Shear-induced splitting of a plume outflow in a stratified enclosed basin

Abstract

In this paper we review the outflow data for the Persian Gulf and report on laboratory experiments with outflows from turbulent plumes falling into a pre-established density stratification in a long box. The experiments show the formation of coarse vertical structure, a result of ‘shear layers’ generated by quasi-stationary internal wave modes. Applying a theory of Wong et al. [J. Fluid Mech., 434(2001) 209-244] for shear layers generated by plume outflows at the bottom of a tank, we interpret the intrusion structure in terms of downward-propagating low frequency waves excited by the outflow. In the presence of an upward mean advection driven by entrainment into the plume and displacement by the outflow a mode becomes stationary in the tank and causes the outflow to split into multiple horizontal (T, S) intrusions with thickness given by the dominant vertical wavelength. Central to this thesis are single-component experiments, which show that double-diffusive convection is not responsible for the coarse structure. When the laboratory plume is given both temperature and salinity contrasts from the surroundings, double-diffusive convection is evident and tends to modify the smooth gradients into interfaces and convecting layers. However, the convection is parasitic on the vertical T, S gradient perturbations generated by the shearing modes. In comparing the laboratory results and theory with the data for the Persian Gulf outflows, we tentatively propose that this outflow, as it passes over the sloped boundary, induces internal waves whose normal mode structure may fold up the outflow and create the coarse layers. We also find that the double-diffusive effects are too small to influence the shearing modes in ocean outflows.