Usually, land is defined as “NaN” value, which can not be applied arithmetic.
It’s useful for changing NaN value to normal value, then you can put some number on it.
yes? use coads_climatology
yes? let new_sst = missing(sst[d=1],999)
yes? shade new_sst[l=1]
yes? list new_sst[l=1]
Now you can see the change of land value from NaN to 999.
Sooo, Simple. 🙂