Figure 18.1 is an example of a base field. Note the cell pointed to by the ``perimeter violation'' arrow. When using option stream_function, it is incorrect18.4 to have two distinct land masses seperated by a minimum of only one ocean T-cell. The reason is that if these two land masses are to remain unconnected, then the value of the stream function at the cell in question can be multivalued: one stream function value is implied by the integral of the pressure gradients around one land mass and the other is implied by the integral around the other land mass. But, since all four surrounding U-cells of the offending ``perimeter violation'' cell are land cells, no flow can exist between the land masses. So the integrals are not independent after all. The way to think of this is that the two seperate land masses are components of a ``meta '' land mass where all components are treated as a single mass. This is achieved by modifying the path of the island perimeters of the land masses so that the ``meta'' land mass has only one perimeter. Therefore ``perimeter violation'' cells are no longer a problem. They are automatically accounted for by constructing ``meta'' land masses. If desired, the ``perimeter violation'' cells can always be changed to land cells.
The difference between filling the ``perimeter violation'' cell in with land and leaving it as part of a ``meta'' land mass is that if the cell is made into land, then tracers cannot be diffused across it. However, as part of a ``meta'' land mass, two lateral faces of the ``perimeter violation'' cell connect with other ocean cells and diffusion of tracer quantities can occur. Therefore, if ``perimeter violation'' cells are left as an ocean cells, two ocean basins which cannot communicate advectively may still be able to communicate diffusively.