Subsections
Model Domain Boundary Condition (jperio)
At the model domain boundaries several choices are offered: closed, cyclic eastwest,
south symmetric across the equator, a northfold, and combination closednorth fold
or cyclicnorthfold. The northfold boundary condition is associated with the 3pole 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 to and from to
, 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 eastwest boundary (jperio=1)
 , first and last rows are set
to zero (closed) whilst the first column is set to the value of the lastbutone column
and the last column to the value of the second one (Fig. 8.3a).
Whatever flows out of the eastern (western) end of the basin enters the western
(eastern) end. Note that there is no option for northsouth 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  and points equator line (, i.e. at the southern
end of the domain). For arrays defined at or points, the first row is set
to the value of the third row while for most of  and point arrays (, ,
, but scalar arrays such as eddy coefficients)
the first row is set to minus the value of the second row (Fig. 8.3b).
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) eastwest cyclic (b) symmetric across the equator boundary conditions.

Northfold (jperio = 3 to )
The north fold boundary condition has been introduced in order to handle the north
boundary of a threepolar 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 point pivot and cyclic eastwest boundary condition
(), as used in ORCA 2, 1/4, and 1/12. Pink shaded area corresponds
to the inner domain mask (see text).

Gurvan Madec and the NEMO Team
NEMO European Consortium20170217