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Stratosphere-troposphere exchange processes are responsible for controlling the distribution of chemically and radiatively important trace gases in the upper troposphere and lower stratosphere. Extensive characterization of exchange processes is critical to the development of our understanding and prediction of the climate system. This study examines the occurrence and dynamical and chemical characteristics related to two primary stratosphere-troposphere exchange processes: Rossby wavebreaking and moist convection. Intrusions of air from the tropical upper troposphere into the extratropical stratosphere above the subtropical jet via Rossby wavebreaking potentially have a significant impact on the composition of the lowermost stratosphere (the stratospheric part of the "middleworld"). We first present an analysis of tropospheric intrusion events observed in aircraft observations using kinematic and chemical diagnostics. The transport processes operating during each event are discussed using high-resolution model analyses and backward trajectory calculations. In situ chemical observations of the tropospheric intrusions are used to estimate the mixing timescales of the observed intrusions through use of a simple box model and trace species with different photo-chemical lifetimes. We estimate that the timescale for an intrusion to mix with the background stratospheric air is 5 to 6 days. Detailed analysis of small-scale features with tropospheric characteristics observed in the stratosphere suggests frequent irreversible transport associated with tropospheric intrusions. We also present a 30-year climatology (1981-2010) of anticyclonically and cyclonically sheared Rossby wave-breaking events along the boundary of the tropics in the 350-500 K potential temperature range from ECMWF ERA-Interim reanalyses. Lagrangian transport analyses show poleward transport at altitudes below and above the 370-390 K layer. Poleward transport at lower levels is in disagreement with previous studies and is shown to be largely dependent on the choice of tropical boundary. In addition, transport analyses reveal three modes of transport for anticyclonic wavebreaking events near the tropical tropopause (380 K): poleward, equatorward, and bidirectional. These transport modes are associated with distinct characteristics in the geometry of the mean flow. Stratospheric intrusions (tropopause folds) are known to be major contributors to stratosphere-troposphere exchange. The specific mixing processes that lead to irreversible exchange between stratospheric intrusions and the surrounding troposphere, however, are not entirely understood. This study presents direct observations of moist convection penetrating into stratospheric intrusions. The characteristics of convective injection are shown by using in situ aircraft measurements, radar reflectivities, and model analyses. Convective injection is observed at altitudes up to 5 km above the bottom of a stratospheric intrusion. Aircraft measurements show that convective injection in stratospheric intrusions can be uniquely identified by coincident observations of water vapor greater than about 100 ppmv and ozone greater than about 125 ppbv. Trajectory analyses show that convective injection can impact transport in both directions: from troposphere to stratosphere and from stratosphere to troposphere. We present a conceptual model of the synoptic meteorological conditions conducive to convective injection in stratospheric intrusions. In particular, convective injection is found to be associated with a "split front" where the upper-level frontal boundary outruns the surface cold front.

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (p. 79-98).

Thesis (Ph. D.)--University of Washington, 1999. / Vita. Includes bibliographical references (p. 205-225).

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.

Atmospheric mass is transported in and out of the stratospheric polar cap region by a wave-driven meridional circulation. Using composites of polar cap pressure anomalies, defined as deviations from the average annual cycle, it is shown that this stratospheric mass flux is accompanied by a similar mass flux near the surface. This 'tropospheric amplification' of the stratospheric signal is introduced as a new way to quantify stratosphere-troposphere coupling. Regression analysis is used to create a vertical profile of atmospheric pressure during a tropospheric amplification event, and the regression slope profile is used as a tool to quantify the amplification. Using data from 5 reanalysis datasets and 11 climate models, it is shown that high-top models, with a model lid of above 1 hPa, are significantly better at reproducing tropospheric amplification than low-top models, due to having more detailed parameterisations of stratospheric processes. However, the regression slope profiles of all models, bar one, are significantly different to the profile of reanalysis data at a 95% confidence level. Tropospheric amplification is also investigated in historical and future simulations from these models, and it is concluded that there is not expected to be a large change in the phenomenon over the next 100 years. The processes needed to reproduce tropospheric amplification can be identified by comparing idealised models of different complexity. A simple dry-core model is not able to reproduce tropospheric amplification, while a model with a comprehensive radiation scheme does produce the basic regression slope profile under certain configurations. The associations between pressure change and mass flux are further investigated using primitive equations. It is found that vertical and horizontal contributions to mass flux act to mostly cancel each other out, leaving a poorly-conditioned residual, and that the horizontal mass flux across the polar cap boundary has both geostrophic and ageostrophic components.

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>

Stratosphere Troposphere exchange (STE) is important to study as it controls the chemical composition of the upper troposphere/lower stratosphere (UTLS) and thus the radiative balance of this region. STE also controls the transport of chemicals into the stratosphere which are vital to ozone depletion. The troposphere and the stratosphere have specific chemical characteristics and the transition region between these regions displays characteristics of both. Ozone and water vapour concentrations can be used as tracers for the characteristics of the troposphere and stratosphere. This thesis develops measures in tracer space, which allow the determination of the strength and depth of atmospheric mixing between the troposphere and the stratosphere in extratropical regions. The application of entropy as a measure of atmospheric mixing as introduced by Patmore and Toumi [2006], is improved in this study. This is a measure of how the ozone and water vapour mixing ratios vary as a result of mixing. An additional metric to give further information on the form of the mixing line in tracer space is also developed. This measure uses the ozone and water vapour mixing ratios at the boundaries of the transition region (BO3 and BH2O). This study uses data from ozonesondes and hygrometers, along with satellite data from the Atmospheric Infrared Sounder (AIRS). The ozone product from AIRS is also validated as part of this study. The entropy, BO3 and BH2O measures from this study, are successfully shown to detect regions of enhanced mixing in comparison studies. A key comparison shows that the measures developed in this study are able to produce comparable conclusions to higher resolution aircraft data, with regards to mixing. The separation of entropy, BO3 and BH2O, into different categories allows mixing processes to be assigned to some of the categories. Mixing is shown to have geographic preference, with some regions having significantly more mixing. Some categories have preference with regards to their location either poleward or equatorward of the jet stream. In addition, some information as to the direction of the vertical transport, whether stratosphere to troposphere or vice versa, is obtained.