Observations of across-shore transport and nutrient and plankton mixing suggest that nonlinear internal waves play a critical role in the maintenance of some inner shelf benthic communities. A number of biological studies hint that internal waves are a means by which nearshore larvae that spend time in the coastal ocean get pushed back to shore where they can settle in their nearshore habitats. Other studies question whether sewage from outfalls, typically placed in the middle of the continental shelf with the assumption that they are far enough out to sea, may also be brought back to shore, unwanted, by internal waves.
Although internal waves potentially have important influence on coastal circulation and ecosystems, their across-shore transport length scales, dispersion rates and the fluxes of buoyancy, momentum and energy have not yet been quantified.
Jim Lerczak and Kipp Shearman are using a combined observational and numerical approach to characterize these nonlinear internal waves in field experiments in Massachusetts Bay, a region where internal waves propagate onshore “like clockwork” during each tide cycle in summer.
For the first time, fluorescent dyes were used to tag the water of internal waves. Fluorescein and rhodamine were injected into the water just before an internal wave packet passed by and then were tracked with a fluorometer towed behind the ship. Lerczak noted, “We devised a new field method for quantifying internal waves and measuring transport. By tagging the packet of water, we can say, ‘Water started here, an internal wave grabbed it, and it was taken onshore this far by the internal wave.’ ”
The two years of field experiments also included drifter studies to follow surface movements and sensors on moorings left in place over the season. Lerczak described the goals of the experiments, “In this analysis, we want to quantify variability of internal wave structure imposed by tidal cycles and differences in stratification. We also want to study how internal waves drive deep flows analogous to rip currents and undertows in the surf zone.”
In collaboration with Alberto Scotti of Univ. of North Carolina, a state-of-the-art, adaptive grid, nonhydrostatic numerical model will be used to simulate nonlinear internal wave generation and evolution to compare the numerical output with field
Article courtesy of the College of Earth, Ocean, Atmospheric Sciences 2010 Research Highlights