Dipole formation and collisions in a stratified fluid

BECAUSE of its relevance to large-scale geophysical flows, two-dimensional turbulence has attracted increasing attention during the past two decades. An important feature of such flows is the so-called inverse energy cascade—the spectral flux of kinetic energy from small to larger scales of motion1,2. Numerical simulations of two-dimensional turbulence have demonstrated the emergence of coherent vortices from a randomly generated initial flow field3,4. These calculations revealed the occurrence of two categories of coherent structures, specifically, the monopolar axisymmetric vortex, characterized by a non-zero angular momentum, and the dipolar vortex, characterized by a non-zero linear momentum and a steady translation; recent experiments5,6 demonstrated the existence of a tripolar vortex structure, characterized by a non-zero angular momentum and a steady rotation of its axis. Here we report on the formation of two-dimensional dipole structures by the gravitational collapse of a compact region of three-dimensionally turbulent, mixed fluid in a quiescent stratified environment. The dipole thus produced seems to be very stable, its robustness being demonstrated by experiments on colliding dipoles.