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.

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This study examines the impact of natural climate variability and long-term climate change on North Atlantic tropical cyclone (TC) tracks. Using data from HURDAT for the period 1950-2007, we categorize Atlantic TCs that form in the Main Development Region into one of three track types: Straight-Moving (SM), Recurving Landfall (RCL), and Recurving Ocean (RCO) TCs. As expected, the SM storms are associated with a westward extension and strengthening of the North Atlantic Subtropical High (NASH) whereas the RCO storms coincide with a weakening of the NASH. The presence of El Nino conditions in the tropical Pacific is found to be associated with a weakening and eastward retreat of the NASH, an increase in the percentage of RCO TCs, and a decrease in the percentage of RCL TCs. Using 6-hourly wind fields from NCEP-NCAR Reanalyses, simulated tracks are computed for each historical storm in the sample using the Beta Advection Model (BAM). Using observed genesis locations, the BAM successfully reproduces the differences in TC tracks between SM, RCO and RCL storm types. When storm genesis is uniformly distributed over the MDR we find that RCL and RCO storms still exhibit a distinct difference in tracks, suggesting that differences in the large-scale steering flow over the tropical Atlantic are primarily responsible for their track differences. However the SM TCs exhibit a more northward track under the uniform genesis experiment, indicating that the more southern and western genesis location of these storms is an important contributor in determining their tracks. The observed difference between TC tracks during El Nino and La Nina events is also reproduced by the BAM under both observed and uniformly seeded genesis experiments, suggesting that it is the changes in the large-scale steering flow over the Atlantic that is responsible the larger percentage of RCO storm tracks during El Nino events. The influence of anthropogenic warming is examined using a 7 member ensemble comparing the 2xCO2 experiment to a pre-industrial control. Increased sea level pressure over the northeast and northwest quadrants of the Atlantic does not alter the average TC track.

Underwater (phreatic) caves are a ubiquitous landform on coastal karst terrain, but the marine geological processes operating in these systems are largely unknown. This dissertation redresses the problem by asking if Bermudian phreatic cave sediments archive sea-level and climate information? An important premise is that coastal cave environments are not identical. They can be categorized based on whether they are terrestrially-influenced (anchialine), completely flooded by saline groundwater (submarine), positioned at sea level (littoral) or in the vadose zone (vadose). For the first time the boundary between modern anchialine and submarine cave environments has been distinguished in Green Bay Cave using a multi-proxy approach (benthic foraminifera, sedimentary organic matter content and carbon isotopic composition - ?13Corg, and grain-size analysis). Twelve push cores were extracted from Green Bay Cave and dated with twenty 14C dates, recovering the first underwater cave succession spanning the Holocene (13 ka to present). Green Bay Cave transitioned through all major cave environments during Holocene sea-level rise (vadose, littoral, anchialine, and submarine), providing a sedimentary model for global cave successions. These relationships provide a novel means to solve Quaternary sea-level and climate problems. For sea level, two examples indicate that the littoral cave can be used as a sea-level indicator, distinguished stratigraphically by microfossil or sedimentary proxies. First, the elevation and timing of when Green Bay Cave was a littoral environment indicates Bermuda experienced an abrupt ~6.4 m sea-level rise at 7.7 ka, coinciding with final collapse of the Labrador sector of the Laurentide Ice Sheet. Second, microfossils preserved in elevated caves at +21 m above modern sea level and dated to marine isotope stage 11 (U-series, amino acid racemization) are consistent with modern Bermudian caves and co-stratigraphic sea level. For climate problems, annual temperature monitoring in Walsingham Cave indicates that cave water is thermally comparable to regional oceanographic conditions in the Sargasso Sea. Three sediment cores dated with sixteen radiocarbon dates indicate that Bermuda’s coldest and stormiest conditions of the last 3.2 ka occurred during the Little Ice Age (proxies: ?18Oc, grain size, bulk organic matter).

The first two empirical orthogonal functions of the winter (DJF) surface air temperature (SAT) over North America are associated with the North Atlantic Oscillation (NAO) and the Pacific/North American (PNA) pattern in the NCEP reanalysis. Lagged correlations between the North American SAT structures and the sea surface temperature (SST) were computed. There is a small lag between the tri-pole SST anomaly pattern of the North Atlantic Ocean and the first SAT mode. The second SAT mode lags the eastern Tropical Pacific SST anomaly by two months, associated with ENSO through the winter. A similar analysis is conducted on the seasonal forecasts to see if the forecast models capture the above links. GCM3 captures the ENSO forcing and has the PNA response. GEM captures the link between the SAT and the tripole SST anomaly pattern in North Atlantic. Although GEM captures the ENSO signal, it cannot form the PNA to further this tropical forcing into North America.

Thesis (M.S.)--Oregon State University, 2010. / Printout. Includes bibliographical references (leaves 61-67). Also available on the World Wide Web.

Thesis (M.S. in Meteorology and M.S. in Physical Oceanography)--Naval Postgraduate School, March 2005. / Thesis Advisor(s): Wendell A. Nuss, Thomas H.C. Herbers. Also available online.

Thesis (M.S.)--University of Delaware, 2005. / Principal faculty advisor: Brian Hanson, Dept. of Geography. Includes bibliographical references.

Four regional Scots pine ring-width chronologies at the northern forest-limit, and in the northern, middle and southern boreal forest belts in Finland cover the last fourteen centuries. Tree-ring statistics and response functions were examined, and tree-ring width variation was also compared to North Atlantic Oscillation (NAO) and volcanic forcing. The tree-ring statistics show evidence of an ecogeographical gradient along a north-south transect. The three northernmost regional chronologies share a positive response to mid-summer temperature, and all four chronologies show positive and significant correlation to early-summer precipitation. Moreover, a positive and significant relationship to winter NAO was detected in three out of four regional chronologies. NAO also drives the common (inter-regional) growth variability. Years of known cool summers caused by volcanic forcing exhibit exceptionally narrow tree rings in the three northernmost regional chronologies.