Parametrisation of the Ocean Surface Boundary Layer: The OSMOSIS scheme

Stephen Belcher1, Alan L Grant2, A. J. George Nurser3, Natasha Sarah Lucas4, Tom Philip Rippeth5, Brodie Pearson6, Jeff Polton7, Matthew D Palmer8, Gillian Mary Damerell9, Karen J. Heywood9, Christian E. Buckingham10 and Alberto Naveira Garabato11, (1)Met Office, United Kingdom, (2)University of Reading, Reading, RG6, United Kingdom, (3)National Oceanography Center, Liverpool, United Kingdom, (4)Bangor University, School of Ocean Sciences, Menai Bridge, United Kingdom, (5)Bangor University, School of Ocean Sciences, Bangor, Wales, United Kingdom, (6)Brown University, Geological Sciences, Providence, RI, United States, (7)National Oceanography Centre, Liverpool, United Kingdom, (8)UK National Oceanography Centr, Liverpool, United Kingdom, (9)University of East Anglia, Norwich, United Kingdom, (10)Université de Bretagne Occidentale, Département de Physique, Brest, France, (11)University of Southampton, Ocean and Earth Science, Southampton, United Kingdom
Our fundamental understanding of the ocean surface boundary layer (OSBL) has seen enormous development over the past 10 years. New simulations, particularly large eddy simulations, have demonstrated the possible importance of wave-driven Langmuir turbulence and interactions between submesoscale and the OSBL. New observational platforms, such as gliders and Autonomous Underwater Vehicles have been used to observe the OSBL, over sustained periods from days to months. The challenge is now to integrate this new understanding into parameterisation schemes suitable for larger-scale models.

The new OSMOSIS parameterisation scheme for the OSBL is a response to this challenge. The scheme determines the depth of the OSBL using a new prognostic equation and then uses non-local flux-gradient relationships to parametrize turbulent transports within the OSBL. The flux-gradient relationships, are based on approximations to the turbulence flux budgets. Data from large eddy simulations is used to motivate and calibrate the scheme. Wave driven Langmuir turbulence is represented and the scheme, and is sufficiently flexible to accommodate representation of restratification by submesoscale circulations.

Comparisons with observations taken during the OSMOSIS project show early promise. The scheme represents well the diurnal variation of the OSBL during light wind conditions, the deepening of the OSBL during winter, and the restratification during spring. The basic scheme indicates significant over-deepening during winter, which signals new physics. Early indications are that submesoscale motions are playing a strong role at this time, and representing them substantially improves the results.

The scheme is currently being implemented and tested in Nucleus for European Modelling of the Ocean (NEMO).