Variational Estimation of the Large-scale Time-dependent Meridional Circulation in the Sun: Proofs of Concept with a Solar Mean Field Dynamo Model太阳中大尺度随时间变化的子午环流的变分估计: 太阳平均场发电机模型的概念证明ASTROPHYSICAL JOURNALIF5.42017-11-1011OAAI