multi-year monthly mean standard deviation

[Simon Krichak]

Dear Experts,

 I am trying to calculate mean multiyear standard deviations from
 monthly mean data on temperature and precip.

To do it I apply the following script:

 'open  ../CRU-DATA/GRADS:1951-2000.pre.ctl '
'open  ../CRU-DATA/GRADS:1951-2000.tmp.ctl '
 'set x 1'
'set y 1'
'set t 1 12'
'set tlsupp year'
'define ztemp=ave(aave(tmp.2,lon=36,lon=54,lat=51,lat=65),t+396,t=528,12)'
'define zprec=ave(aave(pre,lon=36,lon=54,lat=51,lat=65),t+396,t=528,12)/30'
'd
sqrt(ave(aave(pow(ztemp-tmp.2,2),lon=36,lon=54,lat=51,lat=65),t+396,t=528,12))'
'd
sqrt(ave(aave(pow(zprec-pre,2),lon=36,lon=54,lat=51,lat=65),t+396,t=528,12))'

And obtain the following diagnostics:

ga-> stand-deviation.gs
Syntax Error:  Invalid dimension expression
 Cannot use an offset value with a varying dimension
 Varying dimension = 3
Error from AVE:  1st dimension expression invalid
Operation Error:  Error from ave function
 Operation Error:  Error from sqrt function
 Error ocurred at column 1
 DISPLAY error:  Invalid expression
Expression =
sqrt(ave(aave(pow(ztemp-tmp.2,2),lon=36,lon=54,lat=51,lat=65),t+396,t=528,12))
Syntax Error:  Invalid dimension expression
Cannot use an offset value with a varying dimension
 Varying dimension = 3
Error from AVE:  1st dimension expression invalid
Operation Error:  Error from ave function
Operation Error:  Error from sqrt function
 Error ocurred at column 1
 DISPLAY error:  Invalid expression
 Expression =
sqrt(ave(aave(pow(zprec-pre,2),lon=36,lon=54,lat=51,lat=65),t+396,t=528,12))


I do not understand my mistake unfortunately.
Please kindly let me know your recommendations.

Thanks.
                   Simon Krichak