Severe ozone depletion in the polar lower stratosphere has been linked to significant changes in tropospheric circulation patterns in the both hemispheres. Observed Southern Hemisphere circulation changes are easily reproduced in climate models and may be achieved by either increasing ozone depleting substances in a chemistry-climate model(CCM) or by imposing observed ozone losses as a zonally-symmetric perturbation in a prescribed-ozone global circulation model (GCM). In the Northern Hemisphere however, only the CCM method produces a circulation response in agreement with analysis of observations, while the GCM method is unable to produce any significant tropospheric circulation changes from imposing observed zonal-mean Arctic ozone losses. Confidence in a mechanistic link between Arctic stratospheric ozone change and changes in tropospheric circulation is greatly increased if the change can be reproduced using a GCM in addition to being reproducible in a CCM. This thesis demonstrates that by allowing ozone to vary along longitude, and by imposing ozone depletion during a realistic timeframe, the GCM method can produce circulation changes compatible with both the CCM method and observations. An equivalent-latitude coordinate allows the prescribed ozone field, and imposed ozone losses, to follow the polar vortex as it is systematically disturbed or displaced off the pole throughout the winter, producing a realistic circulation response in the troposphere in contrast to when ozone and its imposed losses are zonally-symmetric. Timing the imposed ozone depletion with the breakup of the polar vortex reveals that the appearance of the circulation response is very sensitive to the relative timing of these events and to the pre-existing dynamical state of the polar vortex. These results demonstrate that prescribing ozone as a zonally symmetric climatology within a GCM, as has been recent practice in the literature, is only representative of the Southern Hemisphere and is inappropriate for accurately representing processes within the Arctic stratosphere. Moreover this work demonstrates that these dynamically-evolving zonal asymmetries in ozone, which are not present in zonally-symmetric ozone schemes, play a crucial role in allowing perturbations in the Arctic stratosphere to influence the troposphere and surface conditions.

The classical problem of predicting the atmospheric circulation response to extratropical surface forcing is revisited in the context of the observed connection between autumn snow cover anomalies over Eurasia and the wintertime Northern Annular Mode (NAM). In general circulation model (GCM) simulations with prescribed autumn Siberian snow forcing, a vertically propagating Rossby wave train is generated, driving dynamical stratospheric warming and a negative NAM response that couples to the troposphere. It is shown that unexplained aspects of the evolution of this response can be clarified by examining the time evolution of the phasing, and hence the linear interference, between the wave response and the background climatological wave. When the wave response and background wave are in phase (out of phase), wave activity into the stratosphere is amplified (attenuated) and the zonal mean stratosphere-troposphere NAM response displays a negative (positive) tendency. This effect is probed further in a simplified GCM with imposed lower tropospheric cooling. As in the comprehensive GCM, linear interference strongly influences the NAM response. The transition from linear to nonlinear behaviour is shown to depend on forcing strength. Linear interference also plays a key role in the observed October Eurasian snow cover-NAM connection. It is shown that the time lag between October Eurasian snow anomalies and the peak wave activity flux arises because the Rossby wave train associated with the snow is out of phase with the climatological stationary wave from October to mid-November. Beginning in mid-November, the associated wave anomaly migrates into phase with the climatological wave, leading to constructive interference and anomalously positive upward wave activity fluxes. Current generation climate models do not capture this behaviour. Linear interference is not only associated with stratospheric warming due to Eurasian snow cover anomalies but is a general feature of both Northern and Southern Hemisphere stratosphere-troposphere interactions, and in particular dominated the negative NAM events of the fall-winter of 2009-2010. The interannual variability in upward wave activity flux during the season of strongest stratosphere-troposphere interactions is primarily determined by linear interference of quasi-stationary waves. The persistence of the linear interference component of this flux may help improve wintertime extratropical predictability.

