The mid-field river plume is a transition area where a multitude of geophysical mechanisms act to constrain and transform a strongly inertial jet into a geostrophic coastal current. The near-field jet is the area over which the estuarine outflow is supercritical, where advection and shear mixing dominate the dynamics, where the shape of the estuary mouth is important in determining the shape of the near-field plume flow structure, and where the flow is often directed offshore. This is distinct from the geostrophic far-field river plume where the earth?s rotation, wind, and background flow dominate the evolution of the plume, and the flow is primarily along-shore. Although entrainment of ambient water is high in the near-field, the far-field plume tends to form a coastally-trapped boundary current where entrainment is quite small in the absence of other forcing mechanisms such as winds and tides. Within the mid-field, plume spreading must be arrested, mixing reduced, and the flow directed along-shore. Rotation or background currents may arrest plume spreading, preventing plume acceleration and reducing further mixing in the plume. Frontal processes enhance mixing locally, but have an unknown effect on the core of the plume. The contribution of each of these processes to the spreading and mixing of the plume is expected to depend strongly on the scale of the plume. A quantitative understanding of the relative role of these processes is necessary for determining the dynamical role of river plumes of different scales on shelf circulation and transport. The overall objective of this proposal is to better understand how small scale processes of O(1 km) operating near an estuary mouth transition to larger-scale (O(100 km)) buoyancy driven coastal currents through the intermediate scales of O(10 km). The central hypothesis is that a suite of processes operating in the mid-field plume modify the plume spreading and mixing and thus the transition between the near- and far-field plumes. The proposed work will establish the relative influence of a number of key mechanisms in setting the spatial and temporal scales of the near- and mid-field plume regions. It will constrain the total water mass modification in the initial evolution of the plume and help understand how continental shelf-scale coastal currents are formed. The following three specific objectives will be pursued: 1) Quantify the processes that arrest plume spreading, 2) Relate reduced mixing rates to arrested plume spreading and 3) Determine net mixing in the near- and mid-field regions. The approach will be one that integrates field observations, laboratory experiments and numerical model experiments. Such an approach is necessary to accurately describe and quantify the full array of physical processes that contribute to mixing in natural plumes and to determine how these processes are modified across a broad parameter space. Detailed field measurements in the Merrimack River plume will be complimented with non-rotating and rotating laboratory experiments and laboratory and geophysical scale numerical model experiments. This work will fill an important gap in the ability to relate fresh water discharge to large scale coastal ocean circulation.
The broader impacts of the study are two-fold: better understanding the role of river plumes in coastal oceanography, and education of graduate and undergraduate students. The study will determine how river inflows affect water properties over a large area influenced by plumes and provide better predictive capability to coastal managers. It will fund three graduate students, one at each of the three participating institutions. In addition, the field campaigns will provide a unique learning experience not only for the three funded graduate students, but for additional graduate and undergraduate students. The use of a number of small vessels for synoptic research will require graduate students to assume chief scientist responsibilities on individual vessels, providing a level of autonomy and responsibility rare for larger scale physical oceanographic field programs.
