In terms of physical relevance and the level of complexity of the they can account for, the hierarchy of models in geophysical fluid dynamics is as follows: \[ \text{Three-dimensional Navier-Stokes equations} \] \[ \Downarrow \] \[ \text{Nonhydrostatic models} \] \[ \Downarrow \] \[ \text{Primitive equations (hydrostatic equations)} \] \[ \Downarrow \] \[ \text{Shallow water equations} \] \[ \Downarrow \] \[ \text{Quasi-geostrophic models} \] \[ \Downarrow \] \[ \text{Two-dimensional barotropic equations.} \] Much study is needed for the boundary conditions from both the physical and mathematical points of view.