NCO:ncap2 is the function to do it:
ncap2 -s 'new_var=var1+var2' in_filename.nc out_filename.nc
The output file will have all of the variables that exist in the input file as well as the new_var. Add -O if your input and output files are the same (overwrite).
I do not know what the -s stands for.
BUT the new_var will have the same long_name as the first variable used for summing (i.e. it could make some things a bit confusing).
NCO:ncap2 and .total
ncap2 -s 'summed_variable=variable_to_sum.total($lat,$lon)' in.nc out.nc
Make sure to use single quotes.
If your in.nc==out.cnc then adding -A will save you from having to specify “overwrite” (see this).
ncap2 -A -s 'summed_variable=variable_to_sum.total($lat,$lon)' in.nc out.nc
