# Ocean

### a.      Choose your grid size:

We describe using a low resolution version of the finite volume CESM1 model (1.9x2.5 degree atm/land) with a high resolution ocean (gx1v6).  We recommend that you choose a CESM1-supported grid size for your simulation. The most common supported ocean grid sizes are styled after gx3v5 (100 longitudes and 116 latitudes) and gx1v6 (320 longitudes and 384 latitudes).  Although building a new ocean grid with a non-supported grid size is possible, (see CESM User Guide for discussion) additional changes need to be made in the ocean and ice source code.  Examples and tools described in this document are designed for supported ocean grid sizes.

### b.      Grid pole placement:

The ocean model requires that grid poles be placed over land. Numerically no computation can be done at the convergence point of all longitudes at the grid pole. The ocean model solves this problem by shifting the grid pole away from the geographic pole and placing it over a land mass. (Atmospheric models solve this problem by using numerical filters). Therefore if there is no land at the geographic pole, the numerical pole must be shifted over land elsewhere. As long as land exists poleward of ~65o, our tools should be able to create an ocean grid for POP without code modification.

Pole placement is a subjective process, however, we offer a few helpful tips.

1. Place the grid pole as close to the geographic pole as possible.
2. It can be helpful to place the NH and SH pole on the same longitude.
3. Place the grid pole close to the continental edge (1-2 grid cells) but be sure that you are not creating spurious land cells around the pole disc.  To shift the pole inland, experiment with increasing jcon (reasonable range for jcon=10-20).
4. WARNING:  Grid cell size decreases as the grid converges toward the pole point, with the consequence that if your grid cells are too small as you approach the poles, you may have to decrease the modeling timestep to avoid ocean instabilities. Therefore, try to avoid creating grid cells that are smaller than the CESM default ocean grid at your resolution.  Increasing jcon may help avoid this problem.