How to calculate seasonal means in raster time series?

Maybe create a function that takes a date as input and returns a 
meteorological season, and pass this function to `zapply(by=fun)`.
--Mel.
Dear all,

I am using the raster package to calculate seasonal averages of climatic variables. I usually use zApply to perform temporal calculations on gridded time series.

However, this time I need to calculate seasonal averages according to the meteorological nomenclature, with DJF (=winter in the southern hemisphere: December, January, February), MAM, JJA, and SON. It is similar to zoo's yearqtr, but with a offset of -1 month on each quarter.

That means that December values comes from the previous year.
This is a typical raster I work with:

library(raster)

# Create date sequence
idx <- seq(as.Date("2001/1/1"), as.Date("2010/12/31"), by = "day")

# Create raster stack and assign dates
r <- raster(ncol=5, nrow=5)
s <- stack(lapply(1:length(idx), function(x) setValues(r, runif(ncell(r)))))
s <- setZ(s, idx)

On a raster stack like this, what would be the best strategy to calculate "meteorological" seasonal means?
  Thanks in advance,
  -- Thiago V. dos Santos

PhD student
Land and Atmospheric Science
University of Minnesota

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How to calculate seasonal means in raster time series?

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