Inverse energy cascade — 2D turbulence transfers energy upscale as vortices merge into larger coherent structures
In 2D turbulence, vorticity is conserved and vortices of same-sign circulation attract and merge (Biot-Savart interaction). This drives an inverse energy cascade to larger scales (Kraichnan 1967), opposite to 3D turbulence. Enstrophy cascades forward to small scales.