Observations of coherent transverse wakes in shoaling nonlinear internal waves

Abstract

Space- and time-continuous seafloor temperature observations captured the three-dimensional structure of shoaling nonlinear internal waves (NLIWs) off of La Jolla, California. NLIWs were tracked for hundreds of meters in the cross- and along-shelf directions using a fiber optic Distributed Temperature Sensing (DTS) seafloor array, complemented by an ocean-wave-powered vertical profiling mooring. Trains of propagating cold-water pulses were observed on the DTS array inshore of the location of polarity transition predicted by weakly nonlinear internal wave theory. The subsequent evolution of the temperature signatures during shoaling was consistent with that of strongly nonlinear internal waves with a large Froude number, highlighting their potential to impact property exchange. Unexpectedly, individual NLIWs were trailed by a coherent, small-scale pattern of seabed temperature variability as they moved across the mid- and inner shelf. A kinematic model was used to demonstrate that the observed patterns were consistent with a transverse instability with an along-crest wavelength of ∼10 m – a distance comparable to the cross-crest width of the wave-core – and with an inferred amplitude of several meters. The signature of this instability is consistent with the span-wise vortical circulations generated in three-dimensional direct numerical simulations of shoaling and breaking nonlinear internal waves. The coupling between the small-scale transverse wave-wake and turbulent wave-core may have an important impact on mass, momentum, and tracer redistribution in the coastal ocean.

Airborne Observations of Shoaling and Breaking Internal Waves

“…They found significant spatial variability in the surface signature of sea surface temperature (see Figure 3 of Lenain & Pizzo, 2021) potentially associated with transverse instabilities. Similar transverse structures were also recently observed using a fiber optic distributed temperature sensing seafloor array off La Jolla, CA (Lucas & Pinkel, 2022). However, none of these approaches can quantify the spatio‐temporal evolution of the IW, as they do not provide direct spatial observations of the kinematics during steepening and breaking.…”

High resolution seafloor thermometry for internal wave and upwelling monitoring using Distributed Acoustic Sensing

“…The widespread "V"-shaped thermal signatures over the shelf resemble those of single, well-defined bores propagating onand off-shore at nearly constant speeds. Lucas & Pinkel (2022) [55] also observed similar patterns with DTS measurements in the near-shore and explained them in terms of vertical water oscillations induced by IWs that in turn advect the vertical water temperature gradient (e.g. the thermocline) against a gently sloping bottom.…”

“…DTS and DSTS track variations in the Raman and Brillouin back-scattered spectrum of light, respectively [42]. Previous studies relied on DTS to measure different underwater environments, including lake and near-coastal seafloor temperatures [49,50,51,52,53,54,55]. Some of these studies described internal waves, thermal events and tidal currents.…”

Vertical Momentum Transport by Internal Gravity Waves Above the Equatorial Undercurrent at 140°W

“…During the second leg of the cruise, 25 October to 3 November, a floating 1.8 km optical fiber was trailed behind the Revelle (Figure 1b). The optical array, when connected to a Silixa Ultima Distributed Temperature Sensing System (https://silixa.com/technology/ultima-dts/; Lucas & Pinkel, 2022; Sinnett et al., 2020), quantified horizontal scales of sea surface temperature (SST) variability, potentially detecting signatures of nocturnal convection and deeper internal waves (Supporting Information ). The array independently estimated both wave frequency and one component of wavelength, greatly reducing ambiguity in estimates of wave intrinsic frequency (Figure 1b).…”