[ferret_users] Compute mean yearly temperature from monthly dataset

["markus.mingel@xxxxxxxxx" <markus.mingel@xxxxxxxxx>]

Dear users,
I have a problems and I'm not sure I'm solving it in the right way. 

I have a monthly surface temperature (you can see the features of the axis 
below)


yes? SHOW grid/T/l=1:12 tas
    GRID GHF2
 name       axis              # pts   start                end
 LON       LONGITUDE           96mr   0E                   3.75W
 LAT       LATITUDE            96 i   90S                  90N
 normal    Z
 TIME      TIME              1872 i   16-JAN-1850 12:00    16-DEC-2005 12:00
 
       L     T                   TBOX      TBOXLO                TSTEP (days)
       1>  16-JAN-1850 12:00:00  31         01-JAN-1850 00:00:00    15.5
       2>  15-FEB-1850 00:00:00  28         01-FEB-1850 00:00:00    45
       3>  16-MAR-1850 12:00:00  31         01-MAR-1850 00:00:00    74.5
       4>  16-APR-1850 00:00:00  30         01-APR-1850 00:00:00    105
       5>  16-MAY-1850 12:00:00  31         01-MAY-1850 00:00:00    135.5
       6>  16-JUN-1850 00:00:00  30         01-JUN-1850 00:00:00    166
       7>  16-JUL-1850 12:00:00  31         01-JUL-1850 00:00:00    196.5
       8>  16-AUG-1850 12:00:00  31         01-AUG-1850 00:00:00    227.5
       9>  16-SEP-1850 00:00:00  30         01-SEP-1850 00:00:00    258
      10>  16-OCT-1850 12:00:00  31         01-OCT-1850 00:00:00    288.5
      11>  16-NOV-1850 00:00:00  30         01-NOV-1850 00:00:00    319
      12>  16-DEC-1850 12:00:00  31         01-DEC-1850 00:00:00    349.5


and I would compute from these monthly values the yearly means. According to 
previous post I did the following to compute the yearly values:


! Define monthly calendar time axis.
define axis/t=16-jan-1850:16-dec-2005:1/unit=months/calendar=noleap 
month_noleap

! Regrid on new time axis
let tas_monthly = tas[gt=month_noleap@asn]


! Define yearly calendar time axis. 
define axis/t=16-jan-1850:16-dec-2005:12/unit=months/calendar=noleap 
year_noleap


! Regrid to get annual averages.
DEFINE VARIABLE/title="yearly tas [C]" tas_yearly = tas_monthly
[gt=year_noleap@ave]


So is it right? Otherwise, if I'm wrong, which is the best way to do that? 
I believe there is something wrong, since I get 2 spikes at the first and last 
year, respectively (namely the temperature is 1C lower that it should be). 


Thanks into advance for help

Best Regards
Markus