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Physics of estuaries and coastal seas
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Instabilities in river plume fronts during downwelling-favorable winds
Tuesday 8th @ 1130-1150, Conference Hall
Peng Cheng* , State Key Laboratory of Marine Environment Science, Xiamen University
Runqing Lv, State Key Laboratory of Marine Environment Science, Xiamen University
Presenter Email: pcheng@xmu.edu.cn |
| Instabilities are seldom observed in river plumes due to the narrow width that cannot fit eddies of baroclinic instabilities. While the existence of external forcing likely alter the potential vorticity dynamics of river plume fronts and make the plume go unstable. A series of idealized numerical experiments based on ROMS were conducted to investigate the adjustment of river plume fronts in response to downwelling-favorable wind events. During a wind event, Ekman flow advected denser water over light and leads the plume to be well-mixed. As the wind continued, the plume front became unstable with a dominant mode of symmetric instability (SI). After the wind event, baroclinic instabilities developed during the relaxation. A geostrophic current is symmetrically unstable when its Ertel potential vorticity (PV) takes opposite sign of the Coriolis parameter. Atmospheric and Ekman buoyancy fluxes were considered the processes that reduce PV. When the plume approached well-mixed, because the enhanced mixing increased Ekman depth in the plume the Ekman flow was restricted on the seaward side of the front such that it cannot further reduce PV. This study showed that downwelling-favorable winds enhanced downstream freshwater transport that act as a trigger of SI. A comparison of buoyancy flux driven by Ekman transport and along-shelf freshwater transport confirmed that the latter was more important than the former when Si occurred. The wind-driven along-shelf freshwater transport also acted as a driver of frontogenesis and provided energy source to vertical buoyancy flux and geostrophic shear production. When the wind stopped, the wind-driven freshwater extended the plume width that allowed the baroclinic instabilities to be developed. The wave length of those instabilities agreed with the linear stability theory of baroclinic instabilities. |
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