| Summary: | The exchange of heat, momentum, and chemical species between the stratosphere and troposphere affects weather and climate. Stratosphere-troposphere coupling often refers to weather events. As a result, there are many studies focusing on dynamical coupling between the two layers at daily to seasonal timescales. Less is known about how low frequency climate variability in the Earth system affects stratosphere-troposphere coupling.The purpose of this dissertation is to assess how three forms of low frequency climate variability, the Quasi Biennial Oscillation (QBO), Interdecadal Pacific Variability (IPV), and Atlantic Multidecadal Variability (AMV), influence stratosphere-troposphere coupling. The main tools used are multi-century, high-top, atmospheric global climate model perturbation experiments and reanalysis. Two specific goals are to (1) assess what teleconnections are elicited by each mode of variability and (2) identify how these teleconnections enhance or suppress stratosphere-troposphere coupling. The stratospheric circulation is often associated with wintertime extreme weather events. Benchmarking how these modes of variability affect stratosphere-troposphere coupling may enhance predictability of these events.Studies have assessed the atmospheric response to IPV and AMV individually, but not together. Here, all combinations of IPV and AMV are imposed in a set of perturbation experiments to assess their combined effects on the boreal winter atmosphere. The atmospheric response to IPV dominates the response forced by AMV. In nature, the AMV is currently in its positive phase and the IPV is thought to be transitioning to its positive phase. Therefore, more targeted study is done for the positive AMV experiment, positive IPV experiment, and their combination. Positive IPV promotes a deeper Aleutian Low, which is partially cancelled by the atmospheric response to positive AMV, which promotes ridging over the North Pacific. While the polar stratospheric response to positive IPV is broadly similar with or without positive AMV, the downward propagation of the polar stratospheric warming during positive IPV is significantly reduced when the positive AMV is included.Since the QBO has a known teleconnection with the polar stratosphere, the Holton-Tan effect, a brief attempt is made to subsample the IPV and AMV results by QBO phase. It becomes unclear how the QBO is suppressing or enhancing the IPV and AMV teleconnections. As a result, I pivot towards using a hierarchy of model simulations to isolate the impact of the QBO on planetary waves and the tropospheric and stratospheric circulations. The QBO is found to promote amplification of planetary waves at the tropopause, contrasting with the amplification of planetary waves in the troposphere promoted by IPV and AMV. The QBO forces regional teleconnections. There is consistent evidence that the North Pacific atmosphere is more tightly coupled with the QBO compared to other longitudes. The QBO promotes latitudinal shifting of the tropospheric jet stream, especially in the North Pacific. However, the polar stratospheric response to the QBO moderates this effect. While the QBO promotes an interesting set of teleconnections in the lower stratosphere, the dominant teleconnection between the QBO and polar vortex occurs in the middle stratosphere (30 kilometers) particularly over mid-latitude Asia (60°E-120°E).
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