16.3.1 Grids in two dimensions

When the grid system is contracted to a minimum along one dimension, MOM is essentially reduced to a two dimensional model. For instance, if it is desirable to have a two dimensional model that is a function of latitude $\phi$ and depth z, then setting nxlons=2 and specifying $dx\_lon_1$, $dx\_lon_2$ such that there are two grid cells between bounding surfaces $x\_lon_1$, $x\_lon_2$ will generate imt=4grid cells in the longitudinal direction. Two T cells i=2 and i=3will be calculated and the two extra T cells i=1 and i=4 are for boundaries. Only one U cell, i=2, is not in the boundary. If option cyclic is enabled, then the domain is zonally re-entrant. If the forcing and initial conditions are independent of longitude $\lambda$, then the solution is independent of $\lambda$ and the model is two dimensional in $\phi$ and z. Obviously the relevance of this model depends on the scientific question being posed. This is just to demonstrate how the longitudinal dimension can be contracted to a minimum of imt=4 cells. The memory window should be opened to jmw=jmt to make it as efficient as possible but this will not be as fast as the two dimensional model in $\lambda$ and zdiscussed below because of the short vector lengths in longitude.

A similar contraction can be performed in the latitudinal direction to end up with a two dimensional model in longitude $\lambda$ and depth z. Here again, the minimum number of latitudes is jmt=4 with jrow=2 and jrow=3 being ocean T cells and jrow=1 and jrow=4being land cells. Note that there is only one latitude of ocean U cells. If these U cells are placed at the equator (with the two ocean T cell latitudes placed symmetrically about the equator) and there is no meridional variation in initial conditions or forcing, then the model is two dimensional in $\lambda$ and z. Again, the scientific question being posed needs to be suited to this design. Note that this type of model really flies computationally because of the long vectors in the longitude dimension. The memory window should be opened to jmw=jmt to make it as efficient as possible.

In the vertical, the minimum number of ocean levels is 2. Therefore the bounding surfaces and resolutions can be set to yield km=2. Note that $\kmt < 2$ is not allowed.