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Dynamics of Two-Dimensional Turbulent Bottom Gravity Currents

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  • 1 RSMAS/MPO, University of Miami, Miami, Florida
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Abstract

In light of previous numerical studies demonstrating a strong sensitivity of the strength of thermohaline circulation to the representation of overflows in ocean general circulation models, the dynamics of bottom gravity currents are investigated using a two-dimensional, nonhydrostatic numerical model. The model explicitly resolves the Kelvin–Helmholtz instability, the main mechanism of mixing in nonrotating bottom gravity currents.

A series of experiments were conducted to explore the impact of density difference and slope angle on the dynamics of bottom gravity currents in a nonrotating and homogeneous environment. The features of the simulated currents; that is, a characteristic head at the leading edge and lumped vortices in the trailing fluid, agree qualitatively well with those observed in laboratory experiments. Quantitative comparisons of speed of descent indicate that laboratory results remain valid at geophysical scales.

Two distinct regimes of entrainment of ambient fluid into bottom gravity currents are identified: (i) the laminar entrainment regime is associated with the initial growth of the characteristic head due to the drag exerted by the fresh fluid in front and (ii) the turbulent entrainment is associated with the Kelvin–Helmholtz instabilities. The turbulent entrainment is found to be much stronger than the laminar entrainment, and entrainment in the turbulent regime is less sensitive to the slope angle than that in the laminar regime. The entrainment is quantified as a function of basic parameters of the system, the buoyancy flux and the slope angle, for the purpose of parameterizing the mixing induced by bottom gravity currents.

Corresponding author address: Dr. Tamay M. Özgökmen, RSMAS/MPO, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149-1098. Email: tamay@rsmas.miami.edu

Abstract

In light of previous numerical studies demonstrating a strong sensitivity of the strength of thermohaline circulation to the representation of overflows in ocean general circulation models, the dynamics of bottom gravity currents are investigated using a two-dimensional, nonhydrostatic numerical model. The model explicitly resolves the Kelvin–Helmholtz instability, the main mechanism of mixing in nonrotating bottom gravity currents.

A series of experiments were conducted to explore the impact of density difference and slope angle on the dynamics of bottom gravity currents in a nonrotating and homogeneous environment. The features of the simulated currents; that is, a characteristic head at the leading edge and lumped vortices in the trailing fluid, agree qualitatively well with those observed in laboratory experiments. Quantitative comparisons of speed of descent indicate that laboratory results remain valid at geophysical scales.

Two distinct regimes of entrainment of ambient fluid into bottom gravity currents are identified: (i) the laminar entrainment regime is associated with the initial growth of the characteristic head due to the drag exerted by the fresh fluid in front and (ii) the turbulent entrainment is associated with the Kelvin–Helmholtz instabilities. The turbulent entrainment is found to be much stronger than the laminar entrainment, and entrainment in the turbulent regime is less sensitive to the slope angle than that in the laminar regime. The entrainment is quantified as a function of basic parameters of the system, the buoyancy flux and the slope angle, for the purpose of parameterizing the mixing induced by bottom gravity currents.

Corresponding author address: Dr. Tamay M. Özgökmen, RSMAS/MPO, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149-1098. Email: tamay@rsmas.miami.edu

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