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Freie Universit¨at Berlin (2015)

Modelling the Asian Paleo-hydroclimatic Variability

Fallah Hassanabadi, Bijan

Titre : Modelling the Asian Paleo-hydroclimatic Variability

Modellierung der asiatischen paläohydroklimatischen Variabilität

Auteur : Fallah Hassanabadi, Bijan

Université de soutenance : Freie Universit¨at Berlin

Grade : Doctor rerum naturalium (Dr. rer. nat.) 2015

Résumé
Using a multi-proxy and multi-model approach, this study aims to unravel the characteristics of modern- and palaeo-hydroclimatic variability over Asia. This is designed on different time-scales and diverse geographically distributed regions in Asia. Special emphasis is given to extreme hydro- meteorological events (e.g., mega-droughts). The main focus of this investigation is on climatically sensitive regions of Asia (e.g., monsoon- dominated and westerly-dominated regions). The combination of different model and proxy data leads to an enhanced understanding of the controlling mechanisms of the Asian climate dynamics. In this thesis, palaeoclimate simulations of different time-slices are carried out for selected time periods. The main focus lies in global and regional model simulations, as well as the sensitivity tests using these models. In a first step, existing global simulations for the past 1,000 years are analyzed, concentrating on dynamics of Asian monsoon and Westerlies, and on climate modes like El Niño Southern Oscillation (ENSO), Pacific North Atlantic Oscillation (PNA) and North Atlantic Oscillation (NAO) and their tele-connections with the Asian climate. In this regard, two Paleoclimate Modelling Intercomparison Project Phase III (PMIP3) / Coupled Model Intercomparison Project Phase 5 (CMIP5) climate model ensemble simulations of the past millennium have been analyzed to identify the occurrence of Asian mega-droughts. The Palmer Drought Severity Index (PDSI) is used as the key metric for the data comparison of hydro-climatological conditions. The model results are compared with the proxy data of the Monsoon Asia Drought Atlas (MADA). This study shows that Global Circulation Models (GCMs) are capable to capture the majority of historically recorded Asian monsoon failures at the right time and with a comparable spatial distribution. The simulations indicate that ENSO-like events lead in most cases to these droughts. Both, model simulations and proxy reconstructions, point to less monsoon failures during the Little Ice Age. During historic mega-droughts of the past millennium, the monsoon convection tends to assume a preferred regime described as "break" event in Asian monsoon. This particular regime is coincident with a notable weakening in Pacific Trade winds and Somali Jet. The interesting periods that are run and analyzed include extreme rainfall anomalies within the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). The generated model data are compared with the recently published paleo-data derived from different archives. The global simulations served as boundary conditions for regional climate and its transition from one climate period to another (e.g. from MCA to LIA). For the selected climatic periods typical circulation anomalies responsible for changes in regional climate and the physical mechanisms driving them are identified. Additional sensitivity simulations are carried out with and without Tibetan Plateau to investigate and compare the existing hypotheses on the behavior of Asian summer monsoon due to plateau forcing. The analysis of sensitivity experiments point out to the signicant impacts of Plateau forcing on the atmosphere-ocean tele- connections. It is shown that, in addition to the direct feedbacks of Tibetan Plateau orography on the climate of Asia, such as sensible heat pumping and thermal insulation, other signicant processes exist, which link the Asian summer monsoon to the sea surface temperatures in the North Atlantic Ocean. A removal of the Tibetan Plateau modifiees the wind-driven ocean circulations over the North Atlantic, leading to a decrease of surface heat advection over the North Atlantic Ocean and a decrease of the Atlantic Meridional Overturning Circulation. This, in turn, affects via teleconnections both the monsoon rainfall and the position of the intertropical convergence zone. A climate modelling approach is presented to reproduce the rainfall patterns over Iran due to the climatic forcings during the past 6,000 years. The selected periods are simulated using a spatially high-resolved atmosphere General Circulation Model (GCM). The results show that the winter rainfall patterns over Iran have changed due to the changes in solar insolation to a wetter condition starting around 3,000 yr BP and reaching its maximum during the Medieval Climate Anomaly ca. 1,000 yr BP. The rainfall variability can be explained by the changes in the climate energy balance as a result of changing incoming solar irradiance based on the Milankovitch theory. A shift in the earth energy balance leads to the modulation of the West Asian Subtropical Westerly Jet (WASWJ). The investigations support the hypothesis that during the Holocene a northward shift in the WASWJ contributes to the less cyclonic activities over Iran. This brings less moisture into the region during the winter.

Mots clés  : Paleaoclimate ; Climate Change ; Monsoon ; Arid Central Asia ; Westerlies ; Megadroughts

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