Applying implicit energetics ideas to simulate full-water-column ocean mixing
Robert
Hallberg
NOAA/GFDL
Talk
Energetic considerations have long been used to characterize the dynamics of both the ocean’s surface boundary layer (OSBL) and interior ocean diapycnal mixing. Reichl and Hallberg (2018) developed an implicit integrated energetics approach to model mixing in the OSBL, dubbed ePBL, which captures a wide range of physical boundary layer mixing processes. Subsequent comparisons with other OSBL parameterizations show results with ePBL are remarkably invariant to model resolution and timestep.
These same implicit energetics ideas can be applied to parameterize mixing throughout the ocean water column. Rather than the energy dissipation balancing the local vertical buoyancy flux, as in the traditional Osborn relationship, the energy dissipation can be thought of as balancing the vertically integrated consequences for potential energy throughout the water column due to the local mixing. With finite stratification and an infinitesimal timestep, the two are the same, but the implicit integrated approach works even for unstable parts of the water column and is essentially exact. It also works well in generalized vertical coordinate ocean models with variable vertical resolution. Moreover, as with ePBL for the OSBL, constraints on vertical mixing lengths and rates can be used to gracefully redistribute the turbulent kinetic energy giving rise to a very natural energetically constrained representation of bottom boundary layer mixing and stratified shear mixing in the interior ocean building on the work of Jackson et al. (2008). This presentation describes the issues that are encountered while implementing these ideas in the MOM6 ocean model, and demonstrates the value of this approach for correctly handling situations where the traditional explicit approach gives problematic results.
These same implicit energetics ideas can be applied to parameterize mixing throughout the ocean water column. Rather than the energy dissipation balancing the local vertical buoyancy flux, as in the traditional Osborn relationship, the energy dissipation can be thought of as balancing the vertically integrated consequences for potential energy throughout the water column due to the local mixing. With finite stratification and an infinitesimal timestep, the two are the same, but the implicit integrated approach works even for unstable parts of the water column and is essentially exact. It also works well in generalized vertical coordinate ocean models with variable vertical resolution. Moreover, as with ePBL for the OSBL, constraints on vertical mixing lengths and rates can be used to gracefully redistribute the turbulent kinetic energy giving rise to a very natural energetically constrained representation of bottom boundary layer mixing and stratified shear mixing in the interior ocean building on the work of Jackson et al. (2008). This presentation describes the issues that are encountered while implementing these ideas in the MOM6 ocean model, and demonstrates the value of this approach for correctly handling situations where the traditional explicit approach gives problematic results.
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