def chunk_var_annual(netcdf, var):
    """
    Transform a variable's 3-D data array from months into years.
    
    Parameters
    ----------
    netcdf : Netcdf object
        Netcdf object holding the variable to chunk.
    var : NetcdfVariable object
        Variable to chunk.
        
    Returns
    -------
    List of NumPy ndarrays
        List where each element is a year-chunk of monthly data.
    """
    num_months = netcdf.variables['time'].shape[0]
    num_years = int(num_months / 12)
    if num_months != var.shape[0]:
        raise ValueError('Variable time dimension does not match time variable dimension: {}'.format(var.name))
    chunks = [0] * num_years
    for yr_idx in range(num_years):
        m_0 = yr_idx * 12
        m_1 = m_0 + 12   # m_0 + 11 on paper, but +1 due to numpy slicing.
        chunks[yr_idx] = var.values[m_0:m_1]
    return chunks