The dynamical link between the troposphere and stratosphere and its potential to affect climate

The main issue of this thesis has been to increase our understanding of the mech- anisms by which the stratosphere can affect the tropospheric climate. The dy- namical coupling of tropospheric and stratospheric circulation in the Northern Hemisphere was investigated by applying the new approach Single Wave Analysis which combines a well-known theoretical concept of the coupling mechanism with the statisticai analysis of observational datasets. The isolated features were used to interpret both the coupled modes of variability in tropospheric and strato- spheric geopotential height fields and the changes in the estimated probability density function of these modes. The prominent result of this thesis is that winter seasons characterized either by an anomalously strong or weak polar winter vortex exhibit different tropospheric circulation regimes. Only in the case of a strong stratospheric polar vortex does a downward control of the tropospheric circulation by reflection of waves of zonal wave number (ZWN) one occur. This downward influence on the structure of tropospheric waves is considerably less than the influence of tropospheric dis- turbances on the structure of stratospheric waves of ZWN I and 2. This re- sult confirms our understanding of the coupling of stratosphere and troposphere: Waves in the stratosphere originate in the troposphere, whereas the disturbances in the tropospheric circulation result mainly from internal processes. However, the findings also reveal that the two circulation regimes, characterized either by an preferred exaggeration of an anomalously strong or weak polar winter vortex, exhibit different tropospheric variability structures and are of high relevance to interannual and interdecadal climate variability

Changing lower stratospheric circulation- The role of ozone and greenhouse gases

Stratospheric climate has changed significantly during the last decades. The causes of these changes are discussed on the basis of two different general circulation model experi- ments forced by observed greenhouse gas and ozone concentration. There is a clear and signifi- cant response of the lower stratosphere temperature and geopotential in the model simulations forced by observed ozone changes that is in accord with observed trends in summer in middle and high latitudes of the northern hemisphere. Little effect is seen in the tropics. In spring there occur the strongest anomalies/trends in both hemispheres at polar latitudes; however, the model responseis late by 1 to 2 months and is much weaker than the observed effects. The ozone-forced model in winter of both hemispheres produces slight warming or no change instead of the slight cooling observed. The effects of enhanced greenhouse gases as taken from a transient IPCC sce- nario AGCM run do enhance the cooling in high latitudesin spring, but the effect is much smaller than observed. Hence neither of the two forcings (reduced ozone and increased greenhouse gas- es) in the cold seasonsis able to produce the recent stratospheric and tropospheric trend patterns alone. These trends clearly resemblea natural mode of variability both in the model and in the real worldø This mode associates a strengthened polar night vortex with an enhanced North At- lantic oscillation. The excitation of this mode cannot yet be attributed to anthropogenic forcing

Comment on "Tropospheric temperature response to stratospheric ozone recovery in the 21st century" by Hu et al. (2011)

Abstract. In a recent paper Hu et al. (2011) suggest that the recovery of stratospheric ozone during the first half of this century will significantly enhance free tropospheric and surface warming caused by the anthropogenic increase of greenhouse gases, with the effects being most pronounced in Northern Hemisphere middle and high latitudes. These surprising results are based on a multi-model analysis of CMIP3 model simulations with and without prescribed stratospheric ozone recovery. Hu et al. suggest that in order to properly quantify the tropospheric and surface temperature response to stratospheric ozone recovery, it is necessary to run coupled atmosphere-ocean climate models with stratospheric ozone chemistry. The results of such an experiment are presented here, using a state-of-the-art chemistry-climate model coupled to a three-dimensional ocean model. In contrast to Hu et al., we find a much smaller Northern Hemisphere tropospheric temperature response to ozone recovery, which is of opposite sign. We suggest that their result is an artifact of the incomplete removal of the large effect of greenhouse gas warming between the two different sets of models

Zeitscheibenexperimente mit dem atmosphärischen Zirkulationsmodell T42-ECHAM3 für eine verdoppelte und verdreifachte CO2-Konzentration unter besonderer Beachtung der Änderungen der nordhemisphärischen troposphärischen Dynamik

