Generation Mechanisms of Nonlinear Internal Waves in Coastal Plain Estuaries

University Of Maryland Center For Environmental Sciences, Cambridge MD

Investigators

Abstract

This research aims at significantly advancing our understanding of estuarine mixing processes and improving the predictions of water-quality and biogeochemical cycling in estuaries. Numerical models used in predicting environmental conditions in estuaries often have difficulties in reproducing stratification affecting our ability to accurately predict various biogeochemical processes, including bottom water hypoxia. This may be related to the lack of proper parameterizations for wave-induced mixing in the pycnocline. Nonlinear internal waves are ubiquitous in the stratified coastal ocean and play an important role in generating turbulent mixing and transporting biochemically important materials. However, nonlinear internal waves have been rarely studied in stratified estuaries, except for flows over sills at the entrance to deep fjords. Turbulence and mixing in estuaries has been mainly attributed to bottom boundary layer turbulence whereas little attention has been paid to breaking internal waves. This project will re-analyze experimental data and use numerical models to examine the role of internal waves in estuarine mixing. It will provide educational opportunities for one Ph.D. graduate student who will be trained in numerical ocean modeling, and advanced data analysis techniques. In addition, undergraduate students will be involved in this project during the summer months. Tidal currents in coastal plain estuaries primarily move in the along isobath direction and are thought to be no-conducive to internal wave generation. On the other hand, most coastal plain estuaries feature a deep channel flanked by shallow shoals, with steep bathymetric changes in the cross-channel direction. Moreover, lateral circulation has been observed in many estuaries. It is hypothesized that when the internal Froude number exceeds 1, the interaction between the lateral circulation and channel-shoal bathymetry leads to the generation of an internal lee wave. The latter evolves into a train of nonlinear internal waves through nonlinear steepening and dispersion. It is also hypothesized that under strong tidal or wind forcing conditions the lateral circulation generates frontogenesis and a bottom gravity current which excites upstream propagating nonlinear internal waves. These hypotheses are based on the recent observations of internal lee waves and internal solitary waves in Chesapeake Bay and will be tested using process-oriented and realistic modeling. The previously collected mooring data will be re-analyzed to better quantify the relationship between nonlinear internal waves and estuarine turbulent mixing. A realistic 3-D hydrostatic model of Chesapeake Bay will be run to conduct hindcast simulations and interpret the observational data. The generation, propagation and transformation of internal waves in estuaries will be investigated using hydrostatic and non-hydrostatic models configured over an idealized estuarine channel. The wave generation mechanisms will be explored under a wide range of river flow, tidal and wind forcing conditions. These model runs will be complemented by data analysis to identify the connection between nonlinear internal waves and mixing in the estuary. 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.