Recognizing that the atmosphere and ocean are governed by equations that are isomorphic to one another, we have built a coupled climate model based on a single hydro dynamical core - the MITgcm. Separate atmospheric and oceanic models are rendered by use of appropriate physics overlaid on the dynamics.

Novel features of our approach are:

• The dynamical core of the MITgcm exploits the isomorphism between the
  equations of motion and boundary conditions that govern the atmosphere
  and ocean, in pressure (p) and height coordinates (z) respectively. 
• The vertical coordinate (generic name "r") is scaled by the total fluid 
  column thickness yielding a modified r* vertical coordinate analogous 
  to the atmospheric Eta coordinate. Combined with the finite volume method and a partial cell representation of topography, the method remains accurate near
  steep topography.
• An expanded cubic grid provides a global coverage of the sphere with a relatively uniform resolution (2.8 degree at the equator) and is used in both models, thus avoiding the problem of convergence or meridians as the pole is approached. Careful discretization allows one to accurately treat the whole domain, including the eight singular points at the corner of the cube.
• The use of the same horizontal grid simplifies the coupling procedure, and conservation properties of the coupled model. The 2 components exchange fluxes and surface fields through a single processor coupling interface, using MPI.
• An atmospheric physics package of intermediate complexity (called SPEEDY, Molteni, 2003) enable extended (100's to 1000's of years) integrations to be carried out. Although highly idealized the simplified atmospheric model captures many realistic features of atmospheric circulation.  Other components include a simple land model (2 levels), an instantaneous runoff scheme and thermodynamic sea-ice (in progress).
• The model runs efficiently on a local PC cluster enabling thousands of years of 
  integration to be carried out in days.

We present some preliminary results from our coupled model here.