Title: Recent and future drying of the Mediterranean region
Description: The Mediterranean region has experienced persistent drying since the middle of the 20th Century and global climate models project further drying in the future as a consequence of increasing greenhouse gases. The Mediterranean region is also known to oscillate between decades of relatively wet and dry conditions due to the strong influence of multidecadal North Atlantic Oscillation (NAO). It is therefore of great importance to understand the relationship between forced long-term drying resulting from human influences and those due to natural variability. To this end, we used observations, reanalyses and comprehensive global climate models in this thesis research. The roles of anthropogenic climate change and internal climate variability in causing the Mediterranean region's late 20th Century extended winter drying trend were examined using 20th Century observations as well as 19 coupled climate models from the CMIP3. The drying was strongly influenced by the robust positive trend in the NAO from the 1960s to the 1990s.

Model simulations and observations were used to assess the probable relative roles of radiative forcing and internal variability in explaining the circulation trend that drove much of the precipitation change. It was concluded that the radiatively forced trends were a small fraction of the total observed trends. Instead it was argued that the robust trends in the observed NAO and Mediterranean rainfall during this period were largely due to multidecadal internal variability with a small contribution from the external forcing. Differences between the observed and NAO associated precipitation trends are consistent with those expected as a response to radiative forcing. The radiatively forced trends in circulation and precipitation are expected to strengthen in the current century and these results highlight the importance of their contribution to future precipitation changes in the region. The Mediterranean precipitation climatology and trend were further examined by comparing the newest generation of global climate models (CMIP5) used in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report, to the previous generation (CMIP3) and to observations over the latter half of the 20th Century for both the summer and winter half years.

The observed drying trend since 1950 was predominantly due to winter drying, with very little contribution from the summer. However, in the CMIP5 multimodel mean, the precipitation trend since 1950 is evenly divided throughout the seasonal cycle. This may indicate that in observation, multidecadal internal variability, particularly that associated with the NAO, dominates the wintertime trend. An estimate of the observed externally forced trend showed that winter drying dominated in observations but the spatial patterns were grossly similar to the multimodel mean trend. The similarity was particularly robust in the eastern Mediterranean region, indicating a radiatively forced component being stronger there. These results also revealed modest improvement for the CMIP5 multimodel ensemble in representation of the observed six-month winter and summer climatology. We further explored the detailed mechanisms leading to the NAO-associated precipitation change, such as the role of the change in mean circulation versus that of the storm tracks in the regional moisture budget, which had not been investigated previously.

We employed a moisture budget analysis using 15 CMIP5 models and the ERA-Interim Reanalysis to investigate the relationship between the NAO and the various moisture budget terms for the six-month winter and summer. Compared with the ERA-Interim, the models performed well in their simulation of the relationship between the naturally varying NAO and the large-scale moisture budget. Our results indicated that the shift in the midlatitude transient eddies induced modest moisture convergence, rather than divergence, over the Mediterranean under a positive NAO. The reduction in precipitation in this region during a positive NAO was dominated by the mean moisture divergence, which opposed the transient contribution. There were significant differences between the patterns of NAO-induced moisture budget anomaly and changes due to external radiative forcing. Under radiative forcing there was enhanced evaporation over the Mediterranean Sea, Italy and eastern Europe and drying by the shift in the wintertime storms over nearly all of Europe and the Mediterranean.

Under a positive phase of the NAO, on the other hand, there was modest reduction in evaporation and wetting by the storms over the Mediterranean, and drying over northern Europe. The dependence of the Mediterranean moisture budget on the NAO was similarly explored in the summer half of the year and in this season the models exhibited more disagreement with observations, but otherwise showed the similar results as winter. The stronger anthropogenic induced drying signal over the eastern Mediterranean provided a basis to examine the possible cause and impact of the recent severe and persistent drought in Syria that occurred directly prior to the uprising of 2011. The drought devastated Syrian agriculture, resulting in food shortages, widespread unemployment, the collapse of rural social structure and a mass migration of agricultural refugees to Syria's urban areas. Anger at the government's failure to ameliorate conditions was one spark for the uprising that evolved into civil war. We found that though droughts occur periodically in Syria due to natural causes it is likely that the recent drought was more extreme due to the century long drying trend caused by increased radiative forcing.

It was estimated that the anthropogenic trend made a drought of such severity several times more likely. Droughts as persistent as the recent one are projected to be commonplace in a future warmer world.