sommeria (2021). Physics of Fluids, 33:036602. https://doi.org/10.1063/5.0039251. Cuthbertson, Berntsen, Laanearu and Asplin (2021). Environmental Fluid Mechanics, 21:405-432. https://doi.org/10.1007/s10652-021-09779-5.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221561

Year: 2022