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Dissertation zugänglich unter
URN: urn:nbn:de:gbv:18-92852
URL: http://ediss.sub.uni-hamburg.de/volltexte/2018/9285/

The Tropical Circulation in a Changing Climate

Die tropische Zirkulation unter dem Klimawandel

Plesca, Elina

 Dokument 1.pdf (15.116 KB) 

Freie Schlagwörter (Englisch): climate change , atmospherical circulation , tropical circulation , CO2 forcing
Basisklassifikation: 38.80
Institut: Geowissenschaften
DDC-Sachgruppe: Geowissenschaften
Dokumentart: Dissertation
Hauptberichter: Bühler, Stefan A. (Prof. Dr.)
Sprache: Englisch
Tag der mündlichen Prüfung: 29.06.2018
Erstellungsjahr: 2018
Publikationsdatum: 30.08.2018
Kurzfassung auf Englisch: The tropical atmospheric overturning circulations are likely weakening under increased CO2 forcing. However, insufficient understanding of the circulations’ dynamics diminishes the full confidence in such a response. Here I address two distinct aspects of the circulation change as it is represented in a series of CMIP5 (Coupled Model Intercomparison Project Phase 5) idealized experiments: the robustness of the Walker circulation in a warming climate and the fast response of the tropical overturning circulation to CO2 forcing. The first study investigates the changes in the Pacific Walker circulation under anthropogenic forcing and the sensitivity of its weakening response to internal variability, General Circulation Model (GCM) configuration, and indexing method. The sensitivity to internal variability is analyzed by using a 68-member ensemble of the MPI-ESM-LR model; the influence of model physics is analyzed by using the 28-member CMIP5 multi-model ensemble. Three simple circulation indices, based on mean sea-level pressure, 500 hPa vertical velocity and 200 hPa velocity potential, are computed for each member of the two ensembles. The analysis uses the outputs of the CMIP5 idealized transient climate simulations with 1% per year CO2 increase from pre-industrial level, and investigates the detected circulation response until the moment of CO2 doubling (70 years). Depending on the indexing method, it is found that 50-93% of the MPI-ESM-LR and 54-75% of the CMIP5 ensemble members project significant negative trends in the circulation’s intensity. This large spread in the ensembles reduces the confidence that a weakening circulation is a robust feature of climate change. Furthermore, the similar magnitude of the spread in both ensembles shows that the Walker circulation response is strongly influenced by natural variability, even on a 70-year period. In the second part of this thesis I analyze CMIP5 idealized climate experiments with abrupt quadrupling of atmospheric CO2 to understand the fast response of the tropical overturning circulation to this forcing and the main contributors to this response. I define a measure for the circulation intensity based on pressure velocity in the tropical subsidence regions. In doing so, I play on the advantage of the subsidence regions in being dynamically stable and on the fact that, from a measurement point of view, the observation of these regions is prone to less uncertainty than the convective regions. Also, the subsidence regions are less sensitive to the GCM’s cloud and precipitation parametrization schemes. My method allows to decompose the circulation intensity relative change (with respect to a control state) into a sum of the relative changes in subsidence area, static stability and atmospheric cooling rate. Also, I use aqua planet and realistic planet experiments to look into the effect of the land-sea differentiated heating on the total change in circulation strength. I find that on average the tropical circulation slows down and this change is dominated by the cooling rate reduction, as the other factors show a positive change. The cooling rate reduction results from the direct radiative effect of increased CO2 concentration in the atmosphere. I find that even in a realistic planet setup the circulation change is dominated by the changes in the subsidence regions over the oceans, but the land-sea differentiated heating also contributes to the slow-down of the circulation by driving the vertical expansion of the tropics. A brief analysis of the circulation changes in a coupled climate experiment suggests that the fast response of the circulation is detectable at time scales smaller than 1 year: another mechanism leading to the circulation weakening takes over around this time range, i.e. the increased static stability in response to the surface temperature rise occurring as a result of the CO2 greenhouse effect.


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