Bill sent a schematic of the vertical gridding used in POP.  This gridding is also used in MOM.  The main point is that interface variables, such as vertical diffusivity, have a k-label corresponding to the bottom interface of a k-cell.  

This convention for labeling vertical interface arrays is not unique.  The alternative convention is to have interface arrays labeled k correspond to the top interface of a k-cell.  It turns out that this alternative convention is followed by the GOLD isopycnal model.  It is also the convention that MOM will transition to using.

I therefore recommend that all interface arrays be dimensioned array(1:nk+1).  Doing so will allow for either the POP or new-MOM conventions to be more readily used.  Diffusivities at the top of the top cell and the bottom of the bottom cell will be zero, so we will be wasting some memory.  But the translation between the two conventions will be facilitated by this dimensioning convention.  

Steve