Modelling the Depth of the Ocean Surface Boundary in the OSMOSIS Scheme.

Alan L Grant, University of Reading, Reading, RG6, United Kingdom, Stephen Belcher, Met Office, United Kingdom and A. J. George Nurser, National Oceanography Center, Soton, Southampton, United Kingdom
The depth of the Ocean Surface Boundary Layer (OSBL) is determined by processes, such as entrainment, absorption of solar radiation, shear at the base of the OSBL and submesoscale motions. It is important that the effects of these processes on the depth of the OSBL are represented in parametrizations. The OSMOSIS scheme, developed as part of the Ocean Surface Mixing Ocean Submesoscale Interaction Study, is a parametrization of the OSBL, which uses a prognostic equation to determine the depth to which mixing extends.

The equation for the depth of the OSBL in OSMOSIS scheme is obtained from the potential energy budget of the OSBL. For the entraining OSBL the resulting equation is similar to that used by mixed layer models. This poster shows how the equation can be modified to represent the effects of submesoscale motions or shear on the depth of the OSBL. The effects of submesoscale eddies, which tend to restratify the OSBL, are represented by the parametrization described by Fox-Kemper et al. The parametrization gives the vertical buoyancy flux associated with the submesoscale eddies, which can be incorporated into the equation for the OSBL depth. This poster shows that the modified OSMOSIS scheme reproduces the annual cycle of mixed-layer depth in the North-East Atlantic, observed during OSMOSIS (2012–13), in particular the behaviour of the mixed layer depth during the winter.

The parametrization of shear is based on the results of Grant and Belcher. In this case the OSMOSIS scheme is tested against results from large eddy simulations (LES), that use forcing due to Hurricane Francis. The results from the OSMOSIS scheme agree with the LES for locations on the left and right of the hurricane track, in particular, reproduucing the strong cooling on the right-hand side.