The role of atmospheric moisture transport in major drought episodes

Abstract

Droughts are projected to become more intense and severe in the future as the result of increased evaporation and reduced precipitation. The transport of moisture from the sources to surrounding continents represents an important part of the hydrological cycle and deficits or changes in transport can play a crucial role in determining drought. Because of the important role of moisture transport in the development of drought, understanding the connections between the sources and sinks of moisture in the hydrological cycle is one of the most important challenges that humanity is currently facing. This thesis can be considered as an innovative application of the Lagrangian method used by the Environmental Physics Laboratory group (EPhysLab) in studies of droughts, since several studies concerning climate change have reported important changes in the structure of extreme weather phenomena in many regions and these changes may be related to variations in sources of moisture. In order to investigate the relationship between moisture transport and drought, anomalies in moisture transport during the most severe meteorological drought episodes around the world were investigated. The initial drought analysis was regional and focused on the Danube River Basin (DRB); the analysis was then extended to a continental scale, analysing Central Europe (CEU) and the Mediterranean region (MED). Finally, the 27 Reference Regions (RRs) specified in the 5th Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) were studied in order to expand the investigation to a global scale. First, the multiscalar Standardised Precipitation Evapotranspiration Index (SPEI) was used to identify drought episodes that occurred in the analysed regions. Separate analysis of meteorological droughts over three regions in Europe (DRB, CEU, MED) revealed the impact of the 2003 meteorological drought episodes across the continent. After identifying the drought episodes, anomalies in moisture transport were analysed in order to determine whether there were variations in moisture transport from the sources to the analysed regions during severe episodes of drought. Anomalies in moisture transport were investigated with the Lagrangian FLEXible PARTicle (FLEXPART) dispersion model, which was used to track air masses and locate regions where the particles gain or lose humidity. The climatological moisture sources for the analysed regions were characterized using Lagrangian backward tracking and the main moisture sources that serve as common sources for all three regions were determined to be the Mediterranean Sea, the North Atlantic Ocean, the Caspian Sea, the Black Sea, and terrestrial moisture sources surrounding the area. Once these sources were identified, forward tracking was performed in order to analyse anomalies in moisture contribution to the analysed areas during the most severe episodes of drought. This study determined that a decrease in moisture contribution and precipitation occurred along with the appearance of the 2003 drought episodes, while the episodes ended when primarily Mediterranean Sea (MDS), began to provide moisture to all three regions in Europe. Besides the analysis of anomalies in the transport of moisture, an analysis of moisture contribution from the MDS during meteorological drought episodes over CEU provided a climatological perspective of how moisture transport from a major moisture source may be linked to drought indicators and the occurrence of drought episodes. Finally, by analysing anomalous moisture transport and drought episodes in the 27 IPCC RRs, this study provides an in-depth systematic analysis of the moisture transport processes that trigger drought episodes at the global scale, which is necessary for understanding the driving factors of extreme weather events.