Computing yearly averages from monthly values

Hi Guys
My question is fairly simple. I have say a 3D matrix of the form X(nlat,
nlon, 12) where nlat is the number of spacing along the latitude, nlon
is the number of spacing along the longitudes and 12 is the number of
months. So X has 12 months of data for every lat, long grid. I want to
compute the annual average over each lat, long grid to result in a
matrix AnnAvg(nlat, nlon)

I know I can do it using loops but was wondering if theres any function
or short cut in doing this? I looked up the climatology functions but
couldn't find any

Any help will be useful.

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