How can I get seasonal mean data from long time series data

[Yang Haijun <navyang@xxxxxxxxxxxxxxxxxxxx>]

Hi,

I have data with 40 years x 12 months like below,

     currently SET data sets:
    1> /big/navyang/yhj/XBT/T_xbt_55_94.nc  (default)
 name     title                             I         J         K         L
 TEMP     TEMPERATURE ANOMALY FROM XBT     1:73      1:61      1:11      1:480

How can I get long period seasonal mean data with 40 years x 4 seasons?

Thank you very much.

Haijun Yang