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

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. 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 • 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 SummaryThe 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.

Resolving and Parameterising the Ocean Mesoscale in Earth System Models

“…& At eddy-parameterising resolution, part of the available potential energy extracted by the GM parameterisation can be re-introduced at a large scale. For example, studies have reinjected the energy lost by GM into the momentum equation using a simple anti-viscous term [169][170][171]. We note that this approach (a) could be combined with the new momentum closures which are more physically appropriate than an anti-viscous term [163] and (b) may have implications for computational cost due to a reduction in timestep required to satisfy the CFL criteria [171].…”

Properties and dynamics of mesoscale eddies in Fram Strait from a comparison between two high-resolution ocean–sea ice models

“…Compared to eddies generated e.g. in the Gulf Stream region (Kang and Curchitser, 2013), the lifetime of Fram Strait eddies is rather short.…”

A Kinematic Kinetic Energy Backscatter Parametrization: From Implementation to Global Ocean Simulations

“…The performance of dynamic backscatter was already analyzed and discussed by Juricke et al (2019Juricke et al ( , 2020. Dynamic backscatter is computationally more demanding as it (i) applies an additional subgrid energy equation, (ii) uses several iterations of the smoothing filter, and (iii) necessitates a time step reduction for global setups on eddy-permitting meshes.…”

“…This resolution is called eddy permitting: For a large portion of the globe, it does not allow eddies to be simulated explicitly, but the mesh triangles are smaller or close to the Rossby radius in many places so that eddies can be represented formally. The same mesh was used to test the dynamic backscatter parametrization DBack in Juricke et al (2020).…”

“…This affects the release of available eddy potential energy, further weakening the eddy kinetic energy (EKE). For this reason, the concept of energy backscatter parametrizations was extended to ocean models as a means to reduce the accompanying EKE dissipation (Bachman, 2019;Jansen & Held, 2014;Jansen et al, 2015Jansen et al, , 2019Juricke et al, 2019Juricke et al, , 2020Klöwer et al, 2018). According to this concept, the dissipation of EKE is diagnosed, and partly compensated for at larger scales, either deterministically through negative viscosity or stochastically through random forcing of the momentum equation.…”