New publication by our subproject M3

Abstract

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. Over‐dissipation not only affects the smallest resolved scales, but also the generation of eddies through baroclinic instabilities, impacting the entire wavenumber spectrum. The backscatter scheme returns part of this over‐dissipated energy back into the resolved flow.

We employ backscatter in the FESOM2 multiresolution ocean model with a quasi‐uniform 1/4° 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 over‐energizing 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.

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