Abstract
Estuarine circulation is a vertical circulation that develops along the salinity gradient in estuaries and nearshore coastal waters. Fresher, and therefore less-dense, water flows out of the estuary in the surface layer, while a deeper inflow brings water from the open sea into the estuary. This study uses 7 years of in situ current measurements and hydrographic surveys to verify that this deeper inflow has two modes: a deep inflow that intrudes along the seabed, and a shallow-inflow that penetrates into the subsurface layer. These modes show seasonal variability, i.e., the deep-inflow mode occupies almost all of the winter season, whereas the shallow-inflow mode dominates during the summer. This mode change may play a key role in oxygen and carbon dioxide (CO2) dynamics in estuaries and nearshore coastal waters. When the transition from deep- to shallow-inflow begins in spring, a cold water mass forms on the seabed in the upper estuary. This cold water mass is isolated from heating sources and oxygenated water; consequently, the cold water mass becomes hypoxic and accumulates both inorganic nutrients and CO2 during spring and summer. When the transition from the shallow- to deep-inflow occurs, the CO2, which is trapped in the bottom-water, is emitted to the atmosphere. The mechanism that causes the seasonal mode change in estuarine circulation is driven by the spatial inhomogeneity in the heating or cooling of the lower layer, which generates a horizontal density gradient in the layer. This mechanism highlights the importance of temperature in estuarine dynamics, which has not been extensively studied previously.
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Acknowledgements
We are grateful for the monitoring data provided and supplied by the Nagoya Research and Engineering Office for Port and Airport, Chubu Regional Development Bureau, Ministry of Land, Infrastructure, Transport and Tourism. The monthly hydrographic observations were downloaded from the home pages of the Hydrographic and Oceanographic Department, Japan Coast Guard, and Mie Prefecture Fisheries Research Institute.
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Communicated by Neil Kamal Ganju
Key Points
- The seasonal change in the depths of the seawater inflow associated with estuarine gravitational circulation is elucidated using 7 years of in situ current measurements.
- Spatial inhomogeneity in heating or cooling in the lower layer causes a horizontal density gradient in this layer, and this induces the seasonal change in the inflow depth.
- The change in the inflow depth plays an important role in oxygen and CO2 dynamics in estuaries and nearshore coastal waters.
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Kobayashi, S., Nagao, K., Tsurushima, D. et al. Seasonal Changes of Estuarine Gravitational Circulation: Response to the Annual Temperature Change. Estuaries and Coasts 45, 737–753 (2022). https://doi.org/10.1007/s12237-021-00991-6
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DOI: https://doi.org/10.1007/s12237-021-00991-6