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No description available.

The most recent Intergovernmental Panel on Climate Change assessment report concludes that the Atlantic Meridional Overturning Circulation (AMOC) could weaken substantially but is very unlikely to collapse in the 21st century. However, the assessment largely neglected Greenland Ice Sheet (GrIS) mass loss, lacked a comprehensive uncertainty analysis, and was limited to the 21st century. Here in a community effort, improved estimates of GrIS mass loss are included in multicentennial projections using eight state-of-the-science climate models, and an AMOC emulator is used to provide a probabilistic uncertainty assessment. We find that GrIS melting affects AMOC projections, even though it is of secondary importance. By years 2090-2100, the AMOC weakens by 18% [-3%, -34%; 90% probability] in an intermediate greenhouse-gas mitigation scenario and by 37% [-15%, -65%] under continued high emissions. Afterward, it stabilizes in the former but continues to decline in the latter to -74% [+4%, -100%] by 2290-2300, with a 44% likelihood of an AMOC collapse. This result suggests that an AMOC collapse can be avoided by CO2 mitigation.

Orography plays a major role in the general circulation and climate of the tropics. Although many works have been done on the impact of global orography on summer monsoon, the previous studies have examined the impact on seasonal mean scale or only during the first half of the season. Role of orography on intra-seasonal variability has not been addressed previously. Also, the proximate and remote impacts of orography have not been studied. In this thesis an atmospheric General Circulation Model (GCM) has been used to investigate the impact of global and regional orography on monsoon climate. Two different cumulus schemes have been used to study the sensitivity of the results to the cumulus parameterization scheme. The model was forced with seasonally varying sea surface temperature (SST) for the year 1998. An ensemble simulation of 5 members were performed for each experiment. The simulations showed that the removal of Himalayas or orography over the entire earth caused a delay of about one month in the onset of the monsoon. The delay in monsoon onset was on account of a more stable atmosphere due to intrusion of mid-latitude cold air into the Indian region in the absence of Himalayas. After the onset, the precipitation rate was comparable in control and no-mountain simulations. The seasonal mean (June-September) precipitation over this region decreased by 25% in the no-mountain case as compared to control. A comparison of the impact of east and west Himalaya orography showed that orography west of 80E has more impact on the phase and intensity of summer monsoon precipitation over the Indian region than orography east of 80E. The onset of summer monsoon over the Indian region was delayed by about one month with the removal of Himalaya orography west of 80E, but was delayed by just about one week with the removal of Himalaya orography east of 80E. This is because, the cold air intrusion was more when Himalaya orography west of 80E was removed. Seasonal mean precipitation decreased by 22% and 12% with the removal of orography west and east of 80E respectively. Himalaya orography east of 80E showed more influence on precipitation over the north-east Indian region and East Asia. The removal of orography from the African continent increased the summer monsoon precipitation over the Indian region. This was on account of an increase in the zonal mass flux from the African continent in the absence of East African mountains. This mass flux brings more moisture into the south Asian region and increases precipitation over the Indian region and Bay of Bengal. A higher precipitation over the Bay of Bengal leads to higher wind over the Somalia coast and this acts as a positive feedback to enhance the summer monsoon precipitation by about 28% over the Indian region. The presence of orography only over the African continent resulted in the largest delay in the monsoon onset (by 50 days) and the lowest amount of seasonal precipitation (decrease by 36%) over the Indian region among all the simulations. This is due to further reduction in zonal mass (and hence, moisture) flux toward the Indian subcontinent with the inclusion of African orography when compared with no-global orography simulation. The seasonal mean precipitation decreased by 19% over the Indian region with the removal of American orography. The onset of monsoon was delayed by about 3 weeks in this experiment as compared to control. This delay was due to a relative downward motion in the upper troposphere on account of the shift of the Rossby wave with the removal of American mountains. In this thesis, a new theory has been proposed for monsoon onset based on thermodynamic conditioning (necessary condition) and mechanical trigger (sufficient condition) of the atmosphere. This theory was able to explain the large variation in monsoon onset dates (maximum spread 57 days) in different simulations. The low level circulation was affected more by Himalaya orography west of 80E, which had a profound influence on precipitation over the Indian region. However, upper level circulation was affected more by Himalaya orography east of 80 E. The northward shift of the upper tropospheric westerly jet during the Northern Hemispheric summer was sudden in presence of the Tibetan Plateau and gradual in its absence. This shift was not related to the onset of monsoon over the Indian region. Northward propagation of convection was found to be present even in the absence of global orography. But northward extent of this propagation was delayed without orography on account of the absence of a favorite meridional gradient of moist static energy in the lower troposphere in the early summer season due to intrusion of mid-latitude cold air. Space-time spectral analysis showed that the intensity of eastward moving convectively coupled atmospheric waves, known as Madden-Julian oscillation (MJO), decreased in absence of global orography. Moreover, the presence of orography favor the higher zonal wave number for MJO propagation.

