Linear waves and baroclinic instability in an inhomogeneous-density   layered primitive-equation ocean model
  We consider a multilayer generalization of Ripa's inhomogeneous-density single-layer primitive-equation model. In addition to vary arbitrarily in horizontal position and time, the horizontal velocity and buoyancy fields are allowed to vary linearly with depth within each layer of the model. Preliminary results on linear waves and baroclinic instability suggest that a configuration involving a few layers may set the basis for a quite accurate and numerically efficient ocean model. 
F. J. Beron-Vera, M. J. Olascoaga and J. Zavala-Garay