For the spatial discretisation, a staggered Cgrid is used, see Arakawa and Lamb (1977). The grid consists of prismshaped finite volumes with the edges aligned with coordinates. The reference grid for the tracer points (from now on denoted by Tpoints) is shown in figures 6 and 7. The velocity points are located such that the corresponding velocity components are centralised on the surfaces of the Tpoint reference box, the velocity points (from now on Upoints) at the western and eastern surfaces, the velocity points (from now on Vpoints) at the southern and northern surfaces and the velocity points (from now on Wpoints) at the lower and upper surfaces.


The indexing is carried out with indices in eastern () direction, with indices in northern () direction and with indices in upward () direction, such that each grid point is identified by a triple . A Tpoint and the corresponding eastern Upoint, the northern Vpoint and the above Wpoint have always the same index, see figures 6 and 7. The different grid points cover the following index ranges:
On the Tpoints, all tracers such as temperature , salinity , the general tracers and the density are located. All turbulent quantities such as eddy viscosity and eddy diffusivity are located on the Wpoints.
For curvilinear grids, several arrays for spatial increments and have to be defined:
where . The superscripts in (47) indicate whether a or is centrered at a T, U, V, or Xpoint, respectively. For the locations of the corner points , see figure 8.