The project will investigate tidal influences on sub-tidal momentum dynamics and exchange at the interface between a rotation-influenced, stratified estuarine system and the coastal ocean. Prominence of tidal processes challenges the conventional view that circulation and exchange are dominated by gravitational circulation. Emphasis is on discerning the importance and nature of three tidal mechanisms for residual flow generation: (a) tidal correlations between axial current vertical shear and vertical eddy viscosity, associated with strain of the axial density gradient by vertically sheared axial tidal currents; (b) tidal-mean secondary circulation from ebb-flood differences in differential advection of the axial density gradient by laterally sheared axial tidal flow; and (c) lateral tidal rectification due to lateral structure in correlations between axial and lateral tidal currents. Diagnostic terms that appear in the sub-tidal axial momentum equation for each mechanism will be investigated using observations and numerical simulations. The site is eastern Long Island Sound (LIS), a complex system, but one that is generally representative of wide, deep estuaries. An extensive suite of archived observations will be exploited, including several years of 8-times daily ferry-based acoustic Doppler current profiler (ADCP) transects, time series of hydrographic casts from moored CTD profilers, and year-round biweekly/monthly CTD surveys over the past ~15 years. New fieldwork includes two additional years of ferry sampling, and small vessel towed-undulator/ADCP surveys needed for tidal process identification. The surveys will characterize the three-dimensional spatial structure and tidal cycles of turbulence, shear, and stratification, so filling a recognized need to advance understanding of estuarine physics. Numerical modeling emphasizes realistic forcing functions and calibration of turbulence parameterization and coefficient values such that gross observed features (lateral and vertical structure; amplitudes and phases of tidal cycles) of measured velocity shear, stratification, and turbulence are captured. Process-oriented runs of the realistically configured model will be used to investigate the roles of tidal mechanisms. Scalar fluxes and transport pathways will be investigated with the model using both Eulerian and Lagrangian analysis methods.
Understanding of estuary-coast exchange has applications to water quality management. Findings will be relevant to decision makers at the study site, an urban estuary heavily impacted by surrounding coastal populations. An existing public-oriented educational display, in the passenger area of the ferry serving as one platform for new observations, will be assessed and redesigned by collaboration with URI Office of Marine Programs and Dept. of Environmental and Natural Resource Economics. The display raises awareness of estuarine physical oceanography, and its connection to water quality, among ~300,000 ferry riders per year. Two additional years of velocity and hydrographical grid data products will be posted at the ferry project website (www.gso.uri.edu/foster). A URI undergraduate and a University of Connecticut graduate student will be trained. Results will be incorporated in University of Connecticut oceanography courses for undergraduate and graduate classes.