With the new land mask feature an external file (input / step5mask which is the same format as the newly output work / step5mask) specifies whether to use land or ocean data for each cell. (clearclimatecode.org)
[edit: Yes, Steven Mosher pointed me at one; so expect a post using a real land mask soon] (clearclimatecode.org)
Some of it might be related to the latitudinal masks but you'd probably want to look at the full lat - lon data with consistent land masks to do this properly. (realclimate.org)