Here is a puzzling comparison of two zonal averages from Phil Jones’ CRUTEM3 gridded land data. Red shows the average from 20S to 20N and black shows the average of the 20-30S band (both N and S). These are calculated from gridded data at http://hadobs.metoffice.com/crutem3/data/CRUTEM3.nc.
I did this comparison because I noticed a difference between my own average of 20S-20N gridded data and the archived “low latitude” average (which was 30S to 30N)
The range of differences goes from -1.7 to 4.6 deg C. Explanations welcome.