Dell Rebecca Walsh

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Dell
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Rebecca Walsh
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  • Thesis
    Boundary layer dynamics and deep ocean mixing in Mid-Atlantic Ridge canyons
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2013-02) Dell, Rebecca Walsh
    Physical oceanographers have known for several decades the total amount of abyssal mixing and upwelling required to balance the deep-water formation, but are still working to understand the mechanisms and locations—how and where it happens. From observational studies, we know that areas of rough topography are important and the hundreds of Grand-Canyon sized canyons that line mid-ocean ridges have particularly energetic mixing. To better understand the mechanisms by which rough topography translates into energetic currents and mixing, I studied diffusive boundary layers over varying topography using theoretical approaches and idealized numerical simulations using the ROMS model. In this dissertation, I show a variety of previously unidentified characteristics of diffusive boundary layers that are likely relevant for understanding the circulation of the abyssal ocean. These boundary layers share many important properties with observed flows in abyssal canyons, like increased kinetic energy near topographic sills and strong currents running from the abyssal plains up the slopes of the mid-ocean ridges toward their crests. They also have a previously unknown capacity to accelerate into overflows for a variety of oceanographically relevant shapes and sizes of topography. This acceleration happens without external forcing, meaning such overflows may be ubiquitous in the deep ocean. These boundary layers also can force exchange of large volumes of fluid between the relatively unstratified boundary layer and the stratified far-field fluid, altering the stratification far from the boundary. We see these effects in boundary layers in two– and three–dimensions, with and without rotation. In conclusion, these boundary layer processes, though previously neglected, may be a source of a dynamically important amount of abyssal upwelling, profoundly affecting predictions of the basin-scale circulation. This type of mechanism cannot be captured by the kind of mixing parameterizations used in current global climate models, based on a bottom roughness. Therefore, there is much work still to do to better understand how these boundary layers behave in more realistic contexts and how we might incorporate that understanding into climate models.
  • Thesis
    Abyssal mixing from bottom boundary effects in Mid-Atlantic Ridge flank canyons
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2010-06) Dell, Rebecca Walsh
    This paper begins to explore a previously neglected mechanism for abyssal ocean mixing using bottom boundary layer dynamics. Abyssal mixing and the associated upward buoyancy fluxes are necessary to balance the sinking of dense waters at high latitudes and to close the global overturning circulation. Previous studies have concentrated on the hypothesis that the primary mechanism for this mixing is breaking internal waves generated by tidal flows over rough topography. However, intriguing observations, particularly from the Brazil Basin Tracer Release Experiment, suggest that mixing in the flank canyons of the Mid-Atlantic Ridge generated when strong mean flows interact with the many sills and constrictions within the canyons may represent a dynamically important amount of abyssal mixing. The energy pathways and mechanisms of this mixing are much less clear than in the case of breaking internal waves. This study attempts to clarify this by suggesting an analogy with an idealized diffusive boundary layer over a sloping bottom. This boundary layer is characterized by up-slope flows powered by the buoyancy flux in the fluid far from the boundary. Here we explore the energy budget of the boundary layer, and find that the diffusive boundary layer provides flows that are generally consistent with those observed in submarine canyons. In addition, we derive the vertical velocity in the far-field fluid, analogous to an Ekman pumping velocity, that these boundary layers can induce when the bottom slope is not constant. Finally, we present both theoretical and numerical models of exchange flows between the bottom boundary and the far-field flow when the bottom slope is not constant. These exchange flows provide a mechanism by which boundary-driven mixing can affect the overall stratification and buoyancy fluxes of the basin interior.