Dissertations / Theses on the topic 'Sudden warming'

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2012.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (p. 185-192).
The stratosphere and the troposphere exhibit a strong coupling during the winter months. However, the coupling mechanisms between the respective vertical layers are not fully understood. An idealized spectral core dynamical model is utilized in the present study in order to clarify the coupling timing, location and mechanisms. Since the coupling between the winter stratosphere and troposphere is strongly intensified during times of strong stratospheric variability such as stratospheric warmings, these events are simulated in the described model for the study of stratosphere - troposphere coupling, while for comparison the coupling is also assessed for weaker stratospheric variability. While the upward coupling by planetary-scale Rossby waves in the Northern Hemisphere is well understood, the Southern Hemisphere exhibits traveling wave patterns with a weaker impact on the stratospheric ow. However the tropospheric generation mechanism of these waves is not well understood and is investigated in this study. It is found that in the model atmosphere without a zonally asymmetric wave forcing, traveling waves are unable to induce a significant wave ux into the stratosphere. In the absence of synoptic eddy activity, however, the tropospheric ow is baroclinically unstable to planetary-scale waves, and the generated planetary waves are able to propagate into the stratosphere and induce sudden warmings comparable in frequency and strength to the Northern Hemisphere. While baroclinic instability of long waves may be further strengthened by the addition of moisture, the real atmosphere also exhibits strong synoptic eddy activity, and it will have to be further explored if the atmosphere exhibits periods where synoptic eddies are weak enough to allow for baroclinic instability of long waves. In order to further investigate the coupling between the stratosphere and the troposphere, cases of strong coupling are investigated in the analysis of a Northern Hemisphere - like winter atmosphere. A realistic frequency and strength of sudden warmings is obtained using a zonal wave-2 topographic forcing. An angular momentum budget analysis yields that the Eliassen-Palm (EP) flux is closely balanced by the residual circulation dominated by the Coriolis term on a daily basis, while the change in zonal wind is a small residual between these dominant terms. In the stratosphere, the EP flux term and the Coriolis term balance well in time but not exactly in magnitude, yielding a polar stratospheric weakening of the zonal mean wind as observed during stratospheric warmings. In the troposphere, the loss of angular momentum before a sudden warming induces a weak negative annular mode response, which is amplified by the downward propagating signal about three weeks after the sudden warming. The angular momentum budget does not reveal the mechanism of downward influence, but it nevertheless clarifies the momentum balance of the stratosphere - troposphere system, indicating that the effects of the waves and the residual circulation have to be considered at the same time. Since the annular mode response cannot be directly investigated using the angular momentum budget, the annular mode coupling between the stratosphere and the troposphere is further investigated using a statistical approach. The annular mode response is often framed in terms of Empirical Orthogonal Functions (EOFs), but it is here found that for the stratosphere - troposphere system with its strong vertical pressure gradient, EOFs are strongly dependent on the weighting of the data, while Principal Oscillation Patterns (POPs) are considerably less sensitive to an applied weighting while returning the dominant structures of variability. This encourages further research and application of POP modes for the use of stratosphere - troposphere coupling. These findings represent an improvement of the understanding of stratosphere - troposphere coupling and the results are another step in the direction of finding the mechanism of stratosphere - troposphere coupling and the downward influence after the occurrence of a stratospheric sudden warming, which may influence long-term weather prediction in the troposphere.
by Daniela I. V. Domeisen.
Ph.D.

