Title:
Modelling ocean weather: New pathways to representing mesoscale ocean eddies through ocean kinetic energy backscatter
Abstract:
In this talk I will demonstrate the potential of a kinetic energy backscatter scheme employed, for the first time, in a global ocean simulation. 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 ocean eddies through baroclinic instabilities, impacting the entire wavenumber spectrum. The proposed backscatter scheme returns part of this over-dissipated energy back into the resolved flow.
Backscatter is employed in the FESOM2 multiresolution ocean model with 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, temperature and salinity biases are reduced, most notably in the North Atlantic and the Antarctic Circumpolar Current. In some regions, for example the coastal Gulf Stream and Kuroshio, backscatter leads to a slight over-energizing of the flow. We expect such increased biases to be reduced by further tuning of the scheme and other relevant parameters. However, on average the backscatter already leads to substantial improvements in the 1/4° setup.