The Dynamic Interplay between Submesoscales and Boundary Layer Turbulence

Bodner, Abigail S.

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Year:: 2021
Contributor:: Bodner, Abigail S (creator); Fox-Kemper, Baylor (Advisor); Huber, Christian (Reader); Herbert, Timothy (Reader); Marston, John Bradley (Reader); Maxey, Martin (Reader); Griffies, Stephen (Reader); Brown University. Department of Earth, Environmental, and Planetary Sciences (sponsor)
Genre:: theses
Subject:: Oceanography; Climate modeling; Turbulence
Extent:: , None p.

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Abstract:: The ocean mixed layer plays a key role in the climate system by transferring momentum and tracers, such as heat and carbon, from the atmosphere to the ocean interior. Variations in the mixed layer depth help determine the effectiveness of atmosphere-ocean interactions, and can be attributed to surface forcing, as well as dynamical processes such as turbulent mixing, submesoscale frontal instabilities and mixed layer eddies. Theoretical work and modeling of fronts have been useful in understanding why there are so many submesoscale fronts and filaments in the ocean, but it has been less successful in predicting the scale at which these are found in observations. Current submesoscale parameterizations, which help set mixed layer depth in global climate models, depend on a simplistic scaling of frontal width that is demonstrably wrong in several circumstances. The presence of turbulence and instabilities are likely responsible for keeping fronts at the scale observed, yet a complete understanding of how and why this happens has been a long-standing problem. Building toward a more complete understanding of the processes that set this scale, the interaction between submesoscale fronts and turbulent mixing are investigated in this thesis using several platforms. First, a theoretical approach of perturbation analysis is used to include the effects of parameterized turbulence as a first order correction to classic frontogenesis (frontal sharpening) theory. A modified solution is obtained by using potential vorticity (PV) and surface conditions, which exhibit the complex nonlinear behavior of frontal dynamics. The solution reveals that vertical processes merely delay frontal sharpening, whereas horizontal processes may completely oppose frontogenesis. This qualitative approach is next extended into a more realistic environment, by diagnosing a suite of Large Eddy Simulations (LES) spanning the submesoscale and into the boundary layer (3D) turbulence scale. A surprising result emerges, revealing the limitations of PV below the submesoscale. In models where 3D turbulence is not fully resolved, PV is strongly influenced by grid-scale processes, and becomes contaminated by the least reliable scales. Pre-filtering the velocity and buoyancy fields is found to be essential in linking larger scale PV dynamics to small scale turbulent fluxes. Furthermore, in these simulations a variety of processes--winds, waves, convection, and mixed layer instabilities—are found to compete with frontogenesis. New scaling laws are developed, under different forcing parameter ranges, by relating turbulent fluxes to frontal width by making use of the turbulent thermal wind balance. The final aspect of this thesis discusses implementing the modified frontal width scaling in submesoscale parameterizations in coarse resolution climate models for which sensitivity and changes in model bias are documented.
Notes:: Thesis (Ph. D.)--Brown University, 2021

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Citation

Bodner, Abigail S., "The Dynamic Interplay between Submesoscales and Boundary Layer Turbulence" (2021). Earth, Environmental and Planetary Sciences Theses and Dissertations. Brown Digital Repository. Brown University Library. https://repository.library.brown.edu/studio/item/bdr:emhz8tra/

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Earth, Environmental and Planetary Sciences Theses and Dissertations

Theses and Dissertations for the Earth, Environmental and Planetary Sciences department.
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