The polar vortex is a large scale cyclone located in the middle atmosphere near to the planet’s geographic poles. These vortices form during the hemispheric winter and break down in the spring of the following year. They may also break down in mid winter, causing a sudden stratospheric warming event (SSW). The vortex is thought to be preconditioned leading up to these warming events, resulting in the breakdown of the vortex. Integral diagnostics are used to investigate the stripping of air from the vortex as part of this preconditioning. Contour diagnostics of mass and circulation are calculated using ERA-40 reanalysis data for the stratosphere. The edge of the vortex is easily identifiable in these diagnostics as a high gradient of Ertel’s potential vorticity (PV), and the warming events are also clearly visible. From these the amount of air removed from the vortex is determined from the balance equation of the mass integral. These terms show that there are significant amounts of air removed from the vortex, with several stripping events identifiable in them through the winter, especially in those during which a major sudden warming event occurred. These stripping events can be seen in corresponding PV maps, where tongues of PV can be seen to be stripped from the vortex and mixed into the surrounding surf zone of turbulent air. From the integral diagnostics a Lagrangian measure of the meridional circulation in the stratosphere is also calculated. In the final part of the thesis a shallow water model is used to investigate a quantitative link between forcing and the amount of stripping of the vortex. It is found that when the forcing is large enough there is significant stripping of mass from the vortex. This does not lead to SSWs in all cases, and the total amount of stripping is not found to be proportional to the maximum amplitude of the forcing.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2018.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 139-143).
This thesis investigates the effects of eddy-eddy interactions (EEI) and tropical wind variability on sudden stratospheric warming (SSW) formation in an idealized atmospheric GCM. Chapter 2 introduces a method to produce split and displacement SSWs in comparable amounts using either wavenumber 1 or 2 tropospheric heating perturbations. The results are compared to those obtained with wavenumber 2 topographic forcing. It is shown that the fraction of SSWs forced by anomalously strong tropospheric wave flux in the model is similar to that of SSWs in the observed atmosphere, but that the fractions for splits and displacements are different. Furthermore, a large fraction of SSWs occur without significant anomalous tropospheric wave flux, indicating that stratospheric transmission of climatological tropospheric wave flux plays an important role in SSW formation. Chapter 3 investigates the effects of EEI on SSW formation in the model by reproducing the model runs from Chapter 2 with EEI turned off in parts of the atmosphere. It is found that SSW frequencies can be strongly dependent on EEI throughout the atmosphere, but that EEI are required locally for splits and displacements to occur. Significant changes in SSW frequencies are obtained by turning off EEI locally, without changing the lower stratospheric wave forcing. Chapter 3 shows that while SSW formation can be considered a wave-mean flow interaction to first order, higher order processes are required to accurately reproduce both SSW frequencies and dynamics. The wavenumber 2 heating run used in Chapters 2 and 3 produce spontaneous tropical wind oscillations in the stratosphere. Chapter 4 identifies the source of these oscillations, and investigates the effects of the oscillations on the stratospheric polar vortex. Model runs with suppressed tropical wind variability are compared to the control run of Chapter 2. A slight increase in SSW frequency can be found in the model runs with suppressed tropical variability. It is found that upper stratospheric equatorial wind anomalies are strongly correlated with polar vortex strength, and hypothesized that westerly equatorial wind anomalies in the upper stratosphere can reinforce the conditions that lead to an anomalously strong polar vortex. A mechanism explaining this influence is presented.
by Erik Anders Lindgren.
Ph. D.

A vortex dynamics approach is used to study the underlying mechanisms leading to polar vortex breakdown during stratospheric sudden warmings (SSWs). Observational data are used in chapter 2 to construct climatologies of the Arctic polar vortex structure during vortex-splitting and vortex-displacement SSWs occurring between 1958 and 2002. During vortex-splitting SSWs, polar vortex breakdown is shown to be typically independent of height (barotropic), whereas breakdown during vortex-displacement SSWs is shown to be strongly height dependent (baroclinic). In the remainder of the thesis (chapters 3-7), a hierarchy of models approach is used to investigate a possible resonant excitation mechanism which is responsible for the vortex breakdown seen in our observational study. A single layer topographically forced vortex model is shown to exhibit vortex-splitting behaviour similar to that observed during SSWs. Two analytical reductions, the first a fully nonlinear analytical model of an elliptical vortex in strain and rotation velocity fields, the second a weakly nonlinear asymptotic theory applied to a topographically forced vortex, show that vortex-splitting in the model occurs due to a self-tuning resonance of the vortex with the underlying topography. Resonant excitation of an idealized polar vortex by topographic forcing is then investigated in a three-dimensional quasi-geostrophic model, with emphasis on the vertical structure of the vortex during breakdown. It is shown that vortex breakdown similar to that observed during displacement SSWs occurs due to a linear resonance of a baroclinic mode of the vortex, whereas breakdown similar to that observed during splitting SSWs occurs due to a resonance of the barotropic mode. The role of self-tuning in these resonant behaviours is then discussed in relation to the analytic reductions of the single layer model.

