关于计算月距平

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我下载了ncep的月平均资料，时间长度是1979年到2015年，想要计算每个月的距平值，比如1998年2月份距平，应该是1998年2月份的数据减去1979到2015每年的2月份数据的平均值，在论坛上也参考了别人的帖子，对于modify的用法还是不明白，以下是官网的说明：
modify varname type
This command defines a climatological variable, which is year-independent. varname is a defined grid. There are two options for type:

seasonal    - For creating monthly or multi-monthly climatologies
diurnal     - For creating climatologies over a time period less than a day
Usage Notes

Example

Say you have a 50-year timeseries of monthly mean sea surface temperatures (a variable named sst with 600 time steps) and you want to create a climatology and then look at the monthly anomalies. First, set the time range for 1 to 12, to span a complete year. Second, define the variable "sstclim" which will contain the January mean in the first time step, the February mean in the second time set, etc. Then use 'modify' to turn 'sstclim' into a climatological variable. This means that the calendar year associated with 'sstclim' (the first year in the original sst data set) becomes a wild card. Then you can define the anomaly by subtracting the climatology from the original time series. The commands are as follows:

'set t 1 12'
'define sstclim = ave(sst, t+0, t=600, 12)'
'modify sstclim seasonal'
'set t 1 last'
'define anom = sst - sstclim'

不明白的地方是
'set t 1 12'
'define sstclim = ave(sst, t+0, t=600, 12)'
这里sstclim是怎么样的值，既然 'set t 1 12'，为啥ave（sst, t+0, t=600, 12）里面有时间步长12，t+0又是什么鬼
求大神们指点迷津