[A-OS21] Coastal ocean circulation and material cycle

[AOS21-09] Observations of mixing processes in the vicinity of the Tone River mouth

*Yambe Yakehito¹, Eiji Masunaga¹, Naoki Furuichi², Toshimi Nakajima³, Anne Takahashi³, Sachihiko Itoh³ (1.Ibaraki university, 2.Japan Fisheries Research and Education Agency, 3.AORI, The University of tokyo)

Keywords:Tone River, Coastal oceans, Mixing, River plume

River water plays an important role in marine ecosystems in coastal oceans. It is well known that mixing of fresh river water with oceanic water promotes biological productivity through nutrient supply. Our study area, off the Tone River mouth, is one of the largest fishing grounds around Japan. Mixing processes in this area are essential for understanding the marine ecosystem in this region. However, physical processes in this area have not been understood well due to a lack of oceanographic surveys. This study conducted filed observations around the Tone River month to understand mixing processes of river water and their impacts on physical and biological processes. Results show strongly intensified mixing throughout the water column in the inner part of the river mouth (inside the river). The gradient Richardson number which indicating instability of the water column, shows a instability state within the river mouth due to shear effects. This indicates that saltwater entering the estuary mixes with river water and then flows toward the offshore ocean. Results in offshore area also show strong mixing extending toward offshore, similar to the condition within the river mouth. Therefore, river water was strongly mixed both within the river and coastal ocean. Additionally, we successfully observed various phenomena generating mixing river plumes, such as strong mixing near the bottom and mixing associated with the breaking of internal tides propagating from offshore.