xarray の移動平均は，pandasの移動平均同様にrollingとmeanを使って計算できる．
For example 3-month running mean is calculated as follows.
z500.rolling(min_periods=3, center=True, time=3).mean()
Running average in xarray is calculated as pandas's running average, by using rolling and mean.
For example 3-month running mean is calculated as follows.
z500.rolling(min_periods=3, center=True, time=3).mean()