Prof. Xu Tianhe and his team revealed the causes for gravity changes in the Bohai Sea using satellite measurements. This results have important implications for understanding sea level changes in the Bohai Sea. Prof. Xu is now the leader of research group “Satellite Navigation and Remote Sensing”, from Institute of Space Sciences, Shandong University.
It is argued that attributing the gravity changes in the Bohai Sea would be challenged for GRACE (Gravity Recovery and Climate Experiment) satellite mission due to coarse resolution, which is acknowledged to be >300 km. But the area of the Bohai Sea is only about 80,000 km2. On one hand, ocean mass variations in the Bohai Sea are associated with sediment changes that are deposited by the Yellow River, making it hard to analyze the signal observed by GRACE. On the other hand, the North China Plain, which is adjacent to the Bohai Sea, is now experiencing significant groundwater depletion, amounting to 7 Gt/year. This huge negative signal further compromises the gravity signal from the Bohai Sea.
Figure 1. Mass changes in the Bohai Sea. The blue lines are from GRACE data that are generated by different research groups, e.g., Center for Space Research (CSR). The red line is reconstructed by multisource, including altimeter measurements, and ocean analysis. The strong correlation (R=0.8) is shown between blue line and red line, suggesting that mass seasonal change is mainly caused by sea-water.
Prof. Xu and his team reconstructed GRACE-type signals using satellite altimetry, ocean analysis, and a GRACE land mass estimate (Figure 1). Their results suggest that, on seasonal scale, the Bohai Sea mass change observed by GRACE is mainly induced by sea-water rather than sediment. However they argued that attributing the GRACE secular signal is challenged by high uncertainty in GRACE. They demonstrated that the huge groundwater depletion in the North China Plain masked out the increasing signal in the Bohai Sea observed by GRACE. Dr. Dapeng Mu, The first author, said although the sediment accumulation does occur in the Bohai Sea, GRACE has not been ready to detect such a local signal. This research team tend to attribute the secular trend observed by satellite altimetry to sea-water increase.
This finding was reported by Journal of Geodesy, which is concerned with the whole range of geodetic science, and related interdisciplinary sciences. Dr. Mu are now focused on investigating the causes for sea level change from regional scale to global scale, using satellite observations. This is also a major research interest that is supported by Prof. Xu and his group.
Links to relevant research papers:
1. Dapeng Mu#, Tianhe Xu*, Guochang Xu. An investigation of mass changes in the Bohai Sea observed by GRACE. Journal of Geodesy, 2020, 94: 79.https://doi.org/10.1007/s00190-020-01408-1
2.Dapeng Mu#, Tianhe Xu*, Guochang Xu. Detecting coastal ocean mass variations with GRACE mascons. Geophysical Journal International, 2019, 217(3): 2071–2080.https://doi.org/10.1080/01490419.2020.1711832
3. Dapeng Mu#, Tianhe Xu*, Guochang Xu. Improved Arctic ocean mass variability inferred from time-variable gravity with constraints and dual leakage correction. Marine Geodesy, 2020, 43(3): 269-284.https://doi.org/10.1093/gji/ggz138
Written by：Mu Dapeng
Edited by: Che Huiqing