Extracting monthly means and anomalies

[WILSON Steve <sgw@xxxxxxxxxxxx>]

I have a gisst.nc file with array "sst" monthly from 1871 - 1991.  It's a
4D file:  longitudes(i), latitudes(j), level(k=1), year(l) so it already
has a time axis with values like 1950 , 1950.083 , (for jan, feb 1950) to
1991.833 , 1991.916 , 1992.  

I would like to extract (and later use for plotting):

(a) a mean climatology, say sstm (containing 12 monthly mean values) to
gisst_m.nc, over say years 1950-1991.  This means averaging every 12th
value from the gisst.nc file.

(b) monthly anomalies: i.e., using both "sst" from gisst.nc and "sstm" 
from gisst_m.nc, to calculate monthly anomalies over the same period
1950-1991 and write them out, say to "ssta" in a file gisst_a.nc.

Can you suggest a way to do this?


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