The OSMOSIS Parametrisation of the Ocean Surface Boundary Layer (OSBL) in Global Runs of the NEMO Ocean Circulation Model
This scheme has been implemented into the NEMO ocean circulation model, and global runs have been performed at resolutions of 1° and ¼°. Results generally compare well with observations and with runs with the standard NEMO turbulent kinetic energy (TKE) scheme. In particular, the model simulates well the mixed-layer depths in the Southern Ocean in the Austral summer that are frequently underestimated by other models. However, initial versions of the model gave winter mixed-layers that were too deep, even after reducing the entrainment rate when the OSBL is deep enough for the eddy-turnover timescale to be comparable with the inertial timescale and reducing the entrainment rate for Langmuir turbulence when the OSBL depth is much greater than the depth to which the Stokes drift penetrates. Tests show that including these effects only partly explains the overly deep boundary layers.
In order to take account of the sub-mesoscale eddies that are thought to restratify the winter mixed-layer, the Fox-Kemper parametrisation of sub-mesoscale instabilities can be coupled with the OSBL model, so that the energy budget used in the prognostic equation for the mixed-layer depth takes account of the eddy buoyancy fluxes. The results are promising, giving more realistic winter mixed-layer depths.