seasonal average

[praise praise]

Dear All,

I have monthly data say starts from 1958 Jan till 2005 Dec (ie. t=1 to
576).

I would like to do an EOF analysis for the 46 winters only (say the average
of DJF from 1958/59 to 2004/05).

So, how can I define a new variable representing the 46 winters (Not the
climatology) before I can run the EOF.

I have tried this:
      set t 1 46
     define a=ave(air,t+11,t+13,12)

But clearly, its result was not I want.

thanks

mc

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