University, Diss., 2001--München.

Zugl.: Hamburg, Univ., Diss., 2009

Satellite data have shown that east-west oriented cloud bands, known as Intertropical convergence zone (ITCZ), propagate eastwards along the equator throughout the year and northwards during boreal summer on intraseasonal time scales. The northward propagations over Bay of Bengal have important connection with onset of south Asian monsoon and active-break cycles of the Indian monsoon. Some studies on mean structure of ITCZ have concluded that preferred location of ITCZ is governed by meridional variation of sea surface temperature (SST) while other studies have stressed the importance of heating in the free atmosphere. Studies on the migration of ITCZ have shown that northward migration of maximum convergence zone is due to generation of positive barotropic vorticity north of the convection in the boundary layer due to internal dynamics of the atmosphere. In the present study mean and transient structure of northward migration of ITCZ over Bay of Bengal is simulated with the help of a general circulation model (GCM). The mean ITCZ is found not to occur at SST maximum or SST gradient maxima. A new simple model for the mean state of ITCZ based on moisture budget, linear friction and hydrostatic assumption is proposed. It highlights the relative importance of SST and atmospheric effects in generation of maximum convergence. The large cancellation between the effect of SST on boundary layer and thermodynamic effects in free troposphere is shown to control convergence. The model also shows that latitude and time independent linear friction parameterization in a simple model is able to predict monthly mean location of ITCZ in a GCM. The results give a quantitative understanding about the relative role of surface effects and atmospheric effects in determining location of the mean ITCZ. A simple linear model for understanding the mechanism of instability that governs the northward migration of ITCZ is proposed. Vertical shear in mean winds couples the barotrpic and baroclinic modes in free troposphere in this model. The model is able to predict the correct scale with standard values of friction and diffusion parameters. The mechanism of instability is found to be due to internal dynamics of troposphere. It is shown that direction of propagation is decided by vertical shear in zonal as well as meridional mean winds. This is contrary to the previous studies which conclude that either vertical shear in zonal winds or vertical shear in meridional winds control the direction of propagation.

Context. The Martian methane and water cycle are subject of ongoing research through simulation. Exchange with the subsurface has a potentially strong impact, but is often neglected. Aims. For methane, I determine if adsorption with an increased enthalpy can explain the observed seasonal variations and conflicting observations by the Trace Gas Orbiter and the Curiosity rover. For water, the impact of adsorption and ice formation in the subsurface on the global cycle is studied. A new way of initializing the soil, by running a decoupled subsurface model, is tested. Depths of stable subsurface ice and subsurface water distributions are studied. Methods. A General Circulation Model (GCM) is used with a purely diffusive subsurface model. For methane, different initial states, source scenarios, and decay times are tested. For water, a model without an active atmosphere is implemented to provide an initial state. The effect of the subsurface with this initial state on the full atmospheric water cycle is tested. Results. For methane, a strong influence on the global methane cycle is observed. Seasonal variations measured at Gale Crater are reproduced, but the conflicting observations cannot be explained by adsorption. For water, the new initialization can be used without completely disrupting the water cycle. It leads to a generally wetter atmosphere, in conflict with observations. Found ice table depths do not match well with observations, but ice profiles reproduce previous findings. Conclusion. Methane adsorption is able to partly explain observed variations, but cannot be the only process to influence methane abundances. The new initialization method for water works well in principle, but a more refined model is needed for more realistic results.

