Recently, the research team led by Research Fellow Hongzhou Xu from the Department of Ocean Circulation Observing and Modeling (LOCOM) published a research paper titled "Three-layer circulation in the world deepest hadal trench" online in the international journal "Nature Communications". Based on the 10,000-meter-class hadal mooring observation array, this research discovered for the first time the three-layer circulation structure in the deep layer of the Challenger Deep (CD) and revealed the mechanism driving the circulation structure. This research finding has important significance for understanding the hydrodynamic environment in the ocean’s deepest areas. Dr. Huichang Jiang is the first author of the paper, Dr. Xin Xiao from Ocean University of China is the co-first author, Research Fellow Hongzhou Xu and Professor Chun Zhou from Ocean University of China are the co-corresponding authors, and Professor Jiwei Tian from Ocean University of China is a collaborating author.

The CD is the deepest hadal trench in the world. The low-latitude Circumpolar Deep Water (LCDW) from the Southern Ocean invades the Yap Trench and the Philippine Sea Basin through this choke point (Fig. 1), thus having an important impact on the local environment. Due to the challenge of sampling in the extreme deep, the LCDW transport and ocean circulation structure in the CD remain unclear. To address these issues, the department joined hands with the hadal research team of Ocean University of China to deploy a 10,000-meter-class mooring array in the Challenger Deep for long-term current velocity observations (the upper part of Fig. 2). Through the analysis of ocean current data, we found that there is a three-layer circulation structure below 3,600 meters in the Challenger Deep, which are the westward flow, the cyclonic circulation and the anticyclonic circulation from top to bottom respectively (the lower part of Fig. 2). The westward flow turns eastward in summer, giving evidence for bidirectional connectivity of deep-sea basins, while the cyclonic circulation and the anticyclonic circulation are relatively steady. Our analysis, combining mooring observations, numerical experiments and previous findings, indicated that the LCDW intrusion, local topography and turbulent mixing are crucial for modulating the three-layer circulation. Turbulent mixing plays a key role in driving the anticyclonic circulation in the CD.

Fig. 1 | Location of the Challenger Deep and simplified deep circulation in the Western Pacific Ocean.

Fig. 2 | Diagram of subsurface moorings on the transect 142.5°E (upper) and schematic diagram of the deep circulation dynamics in the Challenger Deep (lower).

We thank the crew of the R/V 'Dongfanghong 2' and 'Tansuoyihao' for the deployment and recovery of the moorings. The numerical model simulation is supported by Analytical Instrumentation Center, Institute of Deep-sea Science and Engineering, Chinese Academy of Sciences, China. We thank Dr. Hans van Haren from the Royal Netherlands Institute for Sea Research and Utrecht University for providing CTD profile data. We deeply cherish the memory of Professor Qiang Xie and thank him for his contribution to this work.

This research was jointly funded by the National Natural Science Foundation of China, the National Key R&D Program of China, the China Postdoctoral Science Foundation, and the Innovational Fund for Scientific and Technological Personnel of Hainan Province.

References：Jiang Huichang#, Xiao Xin#, Xu Hongzhou*, Zhou Chun*, Philip A. Vetter, Yu Liu, Long Tong, Chen Qi'an, Tian Jiwei. (2024). Three-Layer Circulation in the World Deepest Hadal Trench. Nature Communications, 15(1), 8949.