Nonlinear Internal Wave Dynamics
What are internal waves?
Internal waves are waves that propagate along density gradients within a fluid. Similar in many respects to surface gravity waves, which travel along the sharp density interface between air and water, internal waves travel along density gradients within the ocean. Perturbations to these density gradients are restored by gravity, generating a propagating wave.
Globally, internal waves play an important role in the ocean, delivering nutrients to surface waters that stimulate the growth of phytoplankton, the base of the oceanic food web. Generated largely by the interaction of the tide with ocean floor topography and wind, internal waves can carry the energy from these forces across entire ocean basins. As internal waves shoal up onto the continental shelf, they interact with the topography, and in much the same way as a surface gravity steepens and breaks on the shore, internal waves can steepen and dissipate their energy on the shelf. As the internal waves steepen they transform into nonlinear internal waves and can take many forms (e.g. “solitons”, “bores”, “boluses”) – all of which have the potential to bring deep water that has different properties (likely colder, higher in nutrients, lower in oxygen, lower in pH?) across the shelf and into shallower waters.
Why internal bores are NOT boring!
The implications of internal waves to coastal ecosystems are many:
What we’re studying
- Nutrient-rich water transported into shallow waters can fuel primary productivity
- Internal wave driven currents can transport larvae
- Internal wave shear can enhance turbulent mixing on the shelf
- Internal waves can transport low oxygen or low pH water onto the shelf, or potentially ventilate near bed waters that are low in oxygen
- Internal waves can help shape the thermal environment for benthic communities on the shelf (kelp, coral reefs, etc)
Given the important role that internal waves play for many coastal ecosystems, our work examines the dynamics of internal waves on the shelf. Specifically, we are interested in understanding the evolution of internal waves across the shelf, the transport of deep waters onto the shelf by nonlinear internal waves, the spatial and temporal characteristics of turbulent mixing driven by internal waves, and the biological implications for internal wave driven variability on benthic communities.
Internal waves on Reefs?
Currently, we are working researchers at Academia Sinica in Taiwan and at Woods Hole Oceanographic Institution in Massachusetts to study the effect of internal wave forcing on a remote coral atoll in the South China Sea, Dongsha Atoll. Dongsha Atoll is a vibrant reef system that sits in the path of some of the largest internal waves measured in the world’s oceans. We want to understand how these internal waves shape the physical and chemical environment in which these corals thrive. To do this we are deploying a series of instruments on the reef to track how the internal waves bring deep water onto the reef and working with coral ecologists to understand how the corals are responding.
One of the exciting aspects of this study is a new instrument that we are using to capture internal wave dynamics from the fore reef onto the reef flat – a Distributed Temperature Sensing (DTS) System. The DTS system couples a pulsed laser with an optical fiber to measure temperature along the length of the bottom-mounted cable from meter-to-kilometer spatial scales with a sampling period of 30 seconds. These measurements of along-bottom temperature provide an unprecedented, continuous view of the physical dynamics at play in the coastal ocean.
TM DeCarlo, KB Karnauskas, KA Davis, GTF Wong (2015). Climate modulates internal wave activity in the Northern South China Sea Geophysical Research Letters, v. 42, p. 831-838
Davis, KA, JJ Leichter, JL Hench, SG Monismith (2008). Effects of western boundary current dynamics on the internal wave field of the Southeast Florida shelf. Journal of Geophysical Research, v. 113, p. C09010