Quoted from: Wallcraft, Alan J., A. Birol Kara, Harley E. Hurlburt, and Peter A. Rochford. "The NRL Layered Global Ocean Model (NLOM) with an embedded mixed layer submodel: Formulation and tuning." Journal of Atmospheric and Oceanic Technology 20, no. 11 (2003): 1601-1615.  https://journals.ametsoc.org/view/journals/atot/20/11/1520-0426_2003_020_1601_tnlgom_2_0_co_2.xml 

The NLOM uses a primitive equation layered formulation where the equations have been vertically integrated through each Lagrangian layer. Prognostic variables are layer density, layer thickness, and layer volume transport per unit width (layer velocity times layer thickness). The bottom topography is confined to the lowest layer and a finite layer thickness is maintained by mixing across layer interfaces. The NLOM has typically been run without an explicit mixed layer, which is equivalent to assuming that the mixed layer is always inside the upper layer (see appendix A for a detailed description of NLOM in this mode). It includes support for passive tracers and this has now been extended with an “almost passive” embedded well-mixed surface turbulent boundary layer. The version presented here builds upon an earlier implementation for the Indian Ocean (Rochford et al. 2000). It has been extended to the global ocean with significant modifications to accommodate the wider diversity of conditions, such as updated mixed layer parameterizations, use of a floating mixed layer, a new advection scheme, a modified Kraus–Turner model, and improved model temperature profile and forcing fields. A schematic illustration of the global NLOM with an embedded mixed layer is shown in Fig. 1.

One of the major advantages of NLOM over other types of OGCMs such as z-level and sigma-coordinate models is its lower computational cost for the same model domain and horizontal resolution. One reason is that we can use lower vertical resolution to realistically represent the ocean circulation. For example, in the ½° model there are only seven layers in the vertical, including the mixed layer. NLOM is also a single efficient portable and scalable computer code that can run any of the model configurations on a variety of computing platforms (Wallcraft and Moore 1997). As a consequence, a 5-yr simulation using NLOM with an embedded mixed layer takes approximately 7 h of wall-clock time on 32 Cray T3E processors. The mixed layer can reach statistical energy equilibrium in just 2–3 yr. This allows an extensive set of climatologically forced sensitivity experiments to be performed with a global OGCM that would otherwise be computationally infeasible.