Rotational effects on exchange flows across a submerged sill

A Cuthbertson,J Berntsen,J Laanearu,Magdeli Asplin

doi:10.1007/s10652-021-09779-5

A Cuthbertson, J Berntsen + Show 2 more

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https://doi.org/10.1007/s10652-021-09779-5

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Abstract

This paper presents new laboratory-scale numerical simulations of density-driven exchange flows generated across an idealised, submerged sill obstruction under both non-rotating and rotating frames of reference using the Bergen Ocean Model (BOM), a three-dimensional general ocean circulation model. Initial non-rotating BOM simulations are compared directly with previous laboratory data obtained in a large-scale channel facility incorporating an idealised trapezoidal sill. These laboratory experiments demonstrate that the saline intrusion flux across the sill is initially reduced and then eventually fully blocked under increasing net-barotropic flow conditions imposed in the counterflowing upper freshwater layer, with the saline blockage also more evident for reduced sill submergence depths. These parametric dependences are also demonstrated in the equivalent BOM simulations of the non-rotating sill exchange flows, although the numerical model results tend to overpredict both the interfacial velocity and density gradients across the sill (as indicative of suppressed interfacial mixing), as well as the fresh-saline source flux ratio at which full blockage of the saline intrusion occurs. The BOM simulations are then extended to consider rotating sill exchange flow dynamics. In particular, these additional runs demonstrate that Coriolis forces increase the overall blockage of the saline intrusion layer compared to equivalent non-rotating exchange flows, especially when the Rossby number associated with the saline intrusion flow across the sill is considerably less than unity. This effect is largely attributed to the development of Ekman boundary layer dynamics and associated secondary circulations within the bi-directional exchange flows. These are shown to impose strong control on the transverse distribution and extent of the lower saline intrusion flow across the sill and, hence, the parametric conditions under which full saline intrusion blockage is achieved in rotating sill exchange flows.

Highlights

Restricted density-driven exchange flows are generated in oceans, seas and coastal margins when adjacent water bodies with different densities are connected by narrow channels or straits (e.g. Gibraltar, Bosphorus, Baltic Sea), or where natural topographic obstructions such as submerged sills control the intrusion of saline water into fjordic basins (e.g. Norway, Scotland) or oceanic deep-water outflows (e.g. Faroe Bank Channel)
The current study presents new numerical simulations, firstly, of the non-rotating experimental data presented in Cuthbertson et al [3], using the Bergen Ocean Model (BOM) at an equivalent laboratory-scale, before extending these BOM simulations to determine the effect of rotation on exchange flow dynamics across the sill
The study has investigated the development of stratified exchange flows across a submerged sill obstruction, comparing experimental results from a large-scale laboratory study with equivalent scaled numerical simulations using a non-hydrostatic, σ-coordinate numerical model [Bergen Ocean Model (BOM)]

Summary

Introduction

Restricted density-driven exchange flows are generated in oceans, seas and coastal margins when adjacent water bodies with different densities are connected by narrow channels or straits (e.g. Gibraltar, Bosphorus, Baltic Sea), or where natural topographic obstructions such as submerged sills control the intrusion of saline water into fjordic basins (e.g. Norway, Scotland) or oceanic deep-water outflows (e.g. Faroe Bank Channel). In the context of fjordic exchange flows, the presence of a submerged sill can generate a region of restricted exchange (RRE) whereby the partial blockage of tidal intrusions can lead to the suppression of deep water renewal, circulations and mixing in semi-enclosed fjordic basin, with contaminant accumulation and the formation of hypoxic bottom waters [1, 2]. It demonstrated that the primary blockage mechanism was driven by interfacial mixing and mass transfer (i.e. entrainment) from the bottom saline intrusion layer under dominant, net-barotropic, flow conditions in the upper freshwater layer. The main physical mechanism for this mass transfer was the development of shear-layer interfacial instabilities and overturning events that were quantified by calculation of the shear layer thickness (i.e. isopycnal separation) and the corresponding Thorpe overturning length scales [4]

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