The classical problem of predicting the atmospheric circulation response to extratropical surface forcing is revisited in the context of the observed connection between autumn snow cover anomalies over Eurasia and the wintertime Northern Annular Mode (NAM). In general circulation model (GCM) simulations with prescribed autumn Siberian snow forcing, a vertically propagating Rossby wave train is generated, driving dynamical stratospheric warming and a negative NAM response that couples to the troposphere. It is shown that unexplained aspects of the evolution of this response can be clarified by examining the time evolution of the phasing, and hence the linear interference, between the wave response and the background climatological wave. When the wave response and background wave are in phase (out of phase), wave activity into the stratosphere is amplified (attenuated) and the zonal mean stratosphere-troposphere NAM response displays a negative (positive) tendency. This effect is probed further in a simplified GCM with imposed lower tropospheric cooling. As in the comprehensive GCM, linear interference strongly influences the NAM response. The transition from linear to nonlinear behaviour is shown to depend on forcing strength. Linear interference also plays a key role in the observed October Eurasian snow cover-NAM connection. It is shown that the time lag between October Eurasian snow anomalies and the peak wave activity flux arises because the Rossby wave train associated with the snow is out of phase with the climatological stationary wave from October to mid-November. Beginning in mid-November, the associated wave anomaly migrates into phase with the climatological wave, leading to constructive interference and anomalously positive upward wave activity fluxes. Current generation climate models do not capture this behaviour. Linear interference is not only associated with stratospheric warming due to Eurasian snow cover anomalies but is a general feature of both Northern and Southern Hemisphere stratosphere-troposphere interactions, and in particular dominated the negative NAM events of the fall-winter of 2009-2010. The interannual variability in upward wave activity flux during the season of strongest stratosphere-troposphere interactions is primarily determined by linear interference of quasi-stationary waves. The persistence of the linear interference component of this flux may help improve wintertime extratropical predictability.

The upper troposphere-lower stratosphere (UTLS) is a transition region between the troposphere and the stratosphere. During the boreal summer, the UTLS is dominated by large-scale anticyclonic circulations over the Asian and North American monsoon regions, exhibiting complex dynamical and chemical characteristics. Re-cent studies have emphasized the important role of the summer monsoon systemin stratosphere-troposphere exchange of water vapor and chemical species, which strongly influences the atmospheric chemistry and climate system. The transport in the UTLS region occurs in both directions, stratosphere-troposphere transport (STT)and troposphere-stratosphere transport (TST). For example, observational studies indicate localized maxima of tropospheric pollutants and stratospheric water vapor(SWV) in the UTLS, which are controlled by deep convection and large-scale circulation. Meanwhile, stratospheric ozone (O3) can fold into tropospheric air and entrain into the planetary boundary layer (PBL) via deep STT, and thus affect air quality at the surface. In this thesis, we aim at improving the understanding of the transport processes in the UTLS that are linked to monsoon dynamics using observations and modelling tools.<div><br></div><div>First, we investigate the TST transport in association with the Asian summer monsoon. We examine the simulation of SWV in the Community Earth System Model, version 1 with the Whole Atmosphere Community Climate Model as its atmospheric component [CESM1(WACCM)]. CESM1(WACCM) generally tends to simulate a SWV maximum over the central Pacific Ocean instead of over the Asian continent as observed, but this bias is largely improved in the high vertical resolution version. The high vertical resolution model with increased vertical layers in the UTLS is found to have a less stratified UTLS over the central Pacific Ocean compared with the low vertical resolution model. It therefore simulates a steepened potential vorticity gradient over the central Pacific Ocean that better closes the upper-level anticyclone and confines the SWV within the enhanced transport barrier.<br></div><div><br></div><div>We further study the transport pathways connecting the Northern Hemisphere sur-face and the North American (NA) UTLS by diagnosing Boundary Impulse Response idealized tracers implemented at the Northern Hemisphere surface during summer. In ensemble average, air masses enter the NA UTLS region above Central America, and then slowly mix into the higher latitudes. However, fast transport pathways with modal age around two weeks are evident in some tracer ensembles. For these rapid transport pathways, the tracers first reach the UTLS region over the eastern Pacific and the Gulf of Mexico as a result of enhanced deep convection and vertical advection, followed by horizontal transport over the United States by a strengthened UTLS anticyclone circulation.<br></div><div><br></div><div>To the end, we evaluate the downward transport of stratospheric O3via STT using simulation from a state-of-the-art chemistry climate model implemented with an artificial stratospheric ozone tracer (O3S). We find that O3transported from the stratosphere makes a significant contribution to the surface O3variability where back-ground surface O3exceeds 95thpercentile, especially over the western U.S. Maximum covariance analysis is applied to O3anomalies paired with stratospheric O3traceranomalies to identify the stratospheric intrusion and the underlying dynamical mechanism. The first leading mode corresponds to deep stratospheric intrusions in the western and northern tier of the U.S., and intensified north easterlies in the mid-to-lower troposphere along the west coast, which also facilitate the transport to the eastern Pacific Ocean. The second leading mode corresponds to deep intrusions over the Intermountain Regions. Both modes are associated with eastward propagating baroclinic systems, which are amplified near the end of the North Pacific storm tracks, leading to strong descents over the western United States.<br></div>

