Diffusively-driven overturning of a stable density gradient

We present the results of an experimental study on the formation and propagation of thermohaline intrusions from an initial state that is stably stratified in two diffusing components. The intrusions form in a layer that contains compensating horizontal gradients of the two components and that lies above a denser reservoir layer that is homogeneous. In the initial state, a vertical barrier separates the upper layer into two half layers each with a different concentration but with the same density. Differential diffusion of solute from the reservoir into the upper layer initiates intrusions that are separated by an interface and that move in opposite directions. The propagation of these intrusions is augmented by double-diffusive fluxes across the interface. Our experiments show that the formation of a diffusive interface between the two intrusions is a robust feature for all experiments that initially have a stable stratification in both diffusing components. A series of contrasting experiments with a fingering interface between the two intrusions was also performed using an initial density profile that favored fingering between part of the upper layer and the reservoir. A simple model is presented for intrusion propagation and density evolution in the intrusions, and the results are compared to measurements from the experiments. These experiments may help illuminate how thermohaline intrusions form in regions of the ocean where both temperature and salinity are stably stratified.