The Role of Standing and Travelling Waves in Stratosphere-troposphere Coupling

Upward wave activity flux in the mid-latitudes is a strong control on the strength and temperature of the stratospheric polar vortex in the Northern Hemisphere winter. In this thesis, the variability of upward wave activity flux is studied using the ideas of linear interference. This framework separates meteorological fields into a zonally asymmetric climatology-â the stationary waveâ and a wave anomaly. Fluxes are then decomposed into a linear term that measures the spatial coherence of the anomaly and the climatology and an additional nonlinear term. In order to determine the structure of the wave anomalies that interfere with the climatology, a novel spectral decomposition of wave disturbances into standing and travelling components is developed. Unlike previous methods, it explicitly accounts for the covariance between the two components. The decomposition is applied to planetary waves in the Northern Hemisphere winter. It is shown that standing waves explain the majority of the variance of the linear part of the upward wave activity flux. Furthermore, the connection between upward wave activity flux in the troposphere and the strength of the stratospheric polar vortex is shown to be primarily driven by standing waves amplifying and attenuating the climatological wave. The relationship between the linear and nonlinear parts of the upward wave activity flux is investigated. It is shown that there is a roughly quadratic relationship between the two when considering individual wavenumbers, and that this can act as an explanation for the positive skewness of the wavenumber-1 upward wave activity flux distribution. A statistical model is constructed which demonstrates that a westward tilted climatological wave is the key ingredient in having a positively skewed wave activity flux distribution. Last, the analysis is extended to consider the dynamics of tropospheric variability and weather extremes. The standing-travelling wave decomposition is applied to tropospheric weather variability in order to separate the slow and fast drivers of cold winter weather over eastern North America. It is found that standing waves driven from the tropics largely control the large-scale ridge-trough structure over North America, but that eastward travelling synoptic waves can strongly modify it on shorter timescales.Ph.D