In dieser Arbeit wurde die Variabilität der Atmosphäre in einem neuen gekoppelten Klimamodell (ECHO-GiSP) untersucht, welches eine vereinfachte Stratosphärenchemie (bis 80 km Höhe) enthält. Es wurden 2 Simulationen über 150 Jahre durchgeführt. In einer der Simulationen wurde die atmosphärische Chemie modelliert, hatte aber keinen Einfluß auf die Dynamik des Klimamodelles. In der zweiten Simulation wurde hingegen die Wirkung der Chemie auf die Klimadynamik explizit berücksichtigt, die über die Strahlungsbilanz des Modelles erfolgt. Dies ist die erste Langzeitsimulation mit einem voll gekoppelten globalen Klimamodell mit interaktiver Chemie. Die Simulation mit rückgekoppelter Chemie zeigt eine Abschwächung des atmosphärischen Variabilitätsmusters der Arktischen Oszillation (AO). Zudem kommt es in der Troposphäre zu einer Reduzierung der mittleren Windgeschwindigkeiten der gemäßigten Breiten aufgrund verringerter Temperaturgegensätze zwischen den Tropen und den Polargebieten. Auch in der Stratosphäre ergibt sich eine Abschwächung und Erwärmung des Polarwirbels. Diese Auswirkungen der Kopplung zwischen der atmosphärischen Chemie und der Dynamik des Klimamodelles sind eine wichtige Erkenntnis, da in früheren Klimasimulationen die Variabilität der AO oft zu stark ausgeprägt war. In der Stratosphäre reduziert sich infolge des abgeschwächten Polarwirbels auch die großräumige Zirkulation zwischen den beiden Hemisphären der Erde. In der Troposphäre werden hingegen die allgemeine Zirkulation, und damit auch die subtropischen Strahlströme des Windes verstärkt. Zudem kommt es in den Tropen zu Temperaturänderungen durch stratosphärische Ozonschwankungen in Abhängigkeit von der AO. Allgemein verändert sich die Kopplung zwischen Troposphäre und Stratosphäre, einschließlich des durch die Anregung von langen atmosphärischen Wellen erfolgenden vertikalen Energieübertrages aus der Troposphäre in die Stratosphäre. / In this work the atmospheric variability in a new coupled climate model (ECHO-GiSP) was analyzed, which includes a simplified stratospheric chemistry (up to 80 km height). Two simulations of 150 years were performed. In one of those simulations the atmospheric chemistry was modeled without having any influence back on the model dynamics. In the second simulation the impact of the chemistry on climate dynamics, taking place via the models radiation balance, was explicitly recognized. This is the first long term simulation using a fully coupled global climate model with interactive chemistry. The simulation with interactive chemistry shows a weakening of the Arctic Oscillation (AO) pattern of atmospheric variability. At the same time there is a reduction of the mean wind speeds in middle latitudes in the troposphere, which is caused by weaker temperature gradients between the tropics and the polar regions. Also, in the stratosphere a weakening and warming of the polar vortex is obvious. These effects of the coupling between atmospheric chemistry and the dynamics of the climate model are an important result, since in earlier climate simulations the variability of the AO often was overestimated. Due to the weakened polar vortex in the stratosphere also the large scale interhemispheric mean circulation is reduced. On the other hand, the tropospheric meridional mean circulation, and thus also the subtropical jetstreams of the zonal wind are enhanced. Furthermore there are tropical temperature variations in the troposphere and lower stratosphere, which are induced by stratospheric ozone variations associated to the phase of the AO. Generally, the coupling between tropo- and stratosphere is changed, which includes the vertical energy and momentum transfer by ascending planetary waves from the troposphere to the stratosphere.

The sources and sinks of energy for transient waves in the Martian atmosphere are investigated, applying diagnostic techniques developed for the analysis of terrestrial baroclinic waves to output from a Mars General Circulation Model. These diagnostic techniques include the vertically averaged eddy kinetic energy and regression analysis. The results suggest that the primary source of the kinetic energy of the waves is baroclinic energy conversion in localized regions. It is also shown that there exist preferred regions of baroclinic energy conversion. In addition, it is shown that downstream baroclinic development plays an important role in the evolution of the waves and in the baroclinic energy conversion process. This is the first time that evidence for downstream baroclinic development has been found for an atmosphere other than the terrestrial one.