def find_concat_dim(da, possible_concat_dims):
    """ look for available dimensions in dataaray and pick the one
    from a list of candidates

    PARAMETERS
    ----------
    da : xarray.DataArray
        xmitgcm llc data array
    possible_concat_dims : list
        list of potential dims

    RETURNS
    -------
    out : str
        dimension on which to concatenate

    """
    out = None
    for d in possible_concat_dims:
        if d in da.dims:
            out = d
    return out