Sensitivity of the Lorenz energy cycle of the global ocean

Abstract

We re-examine the Lorenz energy cycle (LEC) for the global ocean by assessing its sensitivity to model and forcing differences. We do so by comparing LECs derived from two simulations based on different eddy-rich ocean models, ICON-O and MPI-OM, both driven by NCEP/NCAR reanalysis, and by comparing LECs derived from two simulations generated using ICON-O model but driven by two different reanalyses, NCEP/NCAR and ERA5. Regarding model difference, we find weaker eddy kinetic energy, k_e, in the ICON-O simulation than in the MPI-OM simulation. We attribute this to the higher horizontal resolution of MPI-OM in the Southern Ocean. The weaker k_e in ICON-O is not caused by the lack of eddy available potential energy, p_e, but by the strong dissipation of p_e and the resulting weak conversion from p_e to k_e. Regarding forcing difference, we find that considerably more mechanical energy is generated by the ERA5 forcing, which has a higher spatial-temporal resolution compared to the NCEP/NCAR forcing. In particular, the generation of k_e, which also contains the resolved part of the internal wave spectrum, is enhanced by about 1 TW (40%). However, the dominance of the baroclinic and the barotropic pathways forces the enhanced generation of k_e to be balanced by an enhanced dissipation in the surface layer. The gross features of LEC are insensitive to both model and forcing differences, picturing the ocean as an inefficient “windmill” that converts only a small portion of the inputted mechanical energy into the interior mean and transient circulations.