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Channel: Earth, Atmospheric and Planetary Sciences - Ph.D. / Sc.D.
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Residual overturning circulation and its connection to Southern Ocean dynamics

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Residual overturning circulation and its connection to Southern Ocean dynamics Youngs, Madeleine Kendall. Over the last 20 years, our understanding of the meridional overturning circulation has improved, but primarily in a two-dimensional, zonally-averaged framework. In this thesis, I have pushed beyond this simplification and shown that the additional complexity of meanders, storm tracks, and other zonal asymmetries is necessary to reproduce the lowest-order behavior of the overturning circulation. First I examined the role of basin width for determining whether the Atlantic or Pacific oceans experience deep convection. I used a two layered model and a rectangular single-basin model to show that the basin width, in combination with scalings for the overturning circulation make the overturning relatively weaker in the wider basin, priming it for a convection shut down.; In addition to this large-scale work, I have examined Southern Ocean-like meanders using a hierarchy of idealized models to understand the role of bottom topography in determining how the large-scale circulation responds to climate change scenarios. These are useful because they preserve the lowest-order behavior, while remaining simple enough to understand. I tested the response of the stratification and transport in the Southern Ocean to changes in wind using a highly-idealized two-layer quasi-geostrophic model. In addition to showing that meanders are necessary to reproduce the behavior of the Southern Ocean, I found that strong winds concentrate the baroclinic and barotropic instabilities downstream of the bottom topography and weaken the instabilities elsewhere due to a form-drag process. With weak winds, however, the system is essentially symmetric in longitude, like a flat-bottomed ocean.; This result is consistent with observations of elevated turbulence down-stream of major topography in the Southern Ocean. My next study investigated a more realistic Southern Ocean-like channel, with and without bottom topography, and examined the three-dimensional circulation in order to understand where vertical transport occurs and develop a picture of the pathways taken by each individual water parcel. I found that the vertical transport happens in very isolated locations, just downstream of topography. Finally, I added a biogeochemical model to my simulations and found that carbon fluxes are enhanced near topography, again highlighting the role of zonal asymmetries. Thesis: Ph. D., Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2020; Cataloged from student-submitted PDF of thesis.; Includes bibliographical references (pages 135-145).

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