The eastward winds that form each winter in the Arctic stratosphere are intermittently disrupted by planetary-scale waves propagating up from the surface in events known as stratospheric sudden warmings. It is shown here that following roughly half of these sudden warmings, the winds take as long as three months to recover, during which time the polar stratosphere evolves in a robust and predictable fashion. These extended recoveries, termed here Polar-night Jet Oscillation (PJO) events, are relevant to understanding the response of the extratropical troposphere to forcings such as solar variability and climate change. They also represent a possible source of improvement in our ability to predict weather regimes at seasonal timescales. Four projects are reported on here. In the first, the approximation of stratospheric radiative cooling by a linear relaxation is tested and found to hold well enough to diagnose effective damping rates. In the polar night, the rates found are weaker than those typically assumed by simplified modelling studies of the extratropical stratosphere and troposphere. In the second, PJO events are identified and characterized in observations, reanalyses, and a comprehensive chemistry-climate model. Their observed behaviour is reproduced well in the model. Their duration correlates with the depth in the stratosphere to which the disruption descends, and is associated with the strong suppression of further planetary wave propagation into the vortex. In the third, the response of the zonal mean winds and temperatures to the eddy-driven torques that occur during PJO events is studied. The collapse of planetary waves following the initial warming permits radiative processes to dominate. The weak radiative damping rates diagnosed in the first project are required to capture the redistribution of angular momentum responsible for the circulation anomalies. In the final project, these damping rates are imposed in a simplified model of the coupled stratosphere and troposphere. The weaker damping is found to change the warmings generated by the model to be more PJO-like in character. Planetary waves in this case collapse following the warmings, confirming the dual role of the suppression of wave driving and extended radiative timescales in determining the behaviour of PJO events.

The eastward winds that form each winter in the Arctic stratosphere are intermittently disrupted by planetary-scale waves propagating up from the surface in events known as stratospheric sudden warmings. It is shown here that following roughly half of these sudden warmings, the winds take as long as three months to recover, during which time the polar stratosphere evolves in a robust and predictable fashion. These extended recoveries, termed here Polar-night Jet Oscillation (PJO) events, are relevant to understanding the response of the extratropical troposphere to forcings such as solar variability and climate change. They also represent a possible source of improvement in our ability to predict weather regimes at seasonal timescales. Four projects are reported on here. In the first, the approximation of stratospheric radiative cooling by a linear relaxation is tested and found to hold well enough to diagnose effective damping rates. In the polar night, the rates found are weaker than those typically assumed by simplified modelling studies of the extratropical stratosphere and troposphere. In the second, PJO events are identified and characterized in observations, reanalyses, and a comprehensive chemistry-climate model. Their observed behaviour is reproduced well in the model. Their duration correlates with the depth in the stratosphere to which the disruption descends, and is associated with the strong suppression of further planetary wave propagation into the vortex. In the third, the response of the zonal mean winds and temperatures to the eddy-driven torques that occur during PJO events is studied. The collapse of planetary waves following the initial warming permits radiative processes to dominate. The weak radiative damping rates diagnosed in the first project are required to capture the redistribution of angular momentum responsible for the circulation anomalies. In the final project, these damping rates are imposed in a simplified model of the coupled stratosphere and troposphere. The weaker damping is found to change the warmings generated by the model to be more PJO-like in character. Planetary waves in this case collapse following the warmings, confirming the dual role of the suppression of wave driving and extended radiative timescales in determining the behaviour of PJO events.