The stratospheric winter polar vortex consists of strong westerly winds; this winter flow regime can undergo a complete breakdown during sudden stratospheric warming events. In the Northern Hemisphere these events are often accompanied by the descent of anomalous flow regimes which can result in extreme surface weather. The focus of this thesis is to assess sudden stratospheric warmings and their place in the coupled climate system. Portions of this work are dedicated to algorithm development with the aim of quickly and accurately isolating and subcategorising events. A method is successfully created that is computationally cheap, easy to implement, based on dynamically relevant criteria and has error rates clearly outlined. Impacts on the surface and ocean are assessed with focus on specific subclasses of sudden stratospheric warmings. It is found that there is, on average, stronger surface and oceanic impacts following events that split the polar vortex. The ocean system is impacted via modifications to the implied Ekman heat transport and the net atmosphere-surface heat flux. Furthermore, there is a relationship between the initial location of the disturbed polar vortex and the strength of anomalous flow regime at the surface. Analysis is conducted predominantly using general circulation model output, with direct comparison between an atmosphere-only model and a coupled atmosphere-ocean model. For the coupled model there is a reduction in the number of simulated sudden stratospheric warmings, a result of altered atmospheric wave dynamics. This is partially attributed to a cold bias over the equatorial Pacific. The frequency of sudden stratospheric warmings is found to be insensitive to North Atlantic sea surface temperature anomalies.

Abstract This thesis studies the long-term relation between the solar wind driven energetic particle forcing into the atmosphere and the tropospheric circulation in the Northern Hemisphere winter. The work covers the period of more than one hundred years since the turn of the 20th century to present. The thesis makes a statistical analysis of satellite measurements of precipitating energetic electrons, sunspot number data and geomagnetic activity, and compares them with temperature and pressure measurements made at the Earth's surface. Recent results, both observational and from chemistry climate models, have indicated significant effects in the Earth's middle atmosphere due to the energetic electrons precipitating from the magnetosphere. These effects include the formation of reactive hydrogen and nitrogen oxides in the high latitude mesosphere and the depletion of ozone caused by them. Ozone is a radiatively active and important gas, which affects the thermal structure and dynamics of the middle atmosphere. Accordingly, the depletion of ozone can intensify the large scale stratospheric circulation pattern called the polar vortex. Winter weather conditions on the surface have been shown to be dependent on the polar vortex strength. This thesis shows that there is a significant relation between the average fluxes of medium energy (ten to hundred keVs) precipitating electrons and surface temperatures in parts of the Northern Hemisphere in winter time. Temperatures are positively correlated with electron fluxes in North Eurasia and negatively correlated in Greenland during the period 1980-2010 which is covered by direct satellite observations of precipitating particles. This difference is especially notable when major sudden stratospheric warmings and the quasi-biennial oscillation (QBO), which both are known to affect the polar vortex strength, are taken into account. When extended to the late 19th century, the analysis shows that a similar temperature pattern is predominated during the declining phase of the sunspot cycle. The high speed solar wind streams and energetic particle precipitation typically maximize also at the declining phase of the solar cycle. This specific temperature pattern is related to the variability of the northern annular mode (NAM), which is the most significant circulation pattern in the Northern Hemisphere winter. Before the space era, geomagnetic activity measured by ground observations can be used as a proxy for energetic particle precipitation. Earlier studies have found a significant positive correlation between geomagnetic activity and NAM since the 1960s. We find that, when the QBO measured at 30 hPa height is in the easterly phase, a positive correlation is extended to the beginning of 1900s. We also show that high geomagnetic activity causes a stronger effect in the Northern Hemisphere winter than high sunspot activity, especially in the Atlantic and Eurasia. A comprehensive knowledge of the Earth's climate system and all its drivers is crucial for the future projection of climate. Solar variability effects have been estimated to produce only a small factor to the global climate change. However, there is increasing evidence, including the results presented in this thesis, that the different forms of solar variability can have a substantial effect to regional and seasonal climate variability. With this new evidence, the solar wind related particle effects in the atmosphere are now gaining increasing attention. These effects will soon be included in the next coupled model inter comparison project (CMIP6) as an additional solar related climate effect. This emphasizes the relevance of this thesis.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2017.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 129-136).
