How to get multi-year seasonal or annual mean from a multi-year monthly mean time series

[Yang Haijun <navyang@xxxxxxxxxxxxxxxxxxxx>]

Hi all,

I am working on a data with 40 years x 12 months. I want to get a new data with 40 year x 4 
seasons from the old one. How should I do?

The original data is like below:

 name     title                             I         J         K         L
 TEMP     TEMPERATURE ANOMALY FROM XBT     1:73      1:61      1:11      1:480

Mr.Hankin suggested me see the gt=@MOD syntax at

http://ferret.wrc.noaa.gov/Ferret/Documentation/Users_Guide/v50/chapter4.html#modulo$regridding

and define a seasonal modulo axis with DEFINE GRID. 
http://ferret.wrc.noaa.gov/Ferret/Documentation/Users_Guide/v50/partII.html#V5.0_DEFINE 
AXIS$/EDGES
But I still have no idea to do it. Does anyone give me more detail suggestions?

And does anyone give a explanation to the following fifth line?

yes? define axis/t=1-jan-0001:1-jan-0002:1/unit=days/t0=1-jan-0000 tencoding
yes? let tstep = t[gt=tencoding]
yes? let start_date = tstep[t=15-mar-0001]
yes? let end_date = tstep[t=27-may-0001]
yes? define axis/from_data/T/units=days/name=tax/t0=1-jan-0000/edges/modulo   
     {`start_date,p=7`,`end_date,p=7`,`start_date+365.2425,p=7`}

Thank you very much.

Haijun Yang