The vertical propagation of internal waves through the ocean

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

Overview: The vertical propagation of internal waves through a wide range of density stratifications and vertical shear structures of the ocean is an important problem in physical oceanography. A pressing matter is to develop a better understanding of the flow of internal wave energy that propagates down towards the deep ocean from the base of the mixed layer, as this has the potential to impact ocean mixing and thus play a role in influencing ocean circulation and climate. This issue is currently being very actively pursued in regards to near-inertial waves in the Arctic Ocean due to the retreating summer ice. At present, however, the conclusions of large-scale numerical models are at odds with the observed seasonal cycle of near-inertial wave energy in the deep ocean. Furthermore, there are other potentially important scenarios such as the energy-flux and fate of high-frequency internal waves excited by Langmuir circulation, and the transmission and reflection properties of complex double-diffusive staircase structures, about which little is currently known. Intellectual Merit: The intellectual merit of this project is in the advancement and utilization of a semi-analytical method to reasonably model the propagation of internal waves through arbitrary stratifications and vertical shear. The method is designed to handle the challenging scenarios that can arise in the ocean, in which the vertical scale of variations in the stratification and shear are comparable to the vertical wavelength of the transiting internal waves. The theoretical model will be systematically validated for increasingly complex scenarios through comparison with laboratory experiments and numerical simulations. The laboratory experiments and numerical model will enable investigations of nonlinear regimes the linear theoretical model cannot access. The modeling effort will be closely coordinated with past, present and future field studies of the North Pacific and Arctic Ocean via a number of collaborations, keeping it grounded in what happens in the ocean, and both the Principal Investigator and his graduate student will participate in a 2015 NSF-funded field study. Broader Impacts: The broader impact of this research is insight into the propagation and dissipation of internal waves through complex ocean stratifications, thereby supporting investigations of mechanisms that influence ocean mixing. Specifically, the investigators will improve understanding of, and the ability to predict, the downward propagation and instability of internal waves from the base of the mixed layer and through double-diffusive staircase stratifications. The validated theoretical method, prepared as a MATLAB GUI with user guide and made freely available, will provide a new tool for interpreting field data and assisting in the planning of field studies. The project will support the training of a graduate student, an undergraduate researcher and part of the training of a postdoc at MIT. Graduate students, postdocs and faculty at WHOI, Yale and University of Washington will also be trained in the theoretical method. A fellow of the WHOI Geophysical Fluid Dynamics summer program will be recruited to participate in experiments, and MIT-WHOI Joint Program students will be recruited to produce short and inspiring educational videos on the ocean through collaboration with the MIT+K12 program. The theoretical method and experiments will be incorporated in a graduate course on environmental fluid dynamics. The results of this study will be disseminated through journal articles, invited seminars and a website.