A dynamic, embedded Lagrangian model for ocean climate models, Part II: Idealised overflow tests

Dense gravity current overflows occur in several regions throughout the world and are an important process in the meridional overturning circulation. Overflows are poorly represented in coarse resolution level coordinate ocean climate models. Here, the embedded Lagrangian model formulated in the companion paper of Bates et al. (2012) is used in two idealised test cases to examine the effect on the representation of dense gravity driven plumes, as well as the effect on the circulation of the bulk ocean in the Eulerian model. The results are compared with simulations with no parameterisation for overflows, as well as simulations that use traditional hydrostatic overflow schemes.The use of Lagrangian “blobs” is shown to improve three key characteristics that are poorly represented in coarse resolution level coordinate models: (1) the depth of the plume, (2) the along slope velocity of the plume, and (3) the response of the bulk ocean to the bottom boundary layer. These improvements are associated with the more appropriate set of dynamics satisfied by the blobs, leading to a more physically sound representation. Experiments are also conducted to examine sensitivity to blob parameters. The blob parameters are examined over a large parameter space.

► The representation of overflows in coarse resolution level climate models is assessed.
► The fidelity of using an embedded Lagrangian model to represent overflows is examined.
► Using the Lagrangian sub-model greatly improves the representation of a dense plume.
► The interaction of the Eulerian and Lagrangian models improves simulation fidelity.
► The parameter space of the embedded Lagrangian model is explored.