Wetter-Regime und Persistenz

Betreuer: Stephan Pfahl

Beschreibung:Die Persistenz der bodennahen Temperatur ist eine wichtige Größe zur Charakterisierung von extremen Hitze- oder Kältewellen. In dieser Arbeit soll untersucht werden, wie verschiedene Persistenzmaße (Standardabweichung und Auto-Korrelation sowie Amplitude von Schwankungen von Tag zu Tag der bodennahen Temperatur) mit unterschiedlichen europäischen Wetterregimen zusammenhängen. Für die Wetterregime wird eine Klassifikation von Grams et al. (2017) verwendet.

Voraussetzungen: Voraussetzungen für diese Arbeit sind Interesse an atmosphärischer Dynamik und Statistik. Grundkenntnisse in einer Skriptsprache (R, Python), die zur Datenauswertung verwendet werden kann, sind von Vorteil, können aber auch während der Arbeit erworben werden.

Literatur:

• Grams, C.M., Beerli, R., Pfenninger, S., Staffel, I. and Wernli, H. (2017) Balancing Europe’s wind-power output through spatial deployment informed by weather regimes. Nature Clim. Change 7, 557-562, https://doi.org/10.1038/nclimate3338.

• Li, J. and Thompson, D.W.J (2021). Widespread changes in surface temperature persistence under climate change. Nature 599, 425-430, https://doi.org/10.1038/s41586-021-03943-z.

Der Zusammenhang zwischen Fronten und extremen Niederschlägen

Betreuer: Stephan Pfahl, George Pacey, Florian Ruff

Beschreibung: Extreme Niederschläge treten häufig in Zusammenhang mit Fronten auf. Es ist allerdings nicht klar, wie dabei der genaue räumliche Zusammenhang aussieht, also ob z.B. mehr extreme Niederschläge in der pre-frontalen Luftmasse oder direkt an der Front auftreten. In dieser Arbeit soll dieser Zusammenhang klimatologisch untersucht werden, in dem ein Datensatz zu Kaltfronten in Europa während der warmen Jahreszeit mit Niederschlagsdaten kombiniert wird.

Voraussetzungen: Interesse an Klimatologie und meteorologischen Extremereignissen. Grundkenntnisse in einer Skriptsprache (R, Python), die zur Datenauswertung verwendet werden kann, sind von Vorteil, können aber auch während der Arbeit erworben werden.

Literatur:

• Catto J. L. and Pfahl S. (2013). The importance of fronts for extreme precipitation. J. Geophys. Res. 118, 10791–10801, doi:10.1002/jgrd.50852.

Effects of sea surface temperature change on the global distribution of stable water isotopes

Betreuer: Svetlana Botsyun, Stephan Pfahl

Beschreibung: Stable hydrogen and oxygen isotopes in water vapor (δDv, δ¹⁸Ov) and precipitation (δDp, δ¹⁸Op) are widely used as tracers to improve our understanding of the hydrologic cycle on various time scales, from hourly variations to changes over millions of years (Botsyun et al., 2019; Pfahl and Wernli, 2008). Isotope fractionation is strongly dependent on temperature (Gat, 1996). Moreover, global atmospheric dynamics affecting stable water isotopes (Botsyun et al., 2016) is largely controlled by surface temperature. In this study, we aim to isolate the effects of sea surface temperature (SST) change (both increase and decrease) on stable water isotopes. Preliminary results show that the magnitude of isotopic change for the case of a 5°C decreased SST is > 8‰ and is most pronounced over continents, particularly over mountainous regions.

Vorgehen: In this work, we will first analyze the completed model simulations. Two sensitivity experiments are performed with a high-resolution (∼0.75°) isotope-enabled general circulation model (ECHAM5-wiso). In these simulations, the SST is changed (increased and decreased) by 5°C. All other boundary conditions are kept as pre-industrial (year 1850). In the first step, student will be asked to analyze the global climate and associated isotope changes caused by the shift in SST. In the second step (or at M. Sc. level), additional ECHAM5-wiso experiments will be designed to test the impact of model resolution on the SST-isotope link.