In this thesis, we investigate the effects of planetary waves one and two on the polar stratosphere during boreal winter. We use MERRA reanalysis data and the FMS shallow-water model to compare and contrast their propagation into the stratosphere, their interactions within the stratosphere, and their effects on the polar vortex. The results have implications for the predictability of sudden stratospheric warmings (SSWs), theories on the developments of vortex splits and the role of zonal winds in the tropics. In Chapter 2, we use correlations and regressions to demonstrate that the tropopause affects wavenumber one amplitudes more than wavenumber two. Thus, the statistical predictability of SSWs, based on synoptic events in the mid-troposphere (e.g. blockings), is limited. Composites of extreme heat fluxes reveal that they are likely caused by linear interference of the climatology and anomalies. The phases of anomalous planetary waves align with the climatology only during the largest heat fluxes. In Chapter 3, the effect of wave-wave interactions within the stratosphere is quantified by analyzing eddy energy budgets. The energy transfer from wavenumber one toward wavenumber two plays a key role in the vortex split in January 2013 and several other SSWs. This mechanism might explain the growth of wavenumber two in the stratosphere in nonresonant conditions. However, wave-wave interactions are small in averages over all splits since 1979 suggesting that different processes can lead to vortex splits and that the common SSW definitions do not capture the timing of planetary wave growth. In Chapter 4, we employ a shallow-water model to isolate the effects of wave one and two on the polar vortex over a large range of forcing amplitudes and vortex strengths. We are able to simulate SSW splits, which are unequivocally caused by wave-wave interactions. Furthermore, the initial response of the polar vortex depends strongly on the wavenumber of the forcing.
by Andreas Wolfgang Miller.
Ph. D.

The Earth’s middle atmosphere is comprised of the stratosphere, mesosphere and thermosphere, from approximately 10 to 110 km, or approximately 6 to 68 miles. An understanding of the dynamics and climatological conditions in this region is of vital importance to the aerospace industry and military, which both launch aircraft and spacecraft into this region, as well as researchers who study climate change and the interactions between the atmosphere and the Earth, oceans, and space. Measurements of atmospheric properties (density, temperature, and pressure) in this region are relatively difficult to gather as the middle atmosphere’s altitudes are both too high for weather balloons to reach and too low for satellite. That is why most instruments that acquire data from the middle atmosphere are of the remote sensing variety. Rayleigh-scatter lidar (light detection and ranging) is a remote sensing technique that is particularly effective at acquiring long-term measurements of the middle atmosphere. This work focuses on the design and implementation, over one annual cycle, of a unique Rayleigh lidar, which pushes the upper altitude boundary that is typical of such systems. In addition, a study of the connection between Sudden Stratospheric Warmings and the midlatitude mesosphere using a long-term Rayleigh lidar dataset is presented.

