The OSMOSIS Parametrisation of the Ocean Surface Boundary Layer (OSBL) in Global Runs of the NEMO Ocean Circulation Model

A. J. George Nurser1, Alan L Grant2, Stephen E Belcher3, Mike Bell4, Adam Tobias Blaker1, Alex Megann5 and Catherine Guivarch6, (1)National Oceanography Centre, Southampton, United Kingdom, (2)University of Reading, Reading, RG6, United Kingdom, (3)Univ Reading, Reading, United Kingdom, (4)Met Office, Exeter, United Kingdom, (5)National Oceanography Center, Liverpool, United Kingdom, (6)Met Office, United Kingdom
The OSMOSIS scheme is based on results of Large Eddy Simulations (LES) of the Langmuir turbulence that is thought to dominate in the OSBL. Turbulent transports are represented using flux-gradient relationships. The diffusivity profile, scaled by the (OSBL) depth and a turbulent velocity scale, and “non-local” fluxes of tracers and momentum are specified in the boundary layer. The depth of the OSBL is predicted from a new prognostic equation based on the integrated potential energy budget of the OSBL. The equation represents deepening of the OSBL by entrainment driven by Langmuir or shear turbulence in combination with convective turbulence.

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.