Integrated Calendar

The general circulation of the ocean is forced by surface fluxes of momentum, heat, and freshwater at basin scales. The kinetic and available potential energy sources associated with these external forces drive a circulation which exhibits flow features that vary on a wide range of spatial and temporal scales. Understanding how the different forcing mechanisms lead to the observed large-scale ocean circulation patterns and to what degree do the various smaller scale processes modify them have been long standing problems for oceanographers.
A large fraction of the kinetic energy in the ocean is stored in the mesoscale eddy field. This `balanced' eddy field is expected, according to geostrophic turbulence theory, to transfer energy to larger scales. In order for the general circulation to remain approximately steady, sub-mesoscale instabilities leading to `loss of balance' (LOB) have been hypothesized to take place so that the eddy kinetic energy (EKE) may be transferred to small scales where it can be dissipated.
We examine the kinetic energy pathways in fully resolved direct numerical simulations of flow in a flat-bottomed re-entrant channel, a configuration that resembles the Antarctic Circumpolar Current. The flow is allowed to reach a statistical steady state at which point it exhibits both a forward and an inverse energy cascade. We show that EKE is dissipated preferentially at small scales near the surface via sub-mesoscale instabilities associated with LOB and a forward energy cascade rather than by bottom drag after an inverse energy cascade. These results highlight the importance of sub-mesoscales dynamics to the general circulation of the oceans.