碩士
國立中央大學
太空科學研究所
100
Previous studies show the temperature in the stratosphere dramatically increases by 10 degrees Celsius, before eventually recovering to original values in the winter polar region, which is referred to as a stratospheric sudden warming (SSW). In this research, FORMOSAT-3/COSMIC temperature data is used to observe longitude and time variation, showing a clear SPW 1 structure during 2007-2011. The amplitude of SPW1 is large in the winter polar region and is consistent with the warming day, indicating that SPW1 is the cause of stratospheric sudden warmings. The electron density data shows that the coupling mechanism and effects of SSWs are different for each year. Nonlinear interaction between stationary planetary wave 1 (SPW1)and semi-diurnal westward 2 (SW2) generates semi-diurnal westward 1 (SW1), which propagates upward to ionosphere and affected the density variation in 2009 and 2010. This mechanism does not appear to occur in other years.

In this thesis, the dynamics and coupling in the middle atmosphere over the Southern Hemisphere are investigated using SuperDARN high frequency (HF) radar wind data, satellites, light detection and ranging (LIDAR), the South African National Antarctic Expedition (SANAE) imaging riometer and models. In particular, the study focuses on the unprecedented 2002 major stratospheric warming and its role in coupling the middle atmosphere. The dynamics of the middle atmosphere is investigated in terms of mean wind, temperature, gravity waves and planetary wave activity. Studying the middle atmospheric thermal structure over Southern Africa is an important activity to improve the understanding of atmospheric dynamics of this region. Observation of a middle atmosphere thermal structure over Durban (29.9 S, 31.0 E, South Africa) using LIDAR data collected from April 1999 to July 2004 (277 nights), including closest overpasses of the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) and Halogen Occultation Experiments (HALOE) satellites, and the COSPAR International Reference Atmosphere (CIRA-86) are presented in this thesis. The observations from the LIDAR instrument, satellites and CIRA-86 exhibit the presence of annual oscillation in the stratosphere, whereas in the mesosphere the semi-annual oscillation seems to dominate the annual oscillation at some levels. The stratopause is observed in the height range of 40-55 km for all the instruments, with the stratopause temperatures being 260- 270 K for the LIDAR, 250-260 K for the SABER, and 250-270 K for the HALOE. Data from the LIDAR, satellites and CIRA-86 model indicate almost the same thermal structure of the middle atmosphere over Durban. This indicates a good agreement between LIDAR, satellites and the CIRA-86 model. Mean wind and planetary waves are investigated on a climatological scale in this study. Mean wind observations from the SANAE SuperDARN HF radar are compared with observations from Halley SuperDARN HF radar. There is a good agreement between the observations from the two stations both in the zonal and meridional wind components. Zonal wind is observed to be consistently larger than the meridional wind. The zonal wind is also consistently more eastward at both stations with maxima occurring during the solstice months. High latitude summer zonal mean ow at 94 km is observed to be weaker and more variable compared to the eastward winter mean circulation owing to tropospherically forced planetary waves propagating through the middle atmosphere. The zonal mean wind shows greater seasonal variability than does the meridional mean wind. This seasonal behaviour is reasonably well understood in terms of the upward propagating planetary waves and gravity waves interacting with the mean ow. The Coriolis force also plays an important role in the case of meridional wind component. The climatology of planetary waves both in the zonal and meridional wind components indicates an ampli cation of planetary waves of shorter wavenumbers (s = 3) in the winter months. During summer, long period oscillations (e.g. >10 days) which are dominant in winter disappear, and oscillations with shorter period (e.g. <10 days) become dominant. vi There is a strong planetary wave coupling between the stratosphere and mesosphere-lower thermospheric (MLT) during the year 2002 winter season, whilst the coupling is observed to be relatively weak during the other years. The strong planetary wave coupling in 2002 is understandable because during this year the middle atmosphere winter months were characterised by strong planetary wave activity which led to the rst ever detection of the SSW in the Southern Hemisphere. In the year 2002 winter period the mean circulation in the stratosphere is characterized by a series of planetary wave events that weakened the polar vortex and triggered the sudden stratospheric warming in late September. In particular, in