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Conference Paper

Local mixing intensity determines stratification in a mesotidal estuary – a study of extreme runoff conditions

Authors

Reese,  Nina
IUGG 2023, General Assemblies, 1 General, International Union of Geodesy and Geophysics (IUGG), External Organizations;

Gräwe,  Ulf
IUGG 2023, General Assemblies, 1 General, International Union of Geodesy and Geophysics (IUGG), External Organizations;

Klingbeil,  Knut
IUGG 2023, General Assemblies, 1 General, International Union of Geodesy and Geophysics (IUGG), External Organizations;

Li,  Xiangyu
IUGG 2023, General Assemblies, 1 General, International Union of Geodesy and Geophysics (IUGG), External Organizations;

Lorenz,  Marvin
IUGG 2023, General Assemblies, 1 General, International Union of Geodesy and Geophysics (IUGG), External Organizations;

Burchard,  Hans
IUGG 2023, General Assemblies, 1 General, International Union of Geodesy and Geophysics (IUGG), External Organizations;

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Citation

Reese, N., Gräwe, U., Klingbeil, K., Li, X., Lorenz, M., Burchard, H. (2023): Local mixing intensity determines stratification in a mesotidal estuary – a study of extreme runoff conditions, XXVIII General Assembly of the International Union of Geodesy and Geophysics (IUGG) (Berlin 2023).
https://doi.org/10.57757/IUGG23-0501


Cite as: https://gfzpublic.gfz-potsdam.de/pubman/item/item_5015935
Abstract
As already predicted by the Knudsen relations, estuarine dynamics strongly depend on the freshwater runoff entering an estuary. At the same time, salt mixing can be understood as the driver of estuarine circulation. It enables the transport of water between inflow and outflow layers and therefore closes the estuarine circulation by driving a diahaline exchange flow. A recently derived universal law links salt mixing to freshwater runoff for an estuarine volume bounded by an isohaline surface: it states that on long-term average, the area-integrated mixing across the bounding isohaline surface is directly proportional to the freshwater runoff entering the estuary. However, even though numerous studies predict that periods of extreme runoff will become more frequent with climate change, the direct impact of such periods on estuarine mixing and circulation has not yet been investigated. In this study, we therefore focus on salt mixing and diahaline exchange flow during a low-runoff and an extremely high-runoff period. As a representation of typical mesotidal estuaries, we used a realistic numerical setup of the Elbe estuary in northern Germany. We find that the spatial distribution of diahaline exchange flow is directly linked to the local mixing gradient. Additionally, during the high-runoff period, increased vertical stratification occurs within the estuary, even though estuary-wide mixing scales with the freshwater runoff as predicted by the universal law. Our findings indicate that stratification occurs due to a locally limited mixing intensity, which requires an increased isohaline surface to ensure that the area-integrated mixing still follows the universal law.