Cross-shore momentum, mass, and buoyancy fluxes by shoaling nonlinear internal waves observed from dye-tracking studies and moored observations

Numerical studies have shown that large amplitude internal waves are effective agents for transporting surface waters, buoyancy and momentum in the direction of wave propagation, when the pycnocline depth is less than half the total water depth. This is particularly true when surface currents associated with the waves approach the wave propagation speed. However, the transport by shoaling coastal internal waves has yet to be quantified from field measurements. Further, the influence of these fluxes, due to wave packets that are typically generated every tidal cycle, on sub-tidal circulation and buoyancy fields, is not understood. Here, we use fluorescent dye tracking studies and a set of moored time series to quantify the cross-shore transport of momentum, mass and buoyancy due to shoaling, nonlinear internal waves. The experiment, from which these data were obtained, took place in the summer of 2008 in western Massachusetts Bay, where shoreward propagating, large amplitude internal waves are predictably observed. The array of four moorings spanned a cross-shore distance of 5.4 km, where the water depth shoaled from 70 to 36 m. At each mooring, two acoustic Doppler current profilers (ADCPs) and 12 to 17 temperature and conductivity sensors measured currents and density over the entire water column for a period of 65 days. In addition, bottom pressure and near-bottom stress were measured at each site. These data were used to quantify cross-shore momentum and buoyancy fluxes and flux divergences associated with the shoaling waves. Fluorescein and rhodamine dye tracking took place during two nine-day cruises. Dye was initially injected within the surface mixed layer, in a line parallel to internal wave troughs. Here we contrast subsequent transport of dye by internal waves when the dye is injected prior to the arrival of a wave packet and when the dye is injected within a trough at the leading edge of an internal wave packet. When the dye was injected prior to wave packet arrival, the dye patch was subducted under the leading wave front and the center-of-mass of the patch was transported offshore. In contrast, when the dye patch was injected within a wave trough, the center-of-mass of the patch was transported several kilometers onshore.