This special section is the scientific legacy of the 13th National Ocean Data Assimilation and Numerical Simulation Conference of China, which was held in Changsha, China during December 3–4, 2020 with more than 160 participants from 35 units in China. It continued a series of National Ocean Data Assimilation conferences which began in 2003, which played an important role in the development of ocean data assimilation in China. In this 13th conference, there were 41 oral presentations and 15 posters, which focused on ocean data assimilation methods, ocean reanalysis data, developments and operational application of ocean data assimilation systems, assimilation of new type ocean observations, ocean targeted observation analysis, coupled data assimilation, ocean models and parameter estimation, etc. This conference was hosted by National University of Defense Technology, and co-hosted by the Institute of Atmospheric Physics of Chinese Academy of Sciences, the South China Sea Institute of Oceanology of Chinese Academy of Sciences, the Second Institute of Oceanography of the Ministry of Natural Resources, Ocean University of China, Tianjin University and Hohai University.