SMILES MITgcm Simulations

The modelling aspect of SMILES is a collaborative effort combining the expertise of scientists at the University of Cambridge, Plymouth University, and British Antarctic Survey. Post-cruise, publication-quality models will be run on the UK’s national supercomputer, Archer, and will benefit from a substantial allocation of computer time. At minimum of four high-resolution (O(500 m)) models will be run using surface forcing parameters gleaned from the cruise. These models will be used to study the vertical transport and mixed layer depth coincident with strong frontal gradients, which are associated with the strain fields from larger mesoscale eddies. Passive tracers will also be initialised to study the vertical transport properties associated with these same frontal gradients, which is representative of the transport of atmospheric gases that are fluxed into the oceanic mixed layer. Finally, the high-resolution model will allow for testing of a new class of turbulence parameterizations that must be developed to capture these small dynamics on larger, coarser grids.

An early version of these models has already been tested successfully using a preliminary allocation on Archer. The modelling approach uses a quasi-idealised domain which covers the latitudes from 56* S to 60* S and longitudes from 65* W to 55* W. The domain uses bathymetry and forcing fields obtained from the Southern Ocean State Estimate (SOSE). The initial state, including velocity and density profiles, are also read from SOSE beginning on April 1, 2010. The native SOSE resolution is approximately 18 km in the meridional and 10 km in the zonal directions, so the initial fields are interpolated down to 500 m resolution. Open boundary conditions are used on the north and south sides of the box, and are relaxed to the “climatological” SOSE state with a relaxation timescale of 1 hour. To avoid using open boundary conditions on the upstream, zonal sides of the box, all fields are duplicated, mirrored in the zonal direction, and “stitched” together on the eastern side of the box (see schematic below). The meridional geostrophic velocities are also reversed in sign in this box to accommodate the zonal reversal in density gradient.

The model is integrated with a time step of 15 seconds, and is used to simulate the time period from April 15 (approximately equivalent to the cruise start date in 2015) to May 31, 2010. The surface forcing is updated with a frequency of 2 hours, and is linearly interpolated in time to allow the oceanic flow to smoothly respond without the risk of being spuriously forced. The K-Profile parameterisation (Large et al., 1994) is employed to handle potential convectively unstable regions that may develop due to strong heat loss to the atmosphere.

Full 3D model output is collected at 4-hourly intervals. MATLAB postprocessing scripts are employed to diagnose and extract dynamically important variables, such as relative vorticity. The movie embedded below shows the relative vorticity field from one subregion of the model. Areas of high, cyclonic relative vorticity are apparent around the margins of submesoscale fronts; further investigation of these locations will be the principle objective of this research.