The memory window typically has a minimum size of three latitude rows (jmw=3) which is appropriate for second order accurate numerics. Some options use fourth order numerics or, in some cases, averaging operators which require information from two cells away. All fourth order schemes require option fourth_order_memory_window which is automatically enabled in file size.h when any of the existing fourth order schemes are enabled. These schemes require the minimum size of the memory window to be four latitude rows (jwm=4). Some combination of schemes require the minimum memory window to be of size jmw=5. However, more is required than simply opening up the window to jmw=4 rows. In the minimum size fourth order window, prognostic equations are solved only on row 2. In a second order window with jmw=4, they are solved for rows 2 and 3. For a further description of how the window works with higher order schemes, refer to Section 11.3.2.
Calculations always proceed up to latitude row jrow = jmt - 1 even with higher order schemes!. There are no out of bounds references because meridional indexing is limited to a maximum at latitude jrow=jmt and a maximum corresponding memory window row given by j=min(j+joff,jmt) - joff. To accommodate higher order schemes when a fully open memory window jmw=jmt is used, meridional fluxes are set to zero at latitude jrow=jmt which allows calculations to proceed through latitude row jrow=jmt-1.