CDO

Hello,

I am a little puzzle about the meaning of the message I get when using the command
cdo fldmean example.nc output.nc
which is:
cdo fldmean (Warning): Using constant grid cell area weights for code -2!

I am interested in averaging "pr", the only variable in exmaple.nc that depends on both lat and lon.
cdo describes the grid as:

Warning (scanVarAttributes) : NetCDF: Variable not found - areacella
File format: netCDF2
-1 : Institut Source Param Time Typ Grid Size Num Levels Num
1 : unknown unknown -1 var F32 12960 1 1 1
2 : unknown unknown -2 var F64 1 2 1 1
Horizontal grids :
1 : lonlat > size : dim = 12960 nlon = 144 nlat = 90
lon : first = 1.25 last = 358.75 inc = 2.5 degrees_east circular
lat : first = -89 last = 89 inc = 2 degrees_north
available : xbounds ybounds
2 : generic > size : dim = 1
Vertical grids :
1 : surface : 0

I am tempted to assume that the spatial average of pr is well done, that is, using non-constant weights but am not sure.
Could you please enlighten me?

Thanks a lot in advance.

Kind regards.