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<span class="code">modify <i>varname type</i></span> 
<p class="plaintext"> This command defines a climatological variable, which is 
  year-independent. <span class="code"><i>varname</i></span> is a defined grid. 
  There are two options for <span class="code"><i>type</i></span>: 
  <code>seasonal&nbsp;&nbsp;&nbsp;</code> <span class="plaintext">- For creating 
  monthly or multi-monthly climatologies</span><br>
  <code>diurnal&nbsp;&nbsp;&nbsp;&nbsp;</code> <span class="plaintext">- For creating 
  climatologies over a time period less than a day<br>
<h3>Usage Notes</h3>
<p class="plaintext">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 &quot;sstclim&quot; 
  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: </p>
<p class="code"> 'set t 1 12'<br>
  'define sstclim = ave(sst, t+0, t=600, 12)'<br>
  'modify sstclim seasonal'<br>
  'set t 1 last'<br>
  'define anom = sst - sstclim'<br>