Baroclinic effects and tides on the Cape Hatteras continental shelf

Seasonal variability has been identified on the shelf near Cape Hatteras in the semidiurnal and diurnal frequency bands. Large summertime semidiurnal currents appear to be an M2 internal tide whose propagation shoreward is supported by strong Middle Atlantic Bight (MAB) seasonal stratification. At the southern limit of the MAB, strong MAB stratification gives way to weaker seasonal stratification in the South Atlantic Bight (SAB), and the M2 internal tide propagates shoreward less effectively. Strong diurnal variability appears in K1 and O1 components in summer, achieving magnitudes as large as the M2. The diurnal components are typically much smaller than M2 in winter. However, this summer signal is unlikely to be a diurnal internal tide since at these latitudes (34.5–36.5N) the diurnal frequency is subinertial. Coastally trapped waves (CTWs) are presented as a mechanism to explain the increased variability in the diurnal band under summertime stratification. Alongshore southward propagation of the diurnal variability is evident from moorings on the middle and outer shelf (phase speed of 2.1–2.6 m/s along the 60‐m isobath) as far south as Cape Hatteras, but little energy in this band propagates past Cape Hatteras. While diurnal band CTW propagation will not occur at this latitude under well‐mixed conditions, stratification could cause those frequencies to become available for a given wave number, as described in the work of Brink (1990). Estimates of the Huthnance (1978) stratification and slope parameter A , and the Burger number S , suggest the summertime diurnal signal is consistent with baroclinic CTWs. If so, these are the first observations of diurnal CTWs at Cape Hatteras.