In der vorliegenden Arbeit werden Ergebnisse aus 30-Jahre-Zeitscheibenexperimenten mit dem relativ hochauflösenden Hamburger Allgemeinen Zirkulationsmodell der Atmosphäre T42-ECHAM3 vorgestellt. Die Experimente wurden sowohl für eine CO2-Verdopplung als auch eine CO2-Verdreifachung durchgeführt, wodurch die Aufdeckung eines eventuellen nichtlinearen Verhaltens der Signale mit zunehmender CO2-Konzentration ermöglicht wird. Im Vergleich zu anderen Studien, in denen ein ähnlicher Ansatz gewählt wurde, wird sich auf relativ lange Experimente gestützt und es werden statt nur des Winters alle Jahreszeiten in die Analyse einbezogen. Das verwendete Verfahren zur Bestimmung der statistische Signifikanz der Signale berücksichtigt die interdekadische Variabilität im 100-Jahre- Kontrollexperiment, so daß eine große Sicherheit der Ergebnisse erreicht wird. Besonderer Augenmerk wird auf die Änderungen der nordhemisphärischen troposphärischen Dynamik, deren einzelne Eigenschaften systematisch und in ihrer wechselseitigen Abhängigkeit untersucht werden, gelegt. Die allgemeinen Eigenschaften der Anderungen der 2-m-Temperatur bei einer Erhöhung der CO2-Konzentration wie die globale Temperaturzunahme, die Herausbildung eines deutlichen Land-Meer-Kontrastes der Erwärmung und die starke polare Erwärmung vor allem im Winter stimmen mit den Ergebnissen, die durch andere Forschungsgruppen erhalten wurden, gut überein. Der hydrologische Zyklus intensiviert sich mit zunehmender CO2-Konzentration. Die Änderungen sind sowohl im globalen Mittel als auch in einzelnen Regionen nichtlinear. In den tropischen Hauptkonvergenzzonen steht durch eine größere Freisetzung latenter Wärme in der oberen tropischen Troposphäre mehr Energie für den Antrieb der atmosphärischen Zirkulation zur Verfügung. In allen Jahreszeiten wurden signifikante Anderungen der verschiedenen Eigenschaften der nordhemisphärischen troposphärischen Dynamik wie Strahlstrom, stationäre Wellen, transiente Wellen und barokline und barotrope Wechselwirkung der transienten Wellen mit dem Grundstrom bei einer erhöhten CO2-Konzentration gefunden. Dabei wurde während der Untersuchung festgestellt, daß in den Experimenten alle Eigenschaften auch unter stationären Randbedingungen eine beträchtliche interdekadische Variabilität aufweisen, die es zu beachten galt. Ein wichtiges Ergebnis der Untersuchung ist, daß in den Zeitscheibenexperimenten in allen Jahreszeiten in der subtropischen mittleren Troposphäre Gebiete zu finden sind, in denen die Temperaturzunahme maximal ist. Diese Maxima liegen in den Absinkgebieten der Hadley-Zirkulation und werden durch adiabatische Erwärmung verursacht. Zwischen diesen Maxima der Erwärmung und den Minima der Erwärmung in den Trögen über den nördlichen Ozeangebieten liegen in den mittleren Breiten die Gebiete der stärksten Baroklinitätszunahme bei einer erhöhten CO2-Konzentration. Die Kenntnis dieses Zusammenhangs trägt ztt einem besseren Verständnis bei, wie sich die Änderungen der transienten Wellenaktivität räumlich verteilen. Im Winter dominiert ein stationäres Wellensignal, das über dem Nordpazifrk und Nordamerika bei einer CO2-Erhöhung angeregt wird, die Zirkulationsänderung. Das Signal, das im 2xCO2-Experiment stärker als im 3xCO2-Experiment ist, ähnelt sehr dem Pazifik-Nordamerika-Telekonnektionsmuster (PNA). Das nichtlineare Verhalten mit zunehmender CO2-Konzentration wird durch die nichtlineare Änderung der Wärmeflußkonvergenz durch hochfrequente transiente Wellen, von der die Anregung stationärer Wellenaktivität über dem nördlichen Pazifik abhängt, verursacht. Diese Wärmeflußkonvergenz über dem Nordpazifik verringert sich im Vergleich zum ZxCO2-Experiment wieder im 3xCO2-Experiment. Ursache hierfür ist die starke polare Erwärmung im V/inter, die im 3xCO2- Experiment einer weiteren Zunahme der Baroklinität, die Bedingung für die Genese transienter Wellen ist, entgegenwirkt. In der Ähnlichkeit stationärer Wellensignale bei einer Erhöhung der CO2-Konzentration mit bekannten Telekonnektionsmustern liegt ein Potential für die Untersuchung regionaler Klimaänderungen. In den anderen Jahreszeiten beeinflußt die Hadley-Zirkulation über dem Nordatlantik und Europa am stärksten die Änderungen der nordhemisphärischen troposphärischen Dynamik. Dieser Einfluß ist in den Üb"tgungrjahreszeiten, in denen sich die HadIey-Zirkulation in signifikanter Weise intensiviert, besonders stark ausgeprägt. Übe. dem nördlichen Nordatlantik, wo eine Zunahme der Wärmeflußkonvergenz durch transiente Wellen zu beobachten ist, wird im Frühling ein stationäres Wellensignal angeregt, das sich nach Osten ausbreitet. Es ähnelt dem Westatlantischen (V/A) und dem Eurasischen Telekonnektionsmuster (EU). Im Sommer und Herbst breitet sich ein stationärer Wellenzug von einem Gebiet nahe Grönland ausgehend nach Südosten aus. Eine größere Variabilität durch hochfrequente transiente Wellen ist in den Zeitscheibenexperimenten in einzelnen geographischen Regionen zu beobachten. Die Änderungen sind in den Übergangsjahreszeiten bedeutend größer und stärker signifikant als im V/inter. Im Winter nimmt die Variabilität im 2xCO2-Experiment über dem Nordatlantik und dem Nordpazifik zu, zeigt aber im 3xCO2-Experiment keine zusätzliche deutliche Anderung. In den anderen Jahreszeiten ist das Signal im 3xCO2-Experiment stärker als im 2xCO2-Experiment. Die Gebiete mit einer Zunahme der hochfrequenten transienten Wellen sind im Frühling vor allem Nordamerika und der Nordatlantik und im Sommer der Nordatlantik und das nördliche Europa. Im Herbst nimmt die Variabilität in einem Gebiet, das sich vom nordöstlichen Atlantik über das nördliche Europa bis weit nach Osteuropa hinein erstreckt, sehr stark zu. Mit der vorgelegten Arbeit wurde gezeigt, daß Zeitscheibenexperimente ein geeignetes Mittel sind, um mit einem atmosphärischen Zirkulationsmodell die Anderungen der atmosphärischen Dynamik für eine erhöhte CO2-Konzentration und für entsprechend veränderte untere Randbedingungen zu untersuchen. Zum Verständnis dieser Anderungen wurde ein wichtiger Beitrag geleistet. Die erhaltenen signifikanten Änderungen der nordhemisphärischen troposphärischen Dynamik unterscheiden sich zwischen den einzelnen Jahreszeiten wesentlich. Besonders