The primitive equations were reformulated and discretized to allow the use of any locally orthogonal horizontal grid. This provides alternatives to the standard latitude-longitude grid with its singularity at the North Pole.
This generalization made possible the development of the displaced-pole grid, which moves the singularity arising from convergence of meridians at the North Pole into an adjacent landmass such as North America, Russia or Greenland. Such a displaced pole leaves a smooth, singularity-free grid in the Arctic Ocean. That grid joins smoothly at the equator with a standard Mercator grid in the Southern Hemisphere.
The most recent versions of the code also support a tripole grid in which two poles can be placed opposite each other in land masses near the North Pole to give more uniform grid spacing in the Arctic Ocean while maintaining all the advantages of the displaced pole grids.