Decaying two-dimensional turbulence undergoes statistical heating

An emergent property of decaying two-dimensional turbulence is shown to be a weak but persistent statistical heating, or tendency towards clustering of like-signed vortices. The rate of this heating, which is driven by changes to the vortex population due to vortex mergers and straining events, provides a strong constraint on both vortex decay laws and kinetic theories of vortex interactions. A quasi-equilibrium statistical theory determines the energy spectrum from just the vortex interaction energy and bulk information about the number density of the vortices. The emergent heating rate is shown to determine the upscale transfer of energy, and is a powerful constraint on power law descriptions of the evolving vortex population.