Voraussetzungen: Requirements for this project are an interest in stable water isotopes and paleoclimate, and a willingness to communicate in English (one of the supervisors is not a German speaker). Nevertheless, the final report can be written in German. Experience with scripting languages (Python, Ferret) would be helpful, but can be acquired/improved during the project.

Literatur:

• Botsyun, S., Sepulchre, P., Risi, C. and Donnadieu, Y.: Impacts of Tibetan Plateau uplift on atmospheric dynamics and associated precipitation δ18O, Clim. Past, 12(6), 1401–1420, doi:10.5194/cp-12-1401-2016, 2016.
• Botsyun, S., Sepulchre, P., Donnadieu, Y., Risi, C., Licht, A. and Caves Rugenstein, J. K.: Revised paleoaltimetry data show low Tibetan Plateau elevation during the Eocene, Science (80-. )., 363(6430), eaaq1436, doi:10.1126/science.aaq1436, 2019.
• Gat, J. R.: Oxygen and hydrogen isotopes in the hydrologic cycle, Annu. Rev. Earth Planet. Sci., 24(1), 225–262, 1996.
• Pfahl, S. and Wernli, H.: Air parcel trajectory analysis of stable isotopes in water vapor in the eastern Mediterranean, J. Geophys. Res. Atmos., 113(20), 1–16, doi:10.1029/2008JD009839, 2008.

Charakterisierung verschiedener Blocking-Indizes

Betreuer: Stephan Pfahl

Beschreibung: Zur Identifizierung von blockierenden Hochdrucklagen existieren mehrere Indizes, die auf verschiedene Aspekte der Blockierung abzielen (Hochdruck-Anomalie, Ostwinde) und auch sehr unterschiedliche klimatologischen Häufigkeitsverteilungen von Blockierungen liefern. In dieser Arbeit soll mit Hilfe von Reanalyse-Daten untersucht werden, inwiefern sich die von zwei verschiedenen Indizes am selben Ort detektierten Blocking-Ereignisse in ihrer synoptischen Struktur voneinander unterscheiden und wie groß generell der Überlapp zwischen den Ereignissen ist.

Voraussetzungen: Voraussetzungen für diese Arbeit sind Interesse an atmosphärischer Dynamik und Statistik. Grundkenntnisse in einer Skriptsprache (R, Python), die zur Datenauswertung verwendet werden kann, sind von Vorteil, können aber auch während der Arbeit erworben werden.

Literatur:

• Woollings, T. et al. (2018): Blocking and its response to climate change. Curr. Clim. Change Rep. 4, 287-300, https://doi.org/10.1007/s40641-018-0108-z.

Circumglobal teleconnection pattern in warm and cold climate

Betreuer: Svetlana Botsyun, Stephan Pfahl

Beschreibung: Large-scale atmospheric circulation in the extratropics fluctuates in its own dynamics, involving nonlinear interaction among eddies with various horizontal scales and a wide range of time scales. The circumglobal teleconnection (CGT) is a major teleconnection pattern in the northern hemisphere in boreal summer and is characterized by several geographically-anchored action centers along the upper-tropospheric westerly jet stream (Ding and Wang, 2005). Alternative names for the CGT also exists in literature, such as the “Silk Road pattern”, “European wave train” for the Eurasian segments, and the “Tokyo–Chicago express” for the Pacific segment. The CGT has important impacts on the mid-latitude temperature, precipitation, and extreme weather (Bothe et al., 2011). In addition, the CGT is closely related to the monsoonal circulation in the tropics (Saeed et al., 2011). For example, there is a chain of interactions between the CGT and the Indian monsoon rainfall variability: one interaction involves the wave train that develops over the North Atlantic and modulates the monsoon rainfall activity, and another interaction involves anomalous monsoon heating influencing the downstream development of the CGT (Ding and Wang, 2005). For the paleo case, the changes in CGT under different climatic forcings, its connection to the Asian monsoons, and its impact on surface climate are not clear. In this project, we suggest exploring changes in CGT for the Pliocene (3 Ma) and the Last Glacial Maximum (LGM, 21 ka) compared to pre-industrial. The Pliocene is considered as a past analogue of future warm climate (Haywood et al., 2016) and the LGM is an example of a colder-than-present climate (Braconnot et al., 2007). Possible research questions:

