The Caucasus Region has been affected by an increasing number of heat waves during the last decades, which have had serious impacts on human health, agriculture and natural ecosystems. A dataset of 22 homogenized, daily maximum (T_max) and minimum (T_min) air temperature series is developed to quantify climatology and summer heat wave changes for Georgia and Tbilisi station between 1961 and 2010 using the extreme heat factor (EHF) as heat wave index. The EHF is studied with respect to eight heat wave aspects: event number, duration, participating heat wave days, peak and mean magnitude, number of heat wave days, severe and extreme heat wave days. A severity threshold for each station was determined by the climatological distribution of heat wave intensity. Moreover, heat wave series of two indices focusing on the 90th percentile of daily minimum temperature (CTN90p) and the 90th percentile of daily maximum temperature (CTX90p) were compared. The spatial distribution of heat wave characteristics over Georgia showed a concentration of high heat wave amplitudes and mean magnitudes in the Southwest. The longest and most frequently occurring heat wave events were observed in the Southeast of Georgia. Most severe heat wave events were found in both regions. Regarding the monthly distribution of heat waves, the largest proportion of severe events and highest intensities are measured during May. Trends for all Georgia-averaged heat wave aspects demonstrate significant increases in the number, intensity and duration of low- and high-intensity heat waves. However, for the heat wave mean magnitude no change was observed. Heat wave trend magnitudes for Tbilisi mainly exceed the Georgia-averages and its surrounding stations, implying urban heat island (UHI) effects and synergistic interactions between heat waves and UHIs. Comparing heat wave aspects for CTN90p and CTX90p, all trend magnitudes for CTN90p were larger, while the correlation between the annual time-series was very high among all heat wave indices analyzed. This finding reflects the importance of integrating the most suitable heat wave index into a sector-specific impact analysis.

During the last 50 years Georgia experienced a rising number of severe summer heat waves causing increasing heat-health impacts. In this study, the 10 most severe heat waves between 1961 and 2010 and recent changes in heat wave characteristics have been detected from 22 homogenized temperature minimum and maximum series using the Excess Heat Factor (EHF). A composite and Canonical Correlation Analysis (CCA) have been performed to study summer heat wave patterns and their relationships to the selected predictors: mean Sea Level Pressure (SLP), Geopotential Height at 500 mb (Z500), Sea Surface Temperature (SST), Zonal (u-wind500) and Meridional Wind at 500 mb (v-wind500), Vertical Velocity at 500 mb (O500), Outgoing Longwave Radiation (OLR), Relative Humidity (RH500), Precipitation (RR) and Soil Moisture (SM). Most severe heat events during the last 50 years are identified in 2007, 2006 and 1998. Largest significant trend magnitudes for the number, intensity and duration of low and high-impact heat waves have been found during the last 30 years. Significant changes in the heat wave predictors reveal that all relevant surface and atmospheric patterns contributing to heat waves have been intensified between 1961 and 2010. Composite anomalies and CCA patterns provide evidence of a large anticyclonic blocking pattern over the southern Ural Mountains, which attracts warm air masses from the Southwest, enhances subsidence and surface heating, shifts the African Intertropical Convergence Zone (ITCZ) northwards, and causes a northward shift of the subtropical jet. Moreover, pronounced precipitation and soil moisture deficiency throughout Georgia contribute to the heat wave formation and persistence over Georgia. Due to different large- to mesoscale circulation patterns and the local terrain, heat wave effects over Eastern Georgia are dominated by subsidence and surface heating, while convective rainfall and cooling are observed in the West.