extracting columns from a ts series

For Dec columns use 
window(upwell, start=c(1984, 12), frequency=1)

This works OK

But the other formula not. Maybe is my fault, I was not clear. What I need is:

(Dec(t-1)+Jan(t)+Feb(t)+March(t))/4

Hmm. So the following line should do the trick. I replaced sum() by
mean() --- easy, isn't it?

 mean(window(upwell, start=c(1984, 12), end=c(1985,3)))

Uwe

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