Fine-Structure Contamination: Observations and a Model of a Simple Two-Wave Case

Internal waves advect vertical structure past Eulerian (fixed depth) sensors, leading to “fine-structure contamination,” wherein the intrinsic frequency spectrum is Doppler shifted by the advective motions. Shear, velocity, and isopycnal displacement records collected at low latitude (6.5°S, Coriolis frequency f = 1/4.4 cpd) with an ADCP and a loosely tethered vehicle are used to demonstrate this mechanism in a simple case. In contrast to the usual midlatitude situation where the intrinsic and advective frequencies are broadband and overlapping, the low Coriolis frequency at low latitude allows clear identification of heaving of near-inertial shear layers by the diurnal internal tide. Specifically, Eulerian shear and velocity frequency spectra show peaks at f ± K1, where K1 = 1 cpd is the diurnal tidal frequency. A simple two-wave model illustrates the mechanism and correctly predicts the magnitude of the shifted peaks. The shifted peaks are absent in spectra of quantities computed in an isopycnal-following frame since the advection effect is removed.