5:15 PM - 7:15 PM
[AOS21-P09] Riverine water distribution and vertical mixing in the plume area of Tone River
Keywords:turbulence observation, nutrient cycle, river plume
River water from the Tone River provides nutrients to the surface layer in the coastal area. Although satellite observation captures low salinity water from the river that forms river plume, its structure and temporal variation, and mixing that redistribute materials to ocean are not yet fully understood. While field observations have provided information on salinity levels and extent in river plume, in the inshore area, where river plume is formed, the observation remains insufficient due to fisheries activities. Especially turbulent observations that are essential for understanding the dynamics of the plume have not been conducted. Therefore, with the cooperation of fishermen, we observed temperature, salinity, velocity, turbulence in the inshore area off the Tone River mouth, and analyzed the structure of outflow from Tone River and mixing characteristics.
Our research group has conducted monthly observations using the Daisan-Toyomaru, fishing vessel belonging to Hasaki Fisheries Cooperative since January 2024. This study presents the result conducted in 9–10th July 2024. The two observation lines was set approximately 5 km from the river estuary to northeastward (15–35 m depth). One on the north side and the other on the south side are referred to as Line A and Line B, respectively. Four observation points were set at equal intervals on each line, where Conductivity Temperature Depth (CTD) profiler, Acoustic Doppler Current Profiler (ADCP), Vertical Microstructure Profiler (VMP) measurements were conducted.
Additionally, satellite imagery from Sentinel-2 (10 m resolution, 10 days cycle), Himawari-9 (100 m resolution, 2.5 mins cycle), Shikisai (GCOMC; 250 m–1 km resolution, 2–3 days cycle) were used to examine the condition of the Tone River estuary before and after the observations.
Tone River water discharges from the river mouth that opens northward approximately 1 km wide. Mixing with seawater occurs in the estuary, and the salinity within the outflows were approximately 31.
Ocean color imagery observed by Sentinel-2 confirmed the shape of the river plume. The distribution of the chlorophyll-a concentration from Himawari-9 corresponded with ocean color imagery, which suggests that chlorophyll-a concentration can indicate temporal variability of the plume under clear weather conditions.
In June 2024, the month before the observation, plume outflows were detected 15 times. In most cases, northward outflows were observed in the morning, and after that expanded more. Three plume patterns were identified: (i) narrow northward outflow, (ii) rotating eastward and flowing along the coast of Chiba Prefecture, and (iii) flowing along the coast of Ibaraki Prefecture.
On 9th July, low salinity layer (salinity 31–32) was observed at 1 m depth throughout Line B. Salinity decreased offshore along the line, with the layer thickened. Turbulent kinetic energy dissipation rate ranged from 10-8 to 10-6 W/kg. Significant turbulence were observed beneath the low salinity layer along the line B and above seabed on the estuary side. Vertical distributions of current velocity showed that ambient flows were baroclinic on both days, and especially northwestward on the first day and northeastward on the second day tended to be stronger, corresponding to the tidal condition.
Within the low salinity layer along line B, chlorophyll-a concentration, turbidity and temperature tended to be maximum values, indicating that riverine water exists. At many observations point, the depth where chlorophyll-a concentration and turbulent kinetic energy dissipation rate peaked coincided, suggesting that vertical shear in current velocity was significant at that depth. These results indicate that mixing beneath the plume contributes to the diffusion of riverine water.
Future studies will conduct more detailed turbulent measurements within the low salinity layer to reveal how much mixing occurs within the plume due to forcing. In addition, we will conduct spatially and temporally higher-resolution observations to understand the dynamics of whole plume.
Our research group has conducted monthly observations using the Daisan-Toyomaru, fishing vessel belonging to Hasaki Fisheries Cooperative since January 2024. This study presents the result conducted in 9–10th July 2024. The two observation lines was set approximately 5 km from the river estuary to northeastward (15–35 m depth). One on the north side and the other on the south side are referred to as Line A and Line B, respectively. Four observation points were set at equal intervals on each line, where Conductivity Temperature Depth (CTD) profiler, Acoustic Doppler Current Profiler (ADCP), Vertical Microstructure Profiler (VMP) measurements were conducted.
Additionally, satellite imagery from Sentinel-2 (10 m resolution, 10 days cycle), Himawari-9 (100 m resolution, 2.5 mins cycle), Shikisai (GCOMC; 250 m–1 km resolution, 2–3 days cycle) were used to examine the condition of the Tone River estuary before and after the observations.
Tone River water discharges from the river mouth that opens northward approximately 1 km wide. Mixing with seawater occurs in the estuary, and the salinity within the outflows were approximately 31.
Ocean color imagery observed by Sentinel-2 confirmed the shape of the river plume. The distribution of the chlorophyll-a concentration from Himawari-9 corresponded with ocean color imagery, which suggests that chlorophyll-a concentration can indicate temporal variability of the plume under clear weather conditions.
In June 2024, the month before the observation, plume outflows were detected 15 times. In most cases, northward outflows were observed in the morning, and after that expanded more. Three plume patterns were identified: (i) narrow northward outflow, (ii) rotating eastward and flowing along the coast of Chiba Prefecture, and (iii) flowing along the coast of Ibaraki Prefecture.
On 9th July, low salinity layer (salinity 31–32) was observed at 1 m depth throughout Line B. Salinity decreased offshore along the line, with the layer thickened. Turbulent kinetic energy dissipation rate ranged from 10-8 to 10-6 W/kg. Significant turbulence were observed beneath the low salinity layer along the line B and above seabed on the estuary side. Vertical distributions of current velocity showed that ambient flows were baroclinic on both days, and especially northwestward on the first day and northeastward on the second day tended to be stronger, corresponding to the tidal condition.
Within the low salinity layer along line B, chlorophyll-a concentration, turbidity and temperature tended to be maximum values, indicating that riverine water exists. At many observations point, the depth where chlorophyll-a concentration and turbulent kinetic energy dissipation rate peaked coincided, suggesting that vertical shear in current velocity was significant at that depth. These results indicate that mixing beneath the plume contributes to the diffusion of riverine water.
Future studies will conduct more detailed turbulent measurements within the low salinity layer to reveal how much mixing occurs within the plume due to forcing. In addition, we will conduct spatially and temporally higher-resolution observations to understand the dynamics of whole plume.