Three-Dimensional Nonhydrostatic Overflows: Numerics, Dynamics and Stochastics

Abstract

The thermohaline circulation in the ocean is strongly influenced by dense water formation in polar seas and marginal seas. Such dense water masses are released into the ocean circulation in the form of overflows, which are bottom gravity currents. It is recently found that various models for the ocean circulation are very sensitive to the representation of overflows. Since the ocean thermohaline circulation contributes significantly to the poleward heat transport, thus playing a vital role in climate dynamics, it is of great importance to accurately represent overflow dynamics.
 
In order to develop appropriate process models for overflows, nonhydrostatic 3D simulations of bottom gravity currents are carried out 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. Results from 3D experiments are compared with 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.
 
Morover, by recognizing that oceanic overflows follow the seafloor morphology, which shows a self-similar structure at spatial scales ranging from 100 km to 1 m, the impact of topographic bumps on entrainment in gravity currents is investigated using Nek5000. It is found that a bumpy surface can lead to a significant enhancement of entrainment compared to a smooth surface. The change in entrainment is parameterized as a function of statistical estimates of the amplitude and wavenumber parameters of bumps with respect to the background slope.
 
Finally, uncertainty in the boundary data is taken into account and the impact of randomness is quantified.