Internal waves, which are waves formed between two layers of a stratified water column, are observed to propagate towards the coastline in most continental shelve regions. As these waves shoal, or are slowed by bottom friction, when entering into water depths of approximately one-half of the wave amplitude, their interaction with the bottom causes these waves to undergo transformations. Observations of across-shore transport and nutrient and plankton mixing suggest that nonlinear internal waves (NLIWs) play a critical role in the maintenance of some inner shelf benthic communities. However, the across-shore transport length scales, the dispersion rates and the fluxes of buoyancy, momentum and energy, and, thus, the potentially important influence of these waves on coastal circulation and ecosystems, have not yet been quantified. Oceanographers from Oregon State University and the University of North Carolina, with support from the Woods Hole Oceanographic Institute, will use a combined observational and numerical approach to quantify transport, dispersion, buoyancy and momentum fluxes, and flux divergences of shoaling NLIWs and determine their influence on the low-frequency circulation and density field in Massachusetts Bay. Their field observations will include deployments of a mooring array to quantify nonlinear internal wave fluxes, rapid shipboard surveys to study the evolution of individual waves from their generation point, through the region of shoaling, and to their demise near the coast, and dye injections and shipboard tracking to quantify scales of across-shore transport and rates of dispersion. In addition to field observations, 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 the field observations and understand the detailed, three-dimensional evolution of these waves. Studying this process will provide an objective assessment of the significance of NLIWs relative to other physical processes in the coastal ocean. Understanding the mechanisms which transport and disperse water-borne materials (e.g., nutrients, pollutants, plankton) in the coastal ocean is a high priority objective for coastal physical oceanographic research and can have direct influence on environmental management.
Internal waves propagate along density contrasts in the ocean. For example, a density interface between dense (cold and salty) bottom waters and light (warm and comparatively fresh) surface waters often exists in the ocean and large vertical oscillations of this interface can propagate horizontally across the sea in a manner analogous to surface swell propagating at the surface of the ocean. In coastal regions, internal waves are often observed to propagate toward the coast with large vertical oscillations (amplitudes are often observed to exceed 30 meters). Surface currents associated with these large amplitude waves have the potential to transport material in the water toward the coast. For example, internal waves are hypothesized to be important for transporting larval stage organisms to nearshore adult habitats. However, transport by internal waves is difficult to directly measure in the ocean. The objectives of this research project were to: directly measure transport of waterborne material by shoreward propagating internal waves and to determine the strength and spatial structure of wave-averaged currents driven by internal waves (analogous to rip currents driven by shoaling and breaking surface waves near the coast). Field experiments took place in the summers of 2008 and 2009 in Massachusetts Bay, a region well known for large amplitude, shoreward propagating internal waves. To measure transport by internal waves, fluorescent dye was injected into the surface water prior to the arrival of an internal wave packet. Subsequent motion of the dye was tracked visually and by using profiling instruments from a ship that measure dye concentration, based on fluorescence intensity. Drifters with GPS units were also deployed to track water motions associated with internal waves. In addition, current meters and temperature and salinity sensors were deployed on tripods set on the ocean bottom and along mooring lines to determine how the internal waves were transformed as they propagated into shallow water and to measure the spatial structure and strength of internal ‘rip-currents’ (wave-averaged currents) driven by the internal waves. Dye and drifter measurements from this study are the first direct measurements of transport by internal waves in the ocean and revealed that a single internal wave packet can transport surface waters toward the coast of Massachusetts Bay by as much as 20 km. The transport is dependent on the amplitude of the wave, with longest cross-shore transports associated with large amplitude waves. Internal wave, cross-shore, rip-currents were also strong (horizontal currents as big as 20 cm/s) and dependent on the amplitude of the waves driving the currents, with large waves driving the largest flows. The currents had a three-layer vertical structure. Surface and bottom currents were directed offshore, while currents at the depth of the density interface were directed onshore. This research convincingly shows that in coastal regions, such as Massachusetts Bay, where internal waves are large, shoaling internal waves can be a dominant driving mechanism for cross-shore currents and cross-shore transport of water-borne materials.
