Tropical atmospheric circulation changes under global warming

Bayr, Tobias (2013) Tropical atmospheric circulation changes under global warming (Doctoral thesis/PhD), Christian-Albrechts-Universität Kiel, Kiel, Germany, 95 pp

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This PhD thesis consists of three scientific articles. In the first one the tropical sea level pressure (SLP) response under global warming is investigated in a multi-model ensemble (MMEns) of climate models from the 3rd phase of the Coupled Model Intercomparison Project (CMIP3) and in ERA Interim reanalysis data. In this article we follow the idea to split up the tropical warming into a spatial homogeneous part and a spatial inhomogeneous part. Even though the inhomogeneous warming is roughly 10 times smaller than the homogeneous warming, the tropical SLP changes are strongly related to the inhomogeneous tropospheric temperature (Ttropos) changes under global warming. According to the Bjerknes Circulation Theorem, this strong relationship can be explained with a nearly direct circulation between a warmer and a colder air column, due to weak Coriolis force in the tropics, with rising air and low SLP in the warmer air column and sinking air and high SLP in the colder air column. The inhomogeneous Ttropos warming is dominated by the land-sea warming contrast. Also SLP trends show a land-sea contrast due to the strong link with Ttropos. It is charactericsed by a SLP decrease over South America and Africa and a SLP increase over the Indo-Pacific. This result is confirmed in an idealised sensitivity experiment. The trends of SLP and Ttropos in ERA Interim over the period from 1989 until 2010 are also strongly linked to each other, but here the land-sea warming contrast is not the dominant driver of Ttropos warming, indicating that the trends over the last decades are mostly due to internal variability. The second article presents a method for comparing the spatial patterns of variability on the basis of Empirical Orthogonal Function (EOF) modes. This method is called Distinct Empirical Orthogonal Function (DEOF) analysis and can be used to find differences in the patterns of variability e.g. between past and future climate or between models and observations. It compares the whole multivariate structure of two datasets and finds the patterns with the largest difference in explained variance between those. We illustrate in well-defined artificial examples how changes in the modes of variability (an intensification, a shift or a multivariate change) can be revealed with this method. Further we compare our approach with examples from the literature. This includes changes in the modes of variability under global warming in SLP over the North Atlantic and Europe, SLP of the Southern Hemisphere, surface temperature of the Northern Hemisphere, sea surface temperature of the North Pacific and precipitation in the tropical Indo-Pacific. In the third article the changes in the equatorial zonal circulation cells, with its most prominent part the Walker Circulation, are investigated in a MMEns of CMIP3 and CMIP5 models under global warming. In the mean state we found a weakening of the zonal circulation cells as well as an eastward shift of the Walker cell. The global warming trend has a similar structure to the variability in zonal stream function that is associated with El Nino Southern Oscillation (ENSO). Indeed, two third of the Walker Circulation changes be explained by a trend towards more El-Nino-like conditions. Under global warming the modes of variability show an eastward shift of the dominant mode, which is consistent with the El-Nino-like trend and a spatial non-linearity found in ENSO associated variability. The CMIP3 and CMIP5 MMEns show in nearly all aspects very similar results, which underlines the robustness of these results. The trends in ERA Interim reanalysis data over the period from 1979 to 2012 show a westward shift and a strengthening of the Walker Circulation, hence in contrast to the CMIP MMEns a trend towards more La-Nina-like conditions. The trend in dominant mode of variability can here explain three quarter of the Walker Circulation changes. Thus long-term trends of the Walker Circulation seem to follow, to a large part, the pre-existing dominant mode of internal variability.

Document Type: Thesis (Doctoral thesis/PhD)
Thesis Advisors: Latif, Mojib and Dommenget, Dietmar
Keywords: Climate change, atmospheric circulation, tropics Klimawandel, atmosphärische Zirkulation, Tropen
Research affiliation: OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-ME Maritime Meteorology
Date Deposited: 17 Dec 2013 14:25
Last Modified: 17 Dec 2013 14:25

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