Change default season months and calculate multi-year seasonal mean in CDO

Question

I have a netcdf file containing daily temperature and precipitation converted from ERA5 data for the period 1975-2005. I want to calculate seasonal means over the entire time period but my season duration is different from the default CDO season. CDO specify seasons as DJF, MAM, etc. but I want it for JJAS season. How can I make this change and calculate the seasonal mean?

cdo -yseasmean -select,season=JJAS uk_t2m.nc uk_t2m_yseasmean.nc

cdo(1) select: Process started

cdo(1) select: Processed 3573990 values from 1 variable over 11323 timesteps

cdo    yseasmean: Processed 3573990 values from 1 variable over 3782 timesteps [0.58s 37MB]

 cdo showdate uk_t2m_yseasmean.nc
  2005-08-31 338075-12-20

Is there a better way to do this operation in CDO or using other tool? I am getting two dates on running cdo showdate command, I could not understand why. Any tips would be helpful.

Answer 1

Probably the most direct way is to simply forget about the pre-defined seasons and just select the months you want and then do a timmean to get the average over those months:

cdo -timmean -selmon,5/8 in.nc out.nc