Model Domain Boundary Condition (jperio)

At the model domain boundaries several choices are offered: closed, cyclic east-west, south symmetric across the equator, a north-fold, and combination closed-north fold or cyclic-north-fold. The north-fold boundary condition is associated with the 3-pole ORCA mesh.

Closed, cyclic, south symmetric (jperio = 0, 1 or 2)

The choice of closed, cyclic or symmetric model domain boundary condition is made by setting jperio to 0, 1 or 2 in namelist namcfg. Each time such a boundary condition is needed, it is set by a call to routine lbclnk.F90. The computation of momentum and tracer trends proceeds from $i=2$ to $i=jpi-1$ and from $j=2$ to $j=jpj-1$, $i.e.$ in the model interior. To choose a lateral model boundary condition is to specify the first and last rows and columns of the model variables.

For closed boundary (jperio=0)

, solid walls are imposed at all model boundaries: first and last rows and columns are set to zero.

For cyclic east-west boundary (jperio=1)

, first and last rows are set to zero (closed) whilst the first column is set to the value of the last-but-one column and the last column to the value of the second one (Fig. 8.3-a). Whatever flows out of the eastern (western) end of the basin enters the western (eastern) end. Note that there is no option for north-south cyclic or for doubly cyclic cases.

For symmetric boundary condition across the equator (jperio=2)

, last rows, and first and last columns are set to zero (closed). The row of symmetry is chosen to be the $u$- and $T-$points equator line ($j=2$, i.e. at the southern end of the domain). For arrays defined at $u-$ or $T-$points, the first row is set to the value of the third row while for most of $v$- and $f$-point arrays ($v$, $\zeta$, $j\psi$, but scalar arrays such as eddy coefficients) the first row is set to minus the value of the second row (Fig. 8.3-b). Note that this boundary condition is not yet available for the case of a massively parallel computer (key_mpp defined).

Figure 8.3: setting of (a) east-west cyclic (b) symmetric across the equator boundary conditions.

North-fold (jperio = 3 to $6$)

The north fold boundary condition has been introduced in order to handle the north boundary of a three-polar ORCA grid. Such a grid has two poles in the northern hemisphere (Fig.16.1, and thus requires a specific treatment illustrated in Fig.8.4. Further information can be found in lbcnfd.F90 module which applies the north fold boundary condition.

Figure 8.4: North fold boundary with a $T$-point pivot and cyclic east-west boundary condition ($jperio=4$), as used in ORCA 2, 1/4, and 1/12. Pink shaded area corresponds to the inner domain mask (see text).