Jointly using the GLDAS 2.2 model and GRACE to study the severe Yangtze flooding of 2020

• Daily land surface model data reveal severe TWS surplus in the MLYR basin in 2020. • GRACE and hydrological observations identified the TWS surplus. • The TWS surplus migrated from south to north and caused flood events. • Anomalies in monsoon and circulation in the middle and high latitudes were the reasons. A severe flood occurred in the middle and lower reaches of the Yangtze River (MLYR) in 2020 and caused an obvious terrestrial water storage (TWS) increase. However, the process and cause of this flood have not been fully revealed. To address this problem, we use Global Land Data Assimilation System (GLDAS) −2.2 daily TWS data that assimilate Gravity Recovery and Climate Experiment (GRACE) observations to thoroughly investigate this flood event. Evaluation results show that the GLDAS TWS agrees well with GRACE TWS and water budget results around the MLYR mainstream, which provides confidence in using GLDAS TWS data to monitor floods. The daily GLDAS TWS anomalies and hydrological observations around the MLYR mainstream coincide well in terms of their temporal patterns. These two independent indicators together identify this severe flood event and its process. GLDAS and hydrologic results suggest that this flood event started south of the MLYR mainstream basin in early June and then moved northward, and in July it covered the whole MLYR mainstream basin and intensified along the MLYR mainstream and its north. This flood event reached its peak around the MLYR mainstream basin and north of it around late July, and then retreated progressively till September. GLDAS results show that the largest regional positive TWS anomaly was ∼ 130.8 mm and happened in late July around the MLYR mainstream, influencing a vast area of ∼ 9.2 × 10 5 km 2 in the MLYR basin. The flood center first appeared along the MLYR mainstream, then moved northward, and finally stayed north of the MLYR mainstream. These results provide us a quantitative understanding of it. The water budget analysis demonstrates that the above-normal precipitation around the MLYR mainstream during June-July 2020 is the main cause of this flood. The weather analysis demonstrates that the precipitation increase is mainly due to anomalies in monsoon circulation systems, including the anomalous extension of the Western Pacific subtropical high and South Asia high, anomalous low-level southwesterly wind and upper-level westerly jet. And the precipitation increase is also closely associated with the anomalous blocking high and cold air activities in the middle and high latitudes.