Specifying units for an interpolated function

I have 2-D temperature field that I am importing into my model via an interpolation under global definitions. I want to specify the units of that temperature field as being deg C. How do I do this?

I define my interpolation as theta. When I make a variable called 'T' using the expression 'theta(x,z) [C]' the expression turns orange, and the units column remains blank.

Any help would be appreciated.

-Adam