Understanding drought governing atmospheric processes in the Greater Alpine Region

• Main Author: Haslinger, Klaus
• Other Authors: Blöschl, Günter, TU Wien, Österreich, Wagner, Klaus
• Format: Thesis
• Language: English
• Published: Wien 2020
• Subjects: Dürre; Großraum Alpen; drought; Greater Alpine Region; Arctic; Gar’; Last Peak; North Atlantic; North Atlantic oscillation; Sea ice
• Online Access: https://doi.org/10.34726/hss.2018.61522; https://hdl.handle.net/20.500.12708/7937

This thesis investigates the emergence of drought events in space and time in the Greater Alpine Region and their atmospheric drivers. The aim is to better understand the precipitation suppressing atmospheric processes on different spatial scales, from regional weather patterns and local feedbacks to the influence of global atmospheric and oceanic modes of circulation. Investigations on this topic have traditionally been based on models and/or relatively short observational data. This thesis goes beyond these studies by using a unique dataset in terms of record length and data quality that consists of climatic data from 1801 to 2010. Analyses of drought events and their atmospheric drivers on such long records provide an opportunity for understanding a broader spectrum of drought events and related drivers and processes. The thesis involves the following tasks: (i) identifying spatial characteristics of droughts on various time scales, (ii) analysing joint spatio-temporal features of droughts and (iii) linking droughts to atmospheric processes. Considering the long term perspective of more than 200 years of drought patterns in the GAR we find that the time periods of the 1850s to the 1870s and the 1940s were the driest ones. Analyses of spatial clustering on prescribed accumulation time scales (1, 3, 6, and 12 months) of precipitation deficit show that the Main Alpine Ridge is a major climatic divide for droughts, which does not only apply to daily and monthly but also to multi-monthly time scales. The frequency of droughts on different accumulation time scales shows no trends, but rather exhibits multidecadal variations which are more pronounced at higher accumulation time scales. Interestingly, these variations differ in space. The north and west were more drought prone in the middle of the 19th century, whereas the east was in the last decades. A new method is proposed for detecting atmospheric drought events and their space-time structure that does not prescribe time scales, which is used for analysing the long term evolution of drought frequency, duration, intensity and severity. Our results show variations of these characteristics on multi-decadal time scales, but no trends over the 210 year period are apparent. The 1860s and 1940s stand out as drought rich periods, although the characteristics of individual droughts in these decades are substantially different, indicating different driving mechanisms. Although air temperatures have increased significantly in the past 200 years, we do not find this increase to be significantly correlated with drought duration, intensity or severity. However, we find that dry springs significantly increase temperatures during subsequent summer droughts, which implies a soil moisture-temperature coupling in the warm season. To finally assess the link between observed drought events and atmospheric processes, a daily atmospheric circulation type reconstruction tailored to the Alpine region and various indices describing major modes of variability in the atmosphere and the ocean are analysed. Our results suggest positive Eastern Atlantic/Western Russia conditions as the main large scale ii atmospheric pattern related to anticyclonic circulation and therefore drought in winter and spring, while the North Atlantic Oscillation has no significant impact. In summer a positive soil moisture - precipitation feedback is detected, which is strongest during weak pressure gradient circulation types that favour local convection. The events of the outstanding dry decades of the 1860s and 1940s were triggered by strong precipitation anomalies during spring and enhanced by soil moisture - precipitation feedbacks during summer. The dry springs of the 1860s were caused by circulation characteristics that were quite different from those of the recent decades as a consequence of the last peak of the Little Ice Age and the related large extent of the Arctic sea ice. The dry springs of the 1940s were related to positive sea surface temperature anomalies in the western subtropical Atlantic, triggering distinct Rossby wave trains leading to persistent positive Eastern Atlantic/Western Russia circulation patterns. 85