Three-Dimensional Turbulent Bottom Density Currents From a High-Order Nonhydrostatic Spectral Element Model by

Overflows are bottom gravity currents which supply dense water masses generated in high-latitude and marginal seas into the general circulation. In light of oceanic observations which indicate that mixing of overflows with ambient water masses takes place over small spatial and time scales, and studies with ocean general circulation models, which indicate that the strength of the thermohaline circulation is strongly sensitive to representation of overflows in these models, overflow-induced mixing emerges as one of the prominent oceanic processes. In this study, nonhydrostatic 3D simulations of bottom gravity currents are carried out, as a continuation of an effort to develop appropriate process models for overflows, that would complement analysis of dedicated observations and large-scale ocean modeling. Nek5000, a parallel high-order spectral element Navier-Stokes solver is used as the basis of the simulations. Numerical experiments are conducted in an idealized setting focusing on the start-up phase of a dense water mass released at the top of a sloping wedge. 3D results are compared to results from 2D experiments and laboratory experiments, based on propagation speed of the density front, growth rate of the characteristic head at the leading edge, turbulent overturning length scales and entrainment parameters. Results from 3D experiments are found to be in general agreement with those from laboratory tank experiments. In 2D simulations, the propagation speed is approximately 20% slower, the head growth rate is 3 times larger, Thorpe scales are 30-50% larger, and entrainment parameter is upto 2 times higher than those in the 3D experiments. The differences between 2D and 3D simulations are entirely due to internal factors associated with the truncation of the Navier-Stokes equations for 2D approximation. It is concluded that in the absence of external factors that will trigger 3D circulation patterns, such as topographic features and/or rotation, 2D dynamics still represent a reasonable approximation to the general evolution of bottom gravity currents.