Balancing Backscatter and Diffusion in a 1/4° Forced Global Ocean Model

Mesoscale ocean eddies in 1/4 (Formula presented.) global ocean models lie near the grid scale and are overdamped by viscosity, leading to reduced eddy kinetic energy, weak sea surface height variability, and mean-state biases. Backscatter has been proposed to remedy this problem by re-injecting dissipated energy, but its diabatic consequences are poorly characterized. We first show that using backscatter alone in a 1/4 (Formula presented.) forced CESM2-MOM6 ocean–sea ice model produces unrealistically large southward heat transport. To address this problem, we define a non-dimensional ratio (Formula presented.) (deformation radius over grid spacing) and adopt a nearly step-like resolution function with a threshold of (Formula presented.). Isopycnal height and tracer diffusion act where (Formula presented.) and backscatter acts where (Formula presented.). We compare two backscatter schemes that differ mainly in where they apply backscatter. One applies backscatter broadly in the Southern Ocean, whereas the other confines it near western boundary currents. Both schemes energize the model by 20%–25%. When backscatter is applied in the Southern Ocean, it energizes and barotropizes the flow but also increases southward heat transport, warms surface temperatures, shifts deep winter mixed layers poleward, and reduces Antarctic sea ice. In contrast, limiting backscatter to western boundary currents strongly enhances eddy kinetic energy there while keeping Southern Ocean heat transport, sea surface temperatures, mixed-layer depth, and sea ice much closer to the reference run. These results show strong sensitivity to backscatter placement and, in this configuration, favor confining it to western boundary currents.