Title

The Physics Of Double-Diffusive Convection In The Arctic Ocean

Date of Award

Fall 10-1-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Timmermans, Mary-Louise

Abstract

My dissertation investigates the physics of double-diffusive convection, a small-scale mixing process that causes vertical heat transport from Arctic Ocean waters toward the overlying sea ice cover. This diffusive-convective mechanism is evidenced by its signature staircase, comprised of adjacent pairs of thick mixed layers and thin interfaces in temperature and salinity. The necessary temperature and salinity conditions for double-diffusive convection are prevalent in the Arctic Ocean, where cool and fresh waters sit above the warm and salty Atlantic Water Layer, whose waters have originated from the Atlantic Ocean. At the top of this Atlantic Water Layer, a double-diffusive staircase is often found. In this dissertation, I investigate the physical mechanisms which govern the vertical heat fluxes associated with these Arctic staircases, staircase temporal and spatial evolution, and the fluid dynamical processes underlying the persistence and manifestation of Arctic staircase features. A combination of analytical theory, numerical simulations, and in-situ observations are invoked to both understand the physics governing staircase properties in the Arctic Ocean, as well as to investigate how double-diffusive structures and associated vertical heat transport may be modified in a changing Arctic Ocean. Specifically, I have conducted the first Arctic-wide analysis of double-diffusive staircase properties. I use data from Ice-Tethered Profilers to explore these properties and to quantify vertical double-diffusive heat fluxes, generally of O(0.1)~Wm$^{-2}$, arising from the Arctic's warm Atlantic Water Layer. Through characterization of the double-diffusive staircase across the entire Arctic, I find evidence that staircases are absent in boundary regions, raising the idea that energetic regions of the Arctic, possibly subject to shear-driven turbulence, may lead to the disruption of the double-diffusive staircase. In the next chapter, I describe the development of a mathematical model to explore the effects of shear-driven, intermittent turbulence on double-diffusive processes. This model builds on the existing work of Turner (1968) and Huppert and Linden (1979) to include the effect of salinity diffusivity. Using my model, I find that double-diffusive heat fluxes are reduced in the presence of intermittent turbulence. Moreover, the model shows that staircase formation is inhibited above a critical threshold of turbulence, possibly lending credence to the idea that intermittent turbulence is responsible for the lack of double-diffusive staircases in Arctic boundary regions. In the following chapter, I use acoustic measurements of the Arctic water column, with high spatial and temporal resolution, to explore the finestructure of Arctic staircases. I develop a methodology exploiting acoustic impedance differences, which are manifested by the sharp temperature jumps at staircase interfaces, to explore this finestructure. In particular, I track the propagation of double-diffusive interfaces in time and infer interface thicknesses and local interface stratifications. This work gives insight on how local mixing may affect double-diffusive staircase evolution as well as provides a new methodology for visualizing double-diffusive staircase properties in the Arctic Ocean. Finally, I present preliminary results from my ongoing research, which investigates the physical parameters responsible for double-diffusive layer thicknesses in the Arctic Ocean through an analysis of the Ice-Tethered Profiler dataset in the western Arctic. Here, I explore the roles of bulk-scale density ratios and large-scale oceanographic factors in influencing double-diffusive layer thicknesses. This work will elucidate the relationships between dynamic and thermodynamic processes governing the double-diffusive staircase structure in an oceanographic setting. The studies in this dissertation provide fundamental context and background for future study of double-diffusive convection in the Arctic Ocean, illustrate a novel method for inferring staircase properties, and provide mathematical intuition for highlighting and interpreting the changing staircase structure in a warming Arctic Ocean.

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