große Unterschiede sind zwischen der Winterzirkulation und der Zirkulation in den anderen Jahreszeiten zu finden, in denen grundlegend verschiedene dynamische Prozesse die Signale dominieren. In folgenden Studien sollten deswegen alle Jahreszeiten analysiert werden.In this study, the results of time-slice experiments are presented. These were carried out using the relatively high resolution Hamburg atmospheric general circulation model T42- ECHAM3. The experiments were done both for CO2-doubling and CO2-tripling to investi- Eate a possible nonlinear response to an increase in the CO2-concentration. In comparison to other studies using a similar approach, these time-slice experiments have a relatively long integration length. Additionally, all seasons arc analyzed instead of only the winter. The approach, used to estimate the statistical significance of the responses, takes into account the interdecadal variability of the 100 year control experiment. Therefore there is a great reliability of the results. Particular attention is paid to the changes of the tropo- spheric dynamics in the Northern Hemisphere. The features of these changes are investigated systematically and studied for their mutual dependence. The general features of the near surface temperature changes due to the higher CO2-concentration, such as the global temperature increase, the enhanced land-sea contrast of the warming and the pronounced polar warming in winter, agree well with the results obtained by other research groups. The hydrological cycle intensifies with increased CO2-concen- tration. The changes are nonlinear both in the global mean and in certain regions. Due to a greater release of latent heat in the tropical convergence zones, there is more energy to drive the atmospheric circulation. In all season, significant changes of the various features in the tropospheric dynamics, such as the jet stream, the stationary waves, the transient waves and the baroclinic and barotropic interaction between the transient waves and the base flow, were found in the Northern Hemisphere for an increased CO2-concentration. During the examination it was discovered that all these features show a large interdecadal variability given stationary lower boundary conditions. This had to be taken into consideration. A significant result of this study is that regions with a maximal temperature increase can be found in the subtropical middle troposphere in all seasons for an increased CO2-concentration. These maxima, caused by an adiabatic warming, are located in the areas in which the air descends in the Hadley circulation system. The regions with the strongest baroclinicity increase, in the time-slice experiments, are situated in the midlatitudes between the areas with the maximal temperature increase in the subtropics, and the areas with the minimal temperature increase in the troughs in the high latitudes over the oceans. Knowing about this effect helps to explain the horizontal distribution of the changes in the transient wave activity. In winter, a stationary wave response, located over the North Pacific and North America, dominates the circulation change. This response, which is larger for 2xCO2 than for 3xCO2, is very similar to the Pacific/North America teleconnection pattern (PNA). This nonlinear response depends on the nonlinear change in the heat flux convergence, induced by high frequency transient waves, over the North Pacific. The heat flux convergence, generating stationary wave activity, enhances with the change of the CO2-concentration from 1.xCO2to 2xCO2, but decreases with the change from 2xCO2to 3xCO2. The reason for this is the strong winter polar warming in the 3xCO2-experiment counteracting the additional increase in baroclinicity as a condition for the generation of transient waves. A similarity between a stationary wave response, like this, and well-known teleconnection patterns gives a potential for forecasting regional climate changes. In the seasons other than the winter one, the Hadley circulation over the North Atlantic exerts the greatest influence on changes in the tropospheric dynamics in the Northern Hemisphere. This influence is particularly strong in spring and autumn, when the Hadley circulation intensifies significantly. In spring, a stationary wave response, propagating eastwards, is generated over the northern North Atlantic, where an increase in the heat flux convergence, due to transient waves, can be found. This response is similar to the Western Atlantic (WA) and the Eurasian (EU) teleconnection pattern. In summer and autumn, a wave train propagates south-eastwards. A larger variability, caused by high frequency transient waves, is found in some geographical regions in the time-slice experiments. The changes are more marked and evidently more statistically significant in spring and autumn than in winter. In winter, the variability increases over the North Atlantic and the North Pacific in the ZxCO2-experiment, but shows only a marginally further change in the 3xCO2-experiment. However, in the seasons other than the winter one, the responses are greater in the 3xCO2-experiment than in the 2xCO2-experiment. In spring, the regions with an increased variability are North America and the North Atlantic. In summer, the regions are the North Atlantic and northern Europe. In autumn, there is a very large increase in variability in an area extending from the north-eastern Atlantic over northern Europe to eastern Europe. In this study it has been shown that time-slice experiments, using an atmosphere circulation model , are a suitable approach to investigate the response of the atmospheric dynam- ics to an increased CO2-concentration and correspondingly changed lower boundary conditions. An important contribution has been made to our understanding of the changes in atmospheric dynamics. These significant changes are substantially different in the various seasons. Particularly large differences are found between the winter circulation and the circulations in the other seasons, in which the responses are dominated by substantially different dynamic processes. According to these results, all seasons should be included in any analysis in future studies

