def _dj(scalararray):
    """Return the difference scalararray(j+1) - scalararray(j)

    A priori, for internal use only.

    Parameters
    ----------
    scalararray : xarray.DataArray
        xarray that should be differentiated.

    Returns
    -------
    dj : xarray.DataArray
       xarray of difference, defined at point j+1/2
    """
    dj = scalararray.shift(y=-1) - scalararray
    return dj