the stratosphere there is a presence of a quasi 10-day eastward propagating planetary wave of wavenumber s=1, while in the MLT a quasi 14-day eastward propagating planetary wave of wavenumber s=1 is observed to be dominant. The Eliassen Palm ux (E-P) ux shows that strong planetary wave activity observed in the middle atmosphere originates from the troposphere. Zonal winds at the MLT show reversal approximately 7 days before the reversal at stratosphere, indicating a downwards propagation of circulation disturbance in the middle atmosphere. Eastward zonal winds dominate the winter MLT, but during the 2002 winter there are many periods of westward winds observed compared to the other years. The SABER vertical temperature pro les indicate cooling of the MLT region during the SSW occurrence. Gravity wave horizontal phase velocities and horizontal wavelengths as seen by the SANAE imaging riometer are observed to reduce dramatically over SANAE during the occurrence of the stratospheric warming. The disturbance of the middle atmosphere during the Southern Hemisphere stratospheric warming in year 2002 winter preconditioned the region for gravity waves to propagate upward to the MLT. The potential energy of these gravity waves is observed to increase with height up until they reach the lower thermosphere. At the MLT they lose their energy, thus depositing their momentum, leading to the MLT cooling and mean wind reversal. Keywords: SSW, Planetary waves, Gravity waves, Stratosphere, MLT, SuperDARN radar, Mean wind, Temperature, Middle atmosphere, SANAE.
Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2012.

碩士
國立成功大學
太空天文與電漿科學研究所
100
In this thesis, modifications of the ionospheric tidal signatures during the 2008-2010 stratospheric sudden warming (SSW) events are studied by applying atmospheric tidal analysis to ionospheric electron densities observed using radio occultation soundings of FORMOSAT-3/COSMIC. The tidal analysis indicates that the zonal mean and major migrating tidal components (DW1, SW2 and TW3) decrease around the time of the SSW, with 1.5-4 hour time shifts in the daily time of maximum. The typical ionospheric SSW signature: a semi-diurnal variation of the ionospheric electron density, featuring an earlier commencement and subsidence of EIA, can be reproduced by differencing the migrating tides before and during the SSW period. Our results also indicate that the migrating tides represent ~80% of the ionospheric tidal components at specific longitudes, suggesting that modifications of the migrating tides may be the major driver for producing ionospheric changes observed during SSW events, accounting for greater variability than the nonmigrating tides that have been the focus of previous studies.

Sudden stratospheric warmings (SSWs) are a key driver of winter climate variability in the Northern Hemisphere. SSWs are a disruption of the strong stratospheric westerlies over the winter pole in which the winds in the upper to middle stratosphere, from about 30 to 50 km above the surface, weaken and reverse and the polar cap temperatures increase by up to 50 K in only a few days. These events affect tropospheric conditions for the two months following, on average shifting the North Atlantic storm track equatorward and resulting in a negative Northern Annular Mode and North Atlantic Oscillation at the surface. These changes are associated with colder and drier than average conditions in Northern Europe and Eurasia and warmer and wetter than average conditions across Southern Europe, as well as high temperatures across North Africa, the Middle East, and Central Asia and increased cold air outbreaks in North America and Eurasia. This thesis examines this typical surface response to SSWs in several different contexts. We consider its relationship to other atmospheric phenomena and features, first quantifying its importance relative to the North Atlantic impacts of the El Niño-Southern Oscillation (ENSO) and then examining the role of ozone chemistry in modeling the surface response to SSWs. We also study the variability of the surface signature of SSWs, with the goal of understanding the uncertainty in magnitude and spatial pattern of surface climate patterns following SSWs and the relative roles of different sources of this uncertainty. After providing background and context in the first chapter, the second chapter studies interactions between SSWs and the El Niño phase of ENSO. El Niño affects climate in the North Atlantic and European regions, those most affected by SSWs, through tropospheric and stratospheric pathways. One of these pathways is increased SSW frequency. However, most SSWs (about 90\%) are unrelated to ENSO, and the importance for boreal winter surface climate of this frequency increase compared to other El Niño pathways remains to be quantified. We here contrast these two sources of variability using two 200-member