Ocean Kinetic Energy Backscatter Parametrization on Unstructured Grids: Impact on Global Eddy-Permitting Simulations

In this study we demonstrate the potential of a kinetic energy backscatter scheme for use in global ocean simulations. Ocean models commonly employ (bi)harmonic eddy viscosities causing excessive dissipation of kinetic energy in eddy-permitting simulations. Overdissipation not only affects the smallest resolved scales but also the generation of eddies through baroclinic instabilities, impacting the entire wave number spectrum. The backscatter scheme returns part of this overdissipated energy back into the resolved flow. We employ backscatter in the FESOM2 multiresolution ocean model with a quasi-uniform 1/4 degrees mesh. In multidecadal ocean simulations, backscatter increases eddy activity by a factor 2 or more, moving the simulation closer to observational estimates of sea surface height variability. Moreover, mean sea surface height, temperature, and salinity biases are reduced. This amounts to a globally averaged bias reduction of around 10% for each field, which is even larger in the Antarctic Circumpolar Current. However, in some regions such as the coastal Kuroshio, backscatter leads to a slight overenergizing of the flow and, in the Antarctic, to an unrealistic reduction of sea ice. Some of the bias increases can be reduced by a retuning of the model, and we suggest related adjustments to the backscatter scheme. The backscatter simulation is about 2.5 times as expensive as a simulation without backscatter. Most of the increased cost is due to a halving of the time step to accommodate higher simulated velocities. Plain Language Summary The weather of the oceans is determined by so-called mesoscale eddies, which carry a large portion of the kinetic energy of ocean currents. They are responsible for the transport of heat and dissolved substances; they can affect the large and fast mean currents of the ocean and interact strongly with the atmosphere above. However, these eddies are not well represented in current ocean and climate models. With this study, we apply a new method to better represent the effect of ocean weather in ocean models. We show that this leads to improvements of the simulation of ocean currents and their variability and reduces biases in ocean temperatures and salinity. While increasing the resolution of ocean models also helps to improve the representation of mesoscale eddies, such a resolution increase is computationally expensive. The new backscatter parametrization can help to save computational costs by allowing improved eddy simulations comparable to much higher resolution.