Eddying vs. laminar ocean circulation models and their applications

International audienceMesoscale eddies are ubiquitous and very energetic features of the ocean circulation. They are represented in the high resolution models used for ocean forecasting , but not yet in today's laminar, coarse-resolution ocean components of models of the climate system. However, advances in high performance computing are likely to change this in a near future, as the next decade should see the use of ed-dying models to become more and more frequent in the broader context of the Earth system modelling. This lecture discusses mesoscale eddies in models of different resolution. The course is organised as follows. Section 1 introduces the notion of mesoscale eddies by an illustration of the ubiquity of oceanic eddies from satellite observations. Then, it provides a definition of ocean mesoscale eddies by analogy with the atmospheric synoptic eddies. The main impacts of ocean mesoscale eddies on the general circulation are recalled. Section 2 discusses some ocean model fundamentals that link primitive equations with resolution, parame-terisation and numerics. The separation between resolved and unresolved scales that results from the choice of grid resolution is discussed, a definition of eddying and laminar resolution models is provided, and the notion of subgridscale parame-terization is illustrated with the example of the mesoscale eddies. Section 3 illustrates the tight link that exists between resolution and numerics. Examples are shown where the use of advanced numerical schemes improves model solutions in a more drastic way than increases in resolution. Section 4 uses the DRAKKAR hierarchy of global ocean circulation models (whose resolutions vary between 2° and 1/12°) to illustrate how the changes in resolution impact the realism of model simulations, in terms of mean state and variability. The conclusion summarizes the major items discussed during the class