Local Generation of Nonlinear Internal Waves on the Continental Shelf: Beams to Modes

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This project studies waves that are formed in the interior of the ocean, at the interface of water masses with different temperature and/or salinity properties. Such waves, called internal waves are generated when tidal flows interact with bathymetric irregularities in the interior of the ocean or the steep continental slope. These internal tidal waves tend to concentrate their energy along specific paths that resemble that of a light beam and propagate almost vertically in the water column. During the internal tide propagation, another type wave is generated, with different characteristics, called internal soliton waves. They propagate horizontally and have been linked to vertical mixing, cross-shelf transport of materials and larvae, and sediment mobilization. This study focuses on understanding the transition from principally beam-like internal tides to shorter nonlinear waves, especially in the coastal ocean. Laboratory, experimental work combined with theoretical and modelling activities will be used to understand the life-cycle of these energetic waves and inform modeling and interpretation of the coastal environment advancing our understanding of ocean mixing. Over the course of this work, the principal investigator will advise undergraduate students through the Woods Hole Oceanographic Institution (WHOI) Summer Student Fellowship Program on research topics directly related to the internal wave generation and propagation. Additionally, this study will contribute to the summer program in Geophysical Fluid Dynamics (GFD), held annually at WHOI, and it will involve GFD fellows with aspects of this research. This proposal will also contribute to maintaining the WHOI GFD Laboratory, a valuable resource to the scientific research community.Onshore propagating large-amplitude internal solitary waves are ubiquitous on continental shelves throughout the world's oceans. These waves emerge in packets from the nonlinear steepening of the internal tide as it propagates onshore. An under-appreciated aspect of this process is that the radiated internal tide is often dominated by vertically propagating beams generated on the continental slope, while the emergent solitary waves propagate horizontally with a single vertical mode structure. How the transition from principally beam-like internal tides to shorter nonlinear waves with modal structure occurs is not well understood, especially in the coastal setting. In particular, the time and length scales for emergence and the resulting properties of the solitary waves are of interest and will be examined as part of this study. A combination of laboratory experiments and numerical modeling work will be used to examine internal tidal beam generation at the continental slope and subsequent evolution of the beam as it propagates onto the confining topography of the shelf where it may eventually lead to the emergence of horizontally propagating modal internal solitary-like waves. The work will consider idealized stratifications, topography, and forcing (e.g. as single forcing frequency) in order to focus on the underlying dynamics and improve our understanding of this transition and the conditions that promote, or inhibit, the generation of these waves from the internal tide. The experimental component will be used to explore the generation and initial propagation of an internal wave beam onto a shelf and to provide a test for the subsequent modeling. The modeling will address limitations inherent in the laboratory work and use two complementary approaches. The first is an idealized multi-mode Korteweg-de Vries (KdV) model that generalizes previous linear, hydrostatic models of internal tide generation and captures the solitary wave dynamics. The second is to use the MIT general circulation model that allows exploration of additional factors not considered in KdV-type models.