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Physics of estuaries and coastal seas
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The distribution of age in a coastal river plume
Monday 7th @ 1410-1430, Conference Hall
Yeping Yuan* , Zhejiang University
Alexander R. Horner-Devine, University of Washington
Presenter Email: yyping@zju.edu.cn |
| In the study of geophysical flows, it is essential to understand the transport of nutrients, contaminants, and other particles for predicting changes in water quality in aquatic systems. The chemical and biological processes related to these substances are dependent on the residence time of the particular constituent and often proceed exponentially, making them sensitive to small variations in the age of fluid parcel in the system. In environmental flows, such as river plume systems, vortices and short circuits often cause large deviations from the widely used hydraulic residence time, causing this to be an inaccurate representation of actual residence time. We use a novel technique that resolves the spatial and temporal evolution of the age of fluid parcels in laboratory flows to characterize retention processes in coastal buoyant plumes. An anticyclonic eddy sets up near the river mouth and accumulates a fraction of the river discharge. In the first stages of development, fluid is retained in the core of the bulge, resulting in a spatial age distribution that exhibits a maximum in the core of the bulge and decreases radially. After approximately 10 rotation periods, the bulge becomes unstable, and smaller eddies initiate mixing between the core and outer portions of the bulge, transporting older fluid from the center of the bulge into the coastal current. We found that the age distribution is initially characterized by a single peak that over time degenerates into multiple peaks. The number peaks is predicted by an instability wavenumber determined for baroclinic instability. When the rotation rate is low, retention in the bulge increases and the maximum and average ages are also higher. |
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