Temperature extremes over Georgia : changes, patterns and driving forces

Georgia has been affected by an increasing number of temperature extreme events during the last decades, which had serious impacts on human health, agriculture and natural ecosystems. This cumulative dissertation contains four research articles, which investigate annual and seasonal climatology, changes and driving forces of temperature extremes focusing on summer heat waves over Georgia during the last 50 years. A new dataset of 22 homogenized, daily maximum and minimum air temperature series is developed. Temperature extremes are investigated using ETCCDI extreme indices to quantify annual and seasonal trends for the periods 1961-2010, 1971-2010 and 1981-2010. Pronounced warming trends for all Georgia-averaged trends are observed during the most recent analysis period, whereas the magnitude of trends for night-time indices is larger than those for daytime. Largest trend magnitudes could be proven for warm temperature extremes in summer. In order to identify heat wave events, a well-established heat wave index based on the Excess Heat Factor is used, which considers heat-health as a combined measure of excess heat and heat stress. It is studied with respect to eight heat wave aspects: event number, duration, participating heat wave days and peak and mean magnitude. In addition, a severe Excess Heat Factor threshold for each station was calculated empirically as the 85th percentile of the distribution of positive Excess Heat Factor values based on the observation record of each station. As a result, three new aspects have been developed in this study to be able to differentiate between low-intensity, severe and extreme heat waves and their potential heat-health impacts: the number of heat wave days, severe and extreme heat wave days. The spatial distribution of heat wave characteristics over Georgia shows a concentration of high heat wave intensities in the Southwest. The longest and most frequently occurring heat wave events are observed in the Southeast of Georgia. Most severe events are found in both regions. Trends for all Georgia-averaged heat wave aspects demonstrate significant increases in the number, intensity and duration of low- and high-intensity heat waves. Largest significantly positive trend magnitudes for the number, intensity and duration of low and high-impact heat waves have been found during the last 20 years. As a case study, Tbilisi station is chosen to investigate heat wave changes considering the urban heat island effect. Heat wave trend magnitudes for Tbilisi exceed the Georgia-averages and its surrounding stations, implying urban heat island effects and synergistic interactions between heat waves and these effects. The five longest and most intense heat waves between 1961 and 2010 are detected in the summers of 2007, 2006, 2001, 1998 and 1995. To investigate the relationship between heat waves and their potential driving forces a daily composite analysis and a Canonical Correlation Analysis is carried out regarding the Mean Sea Level Pressure, Geopotential Height at 500mb, Sea Surface Temperature, Zonal and Meridional Wind at 500mb, Vertical Velocity at 500mb, Outgoing Longwave Radiation, Relative Humidity, Precipitation and Soil Moisture during major events. As main driving mechanism for heat waves over Georgia a large anticyclone over the Southern Ural is detected, which blocks westerlies, attracts warm air masses from the Southwest, enhances subsidence and surface heating over Georgia, shifts the African Intertropical Convergence Zone northwards, and causes a northward shift of the subtropical jet. In addition to the synoptic features triggering heat wave formation, it is shown that strong soil moisture and precipitation deficiencies contribute to the heat wave persistence over Georgia