Large-Scale Circulation and Climate Variability

The causes of regional climate trends cannot be understood without considering the impact of variations in large-scale atmospheric circulation and an assessment of the role of internally generated climate variability. There are contributions to regional climate trends from changes in large-scale latitudinal circulation, which is generally organized into three cells in each hemisphere-Hadley cell, Ferrell cell and Polar cell-and which determines the location of subtropical dry zones and midlatitude jet streams. These circulation cells are expected to shift poleward during warmer periods, which could result in poleward shifts in precipitation patterns, affecting natural ecosystems, agriculture, and water resources. In addition, regional climate can be strongly affected by non-local responses to recurring patterns (or modes) of variability of the atmospheric circulation or the coupled atmosphere-ocean system. These modes of variability represent preferred spatial patterns and their temporal variation. They account for gross features in variance and for teleconnections which describe climate links between geographically separated regions. Modes of variability are often described as a product of a spatial climate pattern and an associated climate index time series that are identified based on statistical methods like Principal Component Analysis (PC analysis), which is also called Empirical Orthogonal Function Analysis (EOF analysis), and cluster analysis

On the tropospheric response to anomalous stratospheric wave drag and radiative heating

Observational and numerical evidence suggest that variability in the extratropical stratospheric circulation has a demonstrable impact on tropospheric variability on intraseasonal time scales. In this study, it is demonstrated that the amplitude of the observed tropospheric response to vacillations in the stratospheric flow is quantitatively similar to the zonal-mean balanced response to the anomalous wave forcing at stratospheric levels. It is further demonstrated that the persistence of the tropospheric response is consistent with the impact of anomalous diabatic heating in the polar stratosphere as stratospheric temperatures relax to climatology. The results contradict previous studies that suggest that variations in stratospheric wave drag are too weak to account for the attendant changes in the tropospheric flow. However, the results also reveal that stratospheric processes alone cannot account for the observed meridional redistribution of momentum within the troposphere