Scattering of Baroclinic Kelvin Waves by Sills in Straits

Oregon State University, Corvallis OR

Investigators

Abstract

This project will utilize analytical and numerical modeling to study how shallow sills in channels and straits impact incident coastal Kelvin waves. It is motivated by measurement programs over the past two decades in the passages carrying the Indonesian Throughflow (ITF). Kelvin waves generated in the equatorial Indian Ocean have been shown to penetrate these passages episodically, often reversing branches of the ITF for up to a month at a time. Recent measurements have revealed a vertical structure to these reversals and evidence of vertical phase propagation within the straits associated with passing Kelvin waves. Speculation at present connects the vertical propagation observed within the straits to vertical propagation of the incident waves, but the project will demonstrate that the situation can involve considerable complexity. The work will provide a dynamical foundation for understanding the observations in the ITF passages. A preliminary analytical model has been developed which shows promising results. An incident wave of a single baroclinic mode will be scattered into vertical propagation in the channel and in the basin past the channel. A strait will alter the frequency content of an incident vertically propagating pulse, resulting in vertical propagation characteristics of the transmitted and bypassed pulses that are different from those of the incident pulse. The project will develop the preliminary analytical model, providing justification for the foundational assumptions, error estimates and validity limits. The model predictions will then be analyzed for the most intuitive physical understanding of the various effects. Numerical process models would be used to understand the effects on the model predictions of more realistic dissipation, boundary conditions and bottom topography. Finally, a regional numerical model will be used to attempt to reproduce observations in the ITF passages and explain these in terms of the underlying dynamics revealed by the more idealized models. Intellectual Merit: This study will advance the theory of geophysical fluid dynamics by clarifying the effects that sills in straits and channels have on the baroclinic structures and vertical propagation of incident coastal Kelvin waves. Broader Impacts: The Indonesian Throughflow is a vital link in the upper branch of the global oceanic overturning circulation, which in turn is an important component of Earth's climate system. The Indian Ocean Kelvin waves are known to episodically impact the ITF to the extent of frequently reversing the flow direction, yet the complex vertical and temporal structure of these effects is only now beginning to be understood and they are poorly parameterized in global models. Although this study does not aim at improving the parameterizations of global climate models, its contributions will at least improve the knowledge of what must be parameterized in order to successfully model the global system. The project will provide support for a graduate student and will train the student in analytical and numerical modeling techniques necessary to make further contributions in oceanic dynamics.