Clustering of salt fingers in double-diffusive convection leads to staircaselike stratification

We report on high-resolution, three-dimensional, high-Rayleigh number and low density ratio numerical simulations of fingering convection. We observe a previously unreported phenomenon of self-organization of fingers, that cluster together to form larger-scale coherent structures. The flow ultimately forms density staircases alternating well mixed regions with fingering convective zones. We give evidence that the mechanical mixing induced by the clusters forms the staircases with a mechanism analogous to staircase formation in a stably-stratified, non-convective, stirred fluid.