Remote Versus Local Impacts of Energy Backscatter on the North Atlantic SST Biases in a Global Ocean Model

The use of coarse resolution and strong grid-scale dissipation has prevented global ocean models from simulating the correct kinetic energy level. Recently parameterizing energy backscatter has been proposed to energize the model simulations. Parameterizing backscatter reduces long-standing North Atlantic sea surface temperature (SST) and associated surface current biases, but the underlying mechanism remains unclear. Here, we apply backscatter in different geographic regions to distinguish the different physical processes at play. We show that an improved Gulf Stream path is due to backscatter acting north of the Grand Banks to maintain a strong deep western boundary current. An improved North Atlantic Current path is due to backscatter acting around the Flemish Cap, with likely an improved nearby topography-flow interactions. These results suggest that the SST improvement with backscatter is partly due to the resulted strengthening of resolved currents, whereas the role of improved eddy physics requires further research. Global ocean models often suffer from a lack of kinetic energy, and energy backscatter is a relatively new method designed to put kinetic energy back into the resolved flow field. It is also found to help better simulate the surface temperatures in the North Atlantic Ocean, which strongly affect the weather and climate in eastern North America and Europe but are often poorly represented by numerical simulations. Why models struggle with getting these temperatures right remains unclear, so it is important to determine mechanistic reasons for why backscatter can reduce the problem. In this study, we find that the global impact of backscatter can be separated into individual parts for different ocean regions. This regional separation of the impacts allows us to pin down the key ocean regions that give us the source of improvement and compare the roles of different ocean physical processes emphasized in previous studies. Our results point to the importance of backscatter in strengthening the resolved currents, and a need to better understand the role of eddies and their response to backscatter in the key regions identified. Parameterization of kinetic energy backscatter reduces the North Atlantic sea surface temperature (SST) biasesThe SST response to backscatter is attributed to the effect of backscatter in different regionsRoles of backscatter in strengthening resolved currents versus improving eddy physics are accordingly discussed