1. How do slowly varying components of the climate, such as sea surface temperature (SST), greenhouse gas concentrations, ice sheets, etc. affect large-scale circulation variability and the westerly jet stream?
2. How do changes in the North Atlantic pressure gradient affect the CGT?
3. What is the relationship between the surface climate in Asia and the GCT in warm and cold climates?

Vorgehen: We propose to investigate changes in CGT using General Circulation model (GCM) ECHAM5 experiments with boundary conditions for pre-industrial (year 1850), the Pliocene (~3 Ma), and the Last Glacial Maximum (LGM, ~21 ka). Emphasis is on analyzing completed model simulations rather than creating new model runs. Student will be asked to analyze climatological midlatitude circulation in the Northern Hemisphere based on, for example, high-level geopotential height and winds anomalies. For MSc thesis, a temporal variation of the Northern Hemisphere pressure systems will be studied using the empirical orthogonal function (EOF) analysis.

Voraussetzungen: Requirements for this project are an interest in global atmospheric circulation and paleoclimate and a willingness to communicate in English (one of the supervisors is not a German speaker). Nevertheless, the final report can be written in German. Experience with scripting languages (Python, Ferret, ncl) would be helpful, but can be acquired during the project.

Literatur:

• Bothe, O., Fraedrich, K. and Zhu, X.: Large-scale circulations and Tibetan Plateau summer drought and wetness in a high-resolution climate model, Int. J. Climatol., 31(6), 832–846, doi:10.1002/joc.2124, 2011.

• Braconnot, P., Otto-Bliesner, B., Harrison, S., Joussaume, S., Peterchmitt, J.-Y. J.-Y., Abe-Ouchi, A., Crucifix, M., Driesschaert, E., Fichefet, T., Hewitt, C. D. and others: Results of PMIP2 coupled simulations of the Mid-Holocene and Last Glacial Maximum–Part 1: experiments and large-scale features, Clim. Past, 3(2), 261–277 [online] Available from: http://www.clim-past.net/3/279/2007/cp-3-279-2007-relations.html, 2007.

• Ding, Q. and Wang, B.: Circumglobal teleconnection in the Northern Hemisphere summer, J. Clim., 18(17), 3483–3505, 2005.

• Haywood, A. M., Dowsett, H. J., Dolan, A. M., Rowley, D., Abe-Ouchi, A., Otto-Bliesner, B., Chandler, M. A., Hunter, S. J., Lunt, D. J. and Pound, M.: The Pliocene model intercomparison project (PlioMIP) phase 2: scientific objectives and experimental design, Clim. Past, 12(3), 663–675, 2016.

• Saeed, S., Müller, W. A., Hagemann, S. and Jacob, D.: Circumglobal wave train and the summer monsoon over northwestern India and Pakistan: The explicit role of the surface heat low, Clim. Dyn., 37(5), 1045–1060, doi:10.1007/s00382-010-0888-x, 2011.

