Graduation date: 2006Thermohaline interleaving is an important mechanism for laterally fluxing salt, heat, and nutrients between oceanic water masses. Interleaving is driven by a release of potential energy resulting from the vastly differing diffusivities of heat and salt in seawater. The flows are composed of stacked intrusions that flux more buoyant and less buoyant water in opposite directions. In this paper, we investigate the role of shear instability (caused by this juxtaposed motion) and convective instability on intrusion growth. The model described in Walsh and Ruddick (1998) is upgraded to include an improved convective mixing parameterization and a parameterization of shear induced turbulent mixing. Model results show that shear and convective mixing play a similar and significant role in interleaving dynamics. In the absence of either instability, horizontal diffusivities are increased by approximately 30%. This increase is a product of the relationship between these instabilities and kinetic energy, which is inversely affected by shear and convective instabilities. Sensitivity tests show that the model accurately predicts horizontal diffusivities within an observed range, in spite of a wide range of model parameter values.