I have 9 NC files, corresponding to 9 regions of the world, where longitude is divided by 60, latitude is divided by 120, how can I combine the above 9 files into one global file?
Have tried CDO mergegrid, but can only merge 2 files at a time, is there a way to merge 9 files at a time?
cdo mergegrid is likely the wrong operation for this. You want to try collgrid instead. It sounds like you have the world split into 3x3 equally sized rectangles. Combining these is exactly what collgrid is designed to do. The command should be something like:
cdo collgrid *infile.nc outfile.nc
See page 45 of the user guide for more details https://code.mpimet.mpg.de/projects/cdo/embedded/cdo.pdf