Routes to Dissipation in the Ocean: The 2D/3D Turbulence Conundrum

Turbulence research has reached a mature stage, even though at the most fundamental level turbulence itself remains a mystery. The phenomenology and physical mechanisms of the separate regimes of twoand threedimensional, geostrophic, stratified, gravity-wave, and boundary-layer turbulence are all now fairly completely established. They describe well the nature of geophysical turbulence at various space and time scales and at various locations. However, an unsolved fundamental problem remains: how do these different regimes of turbulence coexist and connect in the ocean? How and where does the energy of the general circulation cascade from the large climatic scales, where most of it is generated, to the small scales, where all of it is dissipated? In particular, how is the dynamical transition made from an anisotropic, 2D-like, geostrophic cascade at large scales—with its strong inhibition of down-scale energy flux—to the more isotropic, 3D-like, down-scale cascades at small scales. This essay discusses these problems, highlighting the physical laws that prevent a simple down-scale energy cascade and the possible connections among the different regimes of geophysical turbulence.