- During four cruises in July and September of 2008 and 2009, the shipboard X-band radar was tuned to detect the modulated bands of surface roughness caused by converging and diverging currents associated with high-frequency nonlinear internal wave (NLIW) packets. The data collected was used to quantify the propagation direction (ϴp), wavelength (λ) and wave speed (c) of thirty high-frequency NLIW trains as they propagated onshore across Massachusetts Bay from their generation site on Stellwagen Bank.
The mean propagation direction of the NLIWs was approximately 243°T. As the waves propagated onshore from the generation site, the observed propagation direction
increased by approximately 20°T over 25km. The average propagation direction in July, 241± 1°T, was slightly more southerly than the average propagation direction in September, 245 ± 1 °T, and the rate of change in July, 0.6 ± 0.1 °T / km, was slightly less than the rate of change in September, 1.1 ± 0.2 °T / km.
The wavelength of the NLIW packets, defined as the average distance between the first and second, and second and third zones of convergence of a NLIW packet, was observed to be approximately 356 m. The average wavelength in September, 372 ± 12m, was larger than the average wavelength in July, 340 ± 9m. There were large seasonal variations in the observed wavelength changes as the NLIWs propagated onshore. During the July cruises, λ increased slightly as the waves propagated onshore (75 m over 25 km), while during September, λ increased by almost a factor of two (325m over 25km). The increase of the wavelength as the NLIWs propagate onshore is likely due to a combination of dispersion and shoaling, and the seasonal differences likely due to seasonal variations in stratification.
Because the NLIW are phase-locked to the barotropic tide, Doppler shifting of the NLIWs by the tidal velocities causes systematic variations in the observed wave speeds across the basin. Once the barotropic tidal velocities are accounted for, the wave speed does not change significantly from season to season (0.53 ± 0.02 m/s for July, 0.55 ± 0.01 m/s for September), or in the cross-shore region studied. The lack of seasonal variations is apparently due to the compensatory effects of the deepening of the pycnocline and the weakening of the stratification that occurred between July and September of both years.