def tn_mean(tasmin, freq='YS'):
    r"""Mean minimum temperature.

    Mean of daily minimum temperature.

    Parameters
    ----------
    tasmin : xarray.DataArray
      Minimum daily temperature [℃] or [K]
    freq : str, optional
      Resampling frequency

    Returns
    -------
    xarray.DataArray
      Mean of daily minimum temperature.

    Notes
    -----
    Let :math:`TN_{ij}` be the minimum temperature at day :math:`i` of period :math:`j`. Then mean
    values in period :math:`j` are given by:

    .. math::

        TN_{ij} = \frac{ \sum_{i=1}^{I} TN_{ij} }{I}
    """

    arr = tasmin.resample(time=freq) if freq else tasmin
    return arr.mean(dim='time', keep_attrs=True)