Intellectual Merit: The front that bounds a tidal river plume represents a region of intense exchange between terrestrial- and oceanic-source waters. It propagates as a gravity current, with a turbulent rotor that both dissipates and radiates internal wave energy as it advects and diffuses freshwater into the coastal waters. White and Helfrich (2008) have outlined a framework in which the dissipative or radiative character of a steady front can be assessed from its density and velocity alone, contrasting the hypothesis that frontal deceleration was necessary for wave release (Nash and Moum, 2005). It is the objective of this project to detail the relationship between waves and turbulence using data from the Columbia River plume system, to test some of White and Helfrich?s (2008) ideas in the field, to connect these with laboratory and numerical experiments, and to extrapolate these to a plume-wide system,. The study focuses on 4 aspects of the plume system in which high-resolution turbulence and internal wave data have been collected but not fully analyzed: (1) Energetics and structure of the gravity current head: are details of the density/velocity structure behind the front sufficient to assess whether a front dissipates energy via turbulence or via radiated waves? And can the combination of dissipation, wave trapping and wave release account for the front?s observed evolution (growth and decay)? (2) How do these work to set mass transport and circulation patterns within the head? How/where are energy and mass transported during wave release? (3) Can mixing within the stratified shear flow behind the front be predicted a priori, and how important is it compared to that at the front, from a perspective of both energetics and cross- isopycnal transport? (4) Can (1-3) be related to the external parameters, such as river flow, tides, coastal currents? In addition to testing lab/numerics-inspired theoretical ideas about gravity current propagation, a complimentary goal is to assess which of the front or the trailing shear flow is responsible for the majority of the plume mixing. This is fundamental, as it dictates whether surface coastal waters or deeper fluid is entrained into the plume, which have important biogeochemical consequences. In addition, if energy is lost from the front via waves, that energy is not available for local mixing but may instead produce turbulence in regions remote from the plume itself.
Broader Impacts: In addition to the analyses of Columbia River plume data outlined above, a second objective is to link recent fieldwork on plumes of different scale to each other, and to laboratory and numerical studies that are simultaneously being conducted. To do this, a 4-day workshop will be held with a focus on providing a flexible and adaptive forum for discussing recent results, exchanging analysis techniques, and collectively synthesizing studies and future directions. Given that there has been a significant recent interest in this area (with numerous current and recently-completed experiments), such a meeting is timely. This grant will support a PhD candidate at OSU, and provide opportunities for student exchanges among synergistic field/lab/numerical projects. Results will be broadly disseminated in scientific journals and made accessible to the public through the web and other media (newspaper articles, etc.) Finally, knowledge of the location and intensity of plume fronts and of large-amplitude waves is of significant interest to the marine community: (1) for navigation, these are a hazard due to velocity convergence and wave steepening, especially over the bar, (2) for fisherman, they represent active feeding grounds, (3) for pollution response, the front represents both a material boundary for surface contaminants, but also a conduit for exchange into deeper/coastal waters. It is hoped that these results will ultimately improve the skill of operational river-plume models.
