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The M2 tide in the Bohai, Yellow, and East China Seas (BYECS) is simulated by assimilating the TOPEX/Poseidon altimetry data with the adjoint method. The bottom friction coefficients (BFC) are supposed to be spatially varying. In this paper, the BFC at some grid points are chosen as the independent BFC and the BFC at the other grid points can be obtained through linear interpolation with these independent BFC. The independent BFC are selected uniformly over each 1° x 1° area and 120 independent BFC are obtained. Twin and practical experiments are carried out to discuss the spatially varying BFC. The results of twin experiments demonstrate that the adjoint method has a strong ability of inverting the prescribed BFC distributions combined with the spatially varying BFC. The prescribed BFC distributions are all successfully inverted. In practical experiments, the simulation precision has been highly increased by optimizing the spatially varying independent BFC. Three amphidromic points and a degenerate one appear in the cotidal charts of M2 tide and the results coincide with the observed M2 tide in BYECS fairly well. [ABSTRACT FROM AUTHOR]