Effects of uplift of the Tian Shan Mountains on Asian climate and stable water isotopes in precipitation

Betreuer: Svetlana Botsyun, Stephan Pfahl

Beschreibung: Stable hydrogen and oxygen isotopes in water vapour and precipitation are widely used as tracers to improve our understanding of the hydrological cycle on a variety of time scales, from hourly variations to changes over millions of years (Botsyun et al., 2019; Pfahl and Wernli, 2008). Stable water isotope proxy data (e.g., lake carbonates in which paleoprecipiation isotope values are preserved) are commonly used for paleoclimate reconstructions. However, mountain uplift affects both the climate and the preserved isotope signal (Botsyun et al., 2016, 2019). Therefore, reconstructing the influence of mountain elevation on stable water isotopes is critical for proper interpretation of paleoclimate data. Interactions between mid-latitude westerlies and the Tian Shan Mountains significantly impact climate patterns in Asia today (Sha et al., 2018) and Tian Shan uplift is hypothesised to play an important role in driving Asian climate change since the last ~ 30 Ma (Wang et al., 2020). Presently, Central Asia receives recycled, enriched with heavy isotopes moisture that has been mainly transported across Eurasia by the westerlies (Caves et al., 2015), but for the time when the Tian Shan Mountains were lower, the isotopic signature across the region remains enigmatic. Our preliminary analysis shows that Tian Shan uplift impact westerly and monsoon circulation in the region that results in precipitation and precipitation δ¹⁸O changes

Vorgehen: This work will focus on analysing completed model simulations rather than creating new model runs. We propose using a high-resolution (∼0.75°) isotope-enabled general circulation model (ECHAM5-wiso). Experiments are conducted with boundary conditions for the pre-industrial (year 1850) and varied elevation of Tian Shan. Student will be asked to analyse climatological mid-latitude circulation change and associated surface hydroclimate change. For the M.Sc. – Lagrangian analysis tool “LAGRANTO'' (Sprenger and Wernli, 2015) will be applies to trace moisture trajectories changes for various Tian Shan elevation scenarios.

Voraussetzungen: Requirements for this project are an interest in stable water isotopes and paleoclimate, and a willingness to communicate in English (one of the supervisors is not a German speaker). Nevertheless, the final report can be written in German. Experience with scripting languages (Python, Ferret) would be helpful, but can be acquired during the project.

Literatur:

• Botsyun, S., Sepulchre, P., Risi, C. and Donnadieu, Y.: Impacts of Tibetan Plateau uplift on atmospheric dynamics and associated precipitation δ18O, Clim. Past, 12(6), 1401–1420, doi:10.5194/cp-12-1401-2016, 2016.
• Botsyun, S., Sepulchre, P., Donnadieu, Y., Risi, C., Licht, A. and Caves Rugenstein, J. K.: Revised paleoaltimetry data show low Tibetan Plateau elevation during the Eocene, Science (80-. )., 363(6430), eaaq1436, doi:10.1126/science.aaq1436, 2019.
• Caves, J. K., Winnick, M. J., Graham, S. A., Sjostrom, D. J., Mulch, A. and Chamberlain, C. P.: Role of the westerlies in Central Asia climate over the Cenozoic, Earth Planet. Sci. Lett., 428, 33–43, 2015.
• Pfahl, S. and Wernli, H.: Air parcel trajectory analysis of stable isotopes in water vapor in the eastern Mediterranean, J. Geophys. Res. Atmos., 113(20), 1–16, doi:10.1029/2008JD009839, 2008.
• Sha, Y., Shi, Z., Liu, X., An, Z., Li, X. and Chang, H.: Role of the Tian Shan Mountains and Pamir Plateau in increasing spatiotemporal differentiation of precipitation over interior Asia, J. Clim., 31(19), 8141–8162, 2018.
• Sprenger, M. and Wernli, H.: The LAGRANTO Lagrangian analysis tool–version 2.0, Geosci. Model Dev., 8(8), 2569–2586, 2015.
• Wang, X., Carrapa, B., Sun, Y., Dettman, D. L., Chapman, J. B., Caves Rugenstein, J. K., Clementz, M. T., DeCelles, P. G., Wang, M., Chen, J., Quade, J., Wang, F., Li, Z., Oimuhammadzoda, I., Gadoev, M., Lohmann, G., Zhang, X. and Chen, F.: The role of the westerlies and orography in Asian hydroclimate since the late Oligocene, Geology, 48(7), 728–732, doi:10.1130/G47400.1, 2020.