Processes Controlling Stratospheric Dynamic Variability, the Implications for Ozone Levels, and the Coupling to the Troposphere and Mesosphere

Stratospheric variability plays an important role in driving the weather and climate of the Earth system. The extent to which various forcing factors explain this variability and the involved mechanisms are not fully understood. This thesis investigates processes controlling the variability of the stratosphere and the implication of this variability on ozone and on circulations in the troposphere and mesosphere. A series of sensitivity simulations with NCAR’s CESM1(WACCM) model was performed to understand how these coupling processes are influenced by different natural and anthropogenic factors. The focus of this thesis is mainly on new aspects of the stratosphere- troposphere coupling mechanism via downward wave coupling (DWC), which is the most direct way by which the stratospheric background wind can affect tropospheric circulation. Based on a series of sensitivity simulations, it is shown that although DWC is suppressed in the absence of the Quasi-Biennial Oscillation (QBO) variability, the tropospheric signal to DWC is enhanced, and vice versa when the sea surface temperature (SST) variability is excluded. This apparent mismatch is explained by the differences in the strength of the synoptic-scale eddy-mean flow feedback and the possible contribution of SST anomalies during DWC events. In particular, a weaker eddy-mean flow feedback in the absence of SST variability is consistent with modest Eady growth rate and synoptic wave source anomalies, which results in decreased synoptic-scale wave divergence. For the first time, the downward influence of DWC on the surface weather is suggested to be related to enhanced baroclinic instability in the troposphere. This thesis also provides the first evidence for an effect of DWC on Arctic stratospheric ozone. A statistically significant decrease in Arctic column ozone is observed towards late winter during years with enhanced DWC. This is attributed to an increased net amount of wave reflection that leads to a cold polar vortex and less ozone transport to the pole. The results establish a new perspective on dynamical processes controlling Arctic ozone variability. Under extreme climate change conditions, a significant reduction in DWC events is detected in the future, with a shift of their timing from early to midwinter. This variation is related to changes of the vertical reflecting surfaces and an increased wave absorption in early winter. The result also indicates that future changes in midwinter surface weather during DWC event are related to changes in baroclinic eddy feedback in the troposphere. In the last part of this thesis, the impact of the Antarctic ozone hole on the vertical coupling of the stratosphere and mesosphere-lower thermosphere (MLT) system is investigated in detail. The results highlight that a proper accounting of both, dynamical and radiative effects, is required in order to correctly attribute the causes of the polar MLT response to the Antarctic ozone hole. This thesis provides an advanced understanding of the mechanisms responsible for the coupling between the troposphere, stratosphere, and beyond in both the upward and downward directions. This knowledge has the potential to improve the representation of middle atmosphere circulation in climate models, and thus to improve predictions of ozone, climate, and even tropospheric weather