Climatological mean of several years

I have a netcdf data set of (monthly timexlatitudexlongitude). No of month is 216, so total 18 years. 1st index represent the january data, 2nd index represent the february data ... 12th index represent december data and again 13th index represent january data, 14th index represent february data so on and so forth. Now I want to find mean of all january data, mean of all february data ... mean of all december data. So after doing this operation the size of the array will be reduce to 12xlatxlon(previously it was 216xlatxlon). How can I do that?

Answer 1

[SWARNENDU PAL]

I have given the wrong dimension. The surrent dimension is 128x64x216 (lonxlatxtime). I want to reduce it to 128x64x12