North Atlantic Slope and Canyon Study; Volume 2

Measuring currents in submarine canyons: Technological and scientific progress in the past 30 years

“…This was observed in Lydonia Canyon (Butman, 1988), where currents in the inertial, semidiurnal, and higher frequency bands increased toward the canyon head and toward the fl oor. At places where the slope of the canyon fl oor is near the critical slope for semidiurnal and inertial frequency (see equation 1), the energy in these two frequencies reached the maximum (Butman, 1988). Hotchkiss and Wunsch (1982) rearranged equation 1 to the form…”

“…When internal waves at the canyon mouth propagate toward the canyon head, the power densities of the currents increase in the alongcanyon direction for both along-and crosscanyon velocities, although the former is far more energetic than the latter (Hotchkiss and Wunsch, 1982). This was observed in Lydonia Canyon (Butman, 1988), where currents in the inertial, semidiurnal, and higher frequency bands increased toward the canyon head and toward the fl oor. At places where the slope of the canyon fl oor is near the critical slope for semidiurnal and inertial frequency (see equation 1), the energy in these two frequencies reached the maximum (Butman, 1988).…”

“…Armed with improved instruments and advanced mooring technologies, researchers have been able to simultaneously collect detailed current velocity profi les and other oceanographic data in submarine canyons from multiple moorings deployed along and across the canyon, with multiple current meters and other sensors attached to each mooring, and for periods of time as long as 2 yr (Hotchkiss and Wunsch, 1982;Ferentinos et al, 1985;Butman, 1988;Hunkins, 1988;Palanques et al, 2005;Khripounoff et al, 2009;Xu and Noble, 2009). These long time-series data that have sampling rates of a few minutes allow a wide variety of techniques to be used to describe, analyze, and interpret the observations.…”

“…The high-resolution, high-frequency data collections also made the determination of detailed spectral characteristics of the canyon fl ows possible. Auto spectra and cross-spectra of the velocity and temperature and salinity data defi ne the spatial (both vertically in water column and horizontally along a canyon) variations of the energy of inertial, tidal, and higher-frequency processes (Hotchkiss and Wunsch, 1982;Butman, 1988;Xu and Noble, 2009). Rotary spectra, a close variant of the spectra of vector variables, use the two circular components, the amplitude and phase of both clockwise and counterclockwise motion, to determine the rotary characteristics in fl ow fi elds (Emery and Thomson, 2001;Jarosz et al, 2007).…”

“…This is because Eulerian means are fundamentally different from Lagrangian means, which are a truer representation of water particle motion. Eulerian means can arise from differential vertical mixing in the bottom boundary layer during up-canyon or down-canyon fl ow, from an asymmetry in the up-canyon and down-canyon fl uctuation caused by the geometry of the canyon, or from a net fl ow that balances a Reynolds fl ux caused by the internal wave fi elds (Butman, 1988). This residual Eulerian current may not indicate a net Lagrangian transport of water or sediment.…”