title: Calculate mean of several xarray variables
body: <p>I have a xarray data array for example:</p>
  
  <pre><code>&lt;xarray.Dataset&gt;
  Dimensions:     (lat: 180, lon: 360, time: 360)
  Coordinates:
    * time        (time) datetime64[ns] 1990-01-01 1990-02-01 ... 2020-01-01
    * lat         (lat) float64 -89.5 -88.5 -87.5 -86.5 ... 87.5 88.5 89.5
    * lon         (lon) float64 0 1 2 3 ... 357 358 359
  Data variables:
      a    (time, lat, lon) float32 dask.array&lt;shape=(360, 180, 360), chunksize=(360, 180, 360)&gt;
      b    (time, lat, lon) float32 dask.array&lt;shape=(360, 180, 360), chunksize=(360, 180, 360)&gt;
      c    (time, lat, lon) float32 dask.array&lt;shape=(360, 180, 360), chunksize=(360, 180, 360)&gt;
      d    (time, lat, lon) float32 dask.array&lt;shape=(360, 180, 360), chunksize=(360, 180, 360)&gt;
  </code></pre>
  
  <p>How can I calculate the mean of <code>a, b, c, d</code> into a new variable <code>e</code>?</p>
  
  <p>I know that <code>dataset.mean()</code> works on coordinate dimensions right? Such as over time, lat or long, but how can I calculate the mean of the variables themselves? Couldn't find much online about this.</p>
  
  <p>Thanks very much!</p>