Impact of topographic roughness on product water mass formation
by bottom gravity currents from a spectral element model

Authors:

Tamay M. Ozgokmen, RSMAS, University of Miami, Miami, USA
                   tozgokmen@rsmas.miami.edu, 305 421 4053

Paul F. Fischer, Argonne National Laboratory, Illinois, USA


In light of the pressing need for development of parameterizations of overflow entrainment in ocean general
circulation models, the behavior of turbulent gravity currents
in the presence of ambient stratification is studied via numerical simulation, for cases in which  equilibrated product water masses are formed.
Numerical experiments are based on the high-order three-dimensional nonhydrostatic spectral element model Nek5000, which
combines the geometrical flexibility of finite element models with
the numerical accuracy of spectral models.

In a recent investigation, it was shown that for the case of constant sloping smooth topography and linear ambient stratification, the gravity current separates from the bottom such that the entrained mass flux is independent of the slope angle. The entrainment mass transport, product mass transport, and product salinity  then depend only on the ambient stratification, and these quantities were approximated as simple algebraic  functions of the ambient stratification parameter that modify the source properties. However,
the separation level of the plume did not differ significantly from the neutral equilibrium level in the absence of mixing.

In this study, we put forth that complex bottom topography can affect significantly the equilibrium level of the plume by acting as a conduit to dilute the near-bottom water masses, which otherwise escape the shear-induced mixing taking place at the interface of the gravity current and the ambient flow. The effect of topographic roughness on the propagation speed, separation level from the bottom, the salinity of the product water masses, and the transport and entrainment of the gravity current is quantified.