"monotonic" means something that consistently increases or decreases in a steady and predictable way. It does not have any fluctuations or changes in its pattern, moving in a straight line without any ups and downs. Full definition
But it is not a simple monotonic function of average specific humidity, because water vapor is never a well mixed gas, its atmospheric lifetime (~ 9 days) is way too short for that. (judithcurry.com)
It is well known that it has been warming on the Antarctic Peninsula, probably for the last 100 years (measurements begin at the sub-Antarctic Island of Orcadas in 1901 and show a nearly monotonic warming trend). (realclimate.org)
Modern kitchen design is quite mundane with monotonic colors, plain colors which are usually very neutral and clear cut sharp edges. (industrystandarddesign.com)