Collaborative Research: Understanding tidal Resonances in the Present-Day and Ice-Age Oceans

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The suggestion that dissipation of tidal energy contributes significantly to mixing of the abyssal oceans, has renewed interest in tides, and their influence on the large scale ocean circulation. The postulated connection between tides, mixing, and the meridional overturning circulation raises interesting questions regarding the role of tides in the past. Two recent studies have demonstrated that the North Atlantic tides of the most recent ice age were substantially larger than they are today. Ice-age tides differ from those of today because tides are resonant phenomena, and are therefore likely to be quite sensitive to changes in sea level and associated changes in basin geometry. The principal objective of this proposal is to address the question of how the astronomical tidal forcing, sea level and basin geometry, and dissipation conspire to produce the observed tidal response. Motivated by recent advances in forward tide modeling, the proposed research seeks to understand tides of both the present-day and of the ice-ages. There are 3 principal elements to the proposed research: (1) Computation of the normal modes of Laplace's tidal equations for the global ocean at a resolution of 1 degree, (2) Simulations with a global forward model of tides to investigate the oceanic response to different forcing frequencies, basin geometries, and sea-levels; and (3) Interpretation of the forward model results in terms of damped-driven oscillator theory, using, as inputs, the eigenfrequencies and associated spatial patterns determined from the normal mode calculation. An improved understanding of tides and how they respond to sea-level change would be an important step toward being able to address a range of issues related to the impact of tides on the large scale circulation. The work will contribute to our understanding of the relative importance of the spatial structure of the forcing, the forcing frequency, and the dissipation, in setting the amplitude of tides in the present day and in the ice ages. The research has broad impact because tides and tidal dissipation affect oceanic circulation (via tidal mixing), sea ice, floating ice shelves, and continental ice sheets. The work has implications for paleoclimate, because of the effects of tidal mixing on oceanic circulation, and also because ice-age tides were very large in the Labrador Sea, site of the Heinrich event iceberg discharges (tides have been proposed as a factor in the iceberg discharges). Finally, this research will provide a framework for predicting the tides of a future with potentially much higher sea levels, with application for coastal cities. The PIs play an active role in teaching, mentoring, and outreach, and this project will provide continued support for those activities, including support for an undergraduate student who will work on this project under the PIs' direction. Results and nume