Explaining timescales associated with jet stream variability

Extreme weather events are often the result of slow-moving, large-scale wave patterns. Greater understanding of these large-scale modes of variability would allow us to anticipate how quasi-stationary modes might depend on changes to the background state in future climate. In this thesis the Empirical Normal Mode (ENM) technique, a technique for extracting dynamical modes of variability from atmospheric timeseries data, is utilised to examine the dependence of mode structure and frequency on jet latitude. The ENM methodology is extended to include the lower troposphere spanned by isentropic surfaces that can intersect the ground. This involves careful accounting of the terms in large amplitude pseudomomentum and pseudoenergy associated with this region of the atmosphere - terms that contribute to the "boundary wave activity" in the limit of small amplitude. In the third chapter, the implementation of the technique itself is validated by testing the characteristic 'intrinsic' phase speed of the ENMs against an empirical phase speed derived from the modes' principal component timeseries, using a set of idealised model experiments simulated using the Reading IGCM2.2. It is found that the phase speed matching conditions are met for the dominant freely propagating baroclinic modes, validating the approach to the calculation of wave activity and some approximations used in deriving relevant wave activity norms. In the fourth chapter, a new series of idealised experiments are devised that possess a jetstream with controllable latitude such that the change in behaviour of the modes of variability with a shift in jet latitude may be examined. The initial and relaxation temperature field in thermal wind balance with a prescribed zonal wind field with jet latitudes ranging from ∼ 40° to ∼ 65° is found, and a sloping tropopause is added in order to maintain baroclinicity. Subsequently, in the fifth chapter, the ENM structures of these experiments are found, and the change in the phase speed of the modes as the jet latitude changes is explored. A quasi-stationary branch of modes is identified which is associated with the most perturbation energy (for each zonal wavenumber) and therefore can propagate most strongly westwards against the background state westerly flow. As the jet is shifted polewards, the wavelength of the most energetic modes remains approximately the same, but they shift to lower zonal wavenumbers due to reduction in the latitude circle circumference.