Collaborative Research: Mixing of river water into the coastal ocean and the role and structure of the outer edge of the discharge

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As river waters enter the coastal ocean they mix with the saltier sea water. This process starts at the seaward boundary of the river discharge (plume) and it affects circulation and flow near the river mouth and throughout the adjacent coastal region. Although the area where this river-ocean interface occurs represents a small region of the ocean, it is very important to ocean mixing and is often poorly resolved in circulation models used to predict environmental conditions in coastal regions. As part of this investigation the researchers will carry out field measurements and utilize numerical models to define variability in the structure of the seaward boundary of the freshwater discharge, and the mixing that occurs there. These processes will be examined for different river discharge and tidal conditions. In addition, this study will assess how much of an ebbing riverine outflow is cycled through the seaward boundary and evaluate the overall importance of this region to mixing the total river discharge to a near-ocean salinity. The outcomes from this work will reveal the details of the physical processes taking place at the riverine-ocean interface and improve the predictive capabilities of large-scale models. In addition, the advances gained from this study will ultimately allow better prediction of the fate and transport of pollutants, nutrients and water masses in the ocean. During the period of this project three graduate student will supported contributing to building scientific capacity.The frontal region of a river plume is the seaward boundary that separates a buoyant coastal discharge from ambient ocean water. Oceanic fronts of all scales act as significant energy sinks over several orders of magnitude through mechanisms that are poorly understood. Although significant efforts have advanced knowledge of plume dynamics over the last several decades, fundamental gaps still exist in our understanding of frontal evolution, and the role of the front in establishing the dynamics in the interior of the plume and in the overall plume mixing budget. The objective of this proposal is to understand the evolution and significance of frontal processes across a range of scales, from small scale propagating discharge fronts to intermediate scale shelf fronts bounding an ebb pulse, and to identify the mechanisms controlling the evolution of a front from engineering scales to geostrophic scales. Our hypothesis is that the relative importance of frontal mixing to overall mixing in the plume is controlled by the ratio of a frontal length scale representing water mass interaction with the front, to the overall plume length scale, and diminishes as the plume front evolves, due to diminishing connectivity between the plume front and the source through the ebb tide. We hypothesize that mechanisms of diminishing connectivity include both weakening of barotropic gradients through the plume as well as Coriolis effects as the plume becomes geostrophic. This work will be pursued using 21st century current, hydrography and microstructure measurements in the plume front and core to assess mixing using various techniques, as well as hydrostatic and non-hydrostatic realistic and idealized numerical models. The field effort is focused on the mid-sized Merrimack and Connecticut rivers, which are well studied and provide an ideal setting to observe frontal evolution in regions with and without strong tidal currents. Results from the field study will allow calibration and validation of models, which can then be used to understand similar frontal processes across a wide swath of parameter space.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.