ensembles of one-year integrations of the Whole Atmosphere Community Climate Model, one ensemble with prescribed El Niño sea surface temperatures (SSTs) and one with neutral-ENSO SSTs. We form composites of wintertime climate anomalies, with and without SSWs, in each ensemble and contrast them to a basic state represented by neutral-ENSO winters without SSWs. This approach allows us to isolate the distinct effects of ENSO and SSWs more clearly than was done in previous work. We find that El Niño and SSWs both result in negative North Atlantic Oscillation anomalies and have comparable impacts on European precipitation, but SSWs cause larger Eurasian cooling. These results indicate the potential impact of a strong El Niño on seasonal forecasting in the North Atlantic as well as the importance of resolving the stratosphere in subseasonal and seasonal forecast models to best capture stratospheric polar vortex variability. In the third chapter, we study the importance of interactive ozone chemistry in representing the stratospheric polar vortex and Northern Hemisphere winter surface climate variability. Modeling and observational studies have reported effects of stratospheric ozone extremes on Northern Hemisphere spring climate. Recent work has further suggested that the coupling of ozone chemistry and dynamics amplifies the surface response to midwinter SSWs. We contrast two 200-year simulations from the interactive and specified chemistry (and thus ozone) versions of the Whole Atmosphere Community Climate Model with constant year-2000 forcings. This experiment is thus designed to clearly isolate the impact of interactive ozone on polar vortex variability. In particular, we analyze the response with and without interactive chemistry to midwinter SSWs, March SSWs, and strong polar vortex events (SPVs). With interactive chemistry, the stratospheric polar vortex is stronger, and more SPVs occur, but we find little effect on the frequency of midwinter SSWs. At the surface, interactive chemistry results in a pattern resembling a more negative North Atlantic Oscillation following midwinter SSWs, but with little impact on the surface signatures of late winter SSWs and SPVs. These results suggest that including interactive ozone chemistry in model simulations is important for representing North Atlantic and European winter climate variability. In the fourth chapter, we turn from models to reanalysis and consider the uncertainty in the surface response to SSWs. While the qualitative features of the mean surface signature of SSWs in the North Atlantic and Europe are well-established, its uncertainties as well as other features of surface climate following SSWs are less well-understood. To address the question of robustness of the mean observed response to SSWs, we use bootstrapping with replacement to construct synthetic SSW composites from SSW events in reanalysis, creating an ensemble of composites comparable to the observed one. We then examine the differences across these synthetic composites. We find that the canonical responses of a negative North Atlantic Oscillation and associated temperature and precipitation anomalies in the North Atlantic and European regions in the months following SSWs are robust. However, the magnitude and spatial pattern of these anomalies vary considerably across the composites. We further find that this uncertainty is unrelated to vortex strength and is instead the result of unrelated tropospheric variability. These results have implications for evaluating the fidelity of forecast models in capturing the surface impact of SSWs, by comparing both the mean impact as well as the contribution from internal variability with observations. Overall, we demonstrate the complexity of interactions of sudden stratospheric warmings with other sources of variability in the Earth system. We find that the state of the polar vortex itself, the strength of downward propagation following the SSW, and the surface response can all be affected in important ways by these other components (e.g. tropospheric variability and Arctic ozone). We close by providing broader context for these results and looking towards continuing and future work in the field.

The Middle and Upper Atmosphere Model has been used to simulate the middle atmosphere variability by changing the lower boundary conditions. For this purpose quasi steady-state simulations were performed for individual days around the sudden stratospheric warmings (SSWs) in January 2009 and 2012. The modelled time series reveal a qualitatively good agreement with reanalysis zonal wind with the observed one by reanalysis at the same height and latitude. A medium-range prediction of SSW events appears possible if planetary waves (PW) in the lower stratosphere are additionally assimilated in form of externally forced travelling PW.

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.