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Title: Interannual variability in stratospheric dynamics : interaction between the QBO and the extratropics
Author: Hampson, J.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2000
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This thesis is concerned with interannual variability in the stratosphere, in particular the tropical oscillation called the Quasi-Biennial Oscillation (QBO) and its interaction with the extratropical circulation. A three dimensional mechanistic model is used to investigate interannual variability, with a tropical QBO being forced in the model by either a relaxation scheme or a simple gravity wave parameterisation scheme. The tropospheric forcing of large-scale planetary waves is represented by imposing a geopotential perturbation at the artificial lower boundary of the model. The influence of the QBO on the extratropics is examined to see if there is a preferred mode of variability in the extratropics, and how the extratropical response to the QBO is affected by the strength of extratropical wave forcing in the model. The influence of other details of the QBO, specifically its phase relationships with the annual cycle, its height structure and its time structure are investigated. The correlation between the QBO and the extratropical circulation is seen to be consistent with the Holton-Tan mechanism for the extratropical QBO; this correlation is modulated by the amplitude of extratropical geopotential wave forcing. The extratropical dynamics are seen to affect phase preference relative to the annual cycle, irregularity in period and asymmetry between different phases of the QBO simulated in the three dimensional model. It is hypothesised that this effect comes about through the extratropical wave driving of a seasonally varying equatorial upwelling. This is investigated further in a one dimensional (equatorial) model and a simple zero dimensional model, by imposing the equatorial upwelling as a function of time. The one dimensional model exhibits phase preference, irregularity and asymmetry, consistent with the hypothesis, though cannot explain all equatorial features of the three dimensional model model. The occurrence of "stalled" descents in the parameterised QBO in the three